Prof. Dr. Sumit Chakraborty

Institute of Geology, Mineralogy and Geophysics
Ruhr-Universität Bochum

Author IDs

  • An experimental study of proton implantation in olivine
    Bissbort, Thilo and Jiang, Qinting and Becker, Hans-Werner and Foteinou, Varvara and Chakraborty, Sumit
    Physics and Chemistry of Minerals 50 (2023)
    Implantation of ions in minerals by high energy radiation is an important process in planetary and materials sciences. For example, the solar wind is a multi-ion flux that progressively modifies the composition and structure of near-surface domains in solar objects, like asteroids. A bombardment of a target by different elements like hydrogen (H) at various energies causes, among other things, the implantation of these particles in crystalline and amorphous materials. It is important to understand the mechanisms and features of this process (e.g., how much is implanted and retained), to constrain its contribution to the chemical budget of solar objects or for planning various material-science applications. Yet, there has been no detailed study on H implantation into olivine (e.g., the quantification of maximum retainable H), a major mineral in this context. We performed experiments on H implantation in San Carlos olivine at 10 and 20 keV with increasing fluences (up to 3×1018 at/cm2). Nanoscale H profiles that result from implantation were analyzed using Nuclear Resonance Reaction Analysis after each implantation to observe the evolution of the H distribution as a function of fluence. We observed that after a systematic growth of the characteristic, approximately Gaussian shaped, H profiles with increasing fluences, a maximum concentration at H ~ 20 at% is attained. The maximum concentration is approximately independent of ion energy, but the maximum penetration depth is a function of beam energy and is greater at higher energies. The shapes of the profiles as well as the maximum concentrations deviate from those predicted by currently available models and point to the need for direct experimental measurements. We compared the depth profiles with predictions by SRIM. Based on observations from this study, we were able to constrain the maximum retainable H in olivine as a function of ion energy. © 2023, The Author(s).
    view abstract10.1007/s00269-023-01234-9
  • How does inherited zircon survive in partially molten mantle: Insights on modes of magma transport in the mantle from nanoscale melt-crystal interaction experiments
    Cambeses, Aitor and Chakraborty, Sumit and Jöns, Niels and Montero, Pilar and Bea, Fernando
    Earth and Planetary Science Letters 601 (2023)
    Inherited zircons found in mantle-derived rocks are increasingly relevant for characterizing the history and petrogenetic processes of these rocks. However, the occurrence of inherited zircons in mafic igneous rocks poses an enigma because, as shown by numerous experiments combined with modelling studies, zircon, e.g., 150 μm size, is expected to dissolve extremely swiftly, e.g., c. 1 day, in the presence of mafic melts. Slow dissolution kinetics may explain zircon survival in fast ascending magmas that scavenged zircons near their final emplacement, but it cannot explain the persistence of inherited zircons that dwelled in the magma-bearing mantle for more extended periods. To understand how these zircons can survive over long times in contact with mafic melts, we performed a series of experiments, including some novel nano-scale experiments, to explore the behaviour of zircons in contact with different volumes of melt. Our results demonstrate that if melt occurs in narrow, spatially restricted domains around zircon grains, they get rapidly saturated with Zr. From this point onwards, the melt acts as a protective layer preventing further dissolution. These results carry important implications for the mode of melt transfer in mantle segments from which inherited zircons have been found: zircon crystals in partially molten mantle systems can only survive long in mush-like zones in which the melt migrates through porous flow, the wetting behaviour of basaltic melt on zircon would appear to further facilitate such survival. Zircon crystals would be rapidly consumed in high melt fraction regions of channelised (e.g., dike) flow. Such regions are therefore constrained to be short-lived (weeks to months) and pulsed in nature. Thus, the presence of zircons in mantle-derived rocks helps to place constraints on modes and timescales of melt transfer in the respective mantle segments. © 2022 Elsevier B.V.
    view abstract10.1016/j.epsl.2022.117911
  • Phase-Field Simulation of Texture Evolution in Magmatic Rocks
    Kundin, J. and Steinbach, I. and Chakraborty, S.
    Journal of Geophysical Research: Solid Earth 128 (2023)
    The tool of phase-field modeling for the prediction of chemical as well as microstructural evolution during crystallization from a melt in a mineralogical system has been developed in this work. We provide a compact theoretical background and introduce new aspects such as the treatment of anisotropic surface energies that are essential for modeling mineralogical systems. These are then applied to two simple model systems—the binary olivine-melt and plagioclase-melt systems - to illustrate the application of the developed tools. In one case crystallization is modeled at a constant temperature and undercooling while in the other the process of crystallization is tracked for a constant cooling rate. These two examples serve to illustrate the capabilities of the modeling tool. The results are analyzed in terms of crystal size distributions (CSD) and with a view toward applications in diffusion chronometry; future possibilities are discussed. The modeling results demonstrate that growth at constant rates may be expected only for limited extents of crystallization, that breaks in slopes of CSD-plots should be common, and that the lifetime of a given crystal of a phase is different from the lifetime of this phase in a magmatic system. The last aspect imposes an inherent limit to timescales that may be accessed by diffusion chronometry. Most significantly, this tool provides a bridge between CSD analysis and diffusion chronometry—two common tools that are used to study timescales of magmatic processes. © 2023. The Authors.
    view abstract10.1029/2023JB026718
  • Thermal and Chemical Evolution of an Archean Collision Zone: Insights from P-T-t History of Mafic Granulites from the Coorg Block, S. India
    Basak, Sampriti and Hasenstab, Eric and Bhowmik, Santanu Kumar and Gerdes, Axel and Dasgupta, Somnath and Münker, Carsten and Kumar, Gr. Ravindra and Chakraborty, Sumit
    Journal of Petrology 64 (2023)
    Knowledge of pressure-temperature-time (P-T-t) evolution of Archean high-grade (deep crustal) metamorphic rocks is important for deciphering the nature of Archean tectonic processes. However, exposures of such rocks are limited in the present rock record. Here, we study a suite of high-grade, mafic rocks that are present along a crustal-scale shear zone (called the Mercara Shear Zone) between two Archean terrains of India, the Coorg block and the Dharwar Craton. Given that the Mercara Shear Zone is dated to be Mesoarchean, these shear zone rocks are well suited to elucidate Archean orogenic processes. Petrological investigation shows that these mafic rocks are characterized by a granulitic assemblage of orthopyroxene, clinopyroxene, plagioclase, quartz and amphibole ± garnet, and with accessory phases such as apatite, ilmenite, magnetite and rutile in some cases. We distinguish the investigated rocks into low-Mg and high-Mg varieties based on their whole-rock composition as well as their mode of occurrence in the field and mineral chemistry. This difference in the bulk composition led to different reaction histories - for example, the low-Mg mafic granulites underwent partial melting while high-Mg granulites were less fertile. Combining these observations with the results of geothermobarometry, phase equilibria modeling, geochronology (U-Pb in zircon and Lu-Hf in garnet) and diffusion modeling, we have reconstructed a multi-stage P-T-t history for these rocks. The first phase (Stage 1) is represented by granulite-grade metamorphism at ~750-900°C and 8-13 kbar during ~3100 Ma (with uncertainties permitting a timing as recent as ~2700 Ma), after which they resided at T <500°C, likely at lower crustal levels (Stage 2). Subsequently, these rocks were reheated to a T of 700-750°C at 7-10 kbar at ~2400 Ma (Stage 3) and then again cooled down to ~500-600°C at 6-8 kbar (Stage 4). Application of diffusion chronometry shows that (1) the cooling rates of these granulites at high temperatures (>600°C) varied in the range of 25-50°C/Ma, and (2) the rocks resided for a long duration (~500 million years) at the Stage 2 metamorphic conditions, i.e. at T <500°C. We infer that such a protracted, high-T metamorphic history involving at least two heating pulses, and the relatively slow cooling rates on the order of 10's °C/Ma are consistent with the operation of peel-back styled orogenesis (an embryonic form of plate tectonics) on an early hotter Earth (Mesoarchean to Paleoproterozoic). Moreover, the controls of bulk rock compositions on reaction histories provide a mechanism for intracrustal differentiation and generating Mg-rich, refractory material that may have eventually formed the lower continental crust over a protracted and pulsed thermal evolution spanning several hundred million years. © 2023 The Author(s). Published by Oxford University Press. All rights reserved.
    view abstract10.1093/petrology/egad026
  • Diffusion chronometry of volcanic rocks: looking backward and forward
    Chakraborty, S. and Dohmen, R.
    Bulletin of Volcanology 84 (2022)
    view abstract10.1007/s00445-022-01565-5
  • Hydrogen Diffusion in Clinopyroxene at Low Temperatures (195°C–400°C) and Consequences for Subsurface Processes
    Bissbort, T. and Lynn, K.J. and Becker, H.-W. and Chakraborty, S.
    Geochemistry, Geophysics, Geosystems 23 (2022)
    view abstract10.1029/2022GC010520
  • Internal stress-induced recrystallization and diffusive transport in CaTiO3-PbTiO3solid solutions: A new transport mechanism in geomaterials and its implications for thermobarometry, geochronology, and geospeedometry
    Beyer, C. and Chakraborty, S.
    American Mineralogist 106 (2021)
    We conducted a series of high-temperature experiments where single crystals of CaTiO3 were embedded in PbTiO3 powder for durations of 4 to 502 h at temperatures between 753 and 1207 °C. Combined with results from a previous study (Beyer et al. 2019), these experiments allow us to explore the influence of chemical potential gradients on the mechanisms of incorporation of Pb in CaTiO3. Unlike in the previous study where Pb diffused into CaTiO3, here we find that the rims of the CaTiO3 crystals recrystallize to form a polycrystalline aggregate of [PbxCa(1-x)]TiO3 solid solutions. The width of the recrystallized front increases with run duration, and the contact to the single crystal becomes progressively wavy. The concentration of Pb decreases within the recrystallized front toward the interior of the single crystal, and the newly formed crystals are of different chemical compositions and orientations and are themselves chemically zoned. There is a discontinuous jump in Pb-concentration at the contact of the recrystallized front with the single crystal (termed "the migrating interface"). The development of chemical concentration gradients, combined with the fact that the width of the front grows as the square-root of time indicates a role of diffusion in the process; the formation of new crystals with different composition and no orientation relation to the precursor, and the jump in concentration at the boundary between the newly formed crystals and the single crystal indicates a dissolution-precipitation type process. Thus, this is a novel mechanism where diffusion, as well as dissolution-precipitation (in the sense that the structure of the single crystal is destroyed and replaced by new crystals), occurs simultaneously in a coupled manner and neither is rate-determining. The observations in this experimental study provide several insights into the mechanisms of chemical transformation in such non-metallic materials: (1) chemical differences can trigger mechanical deformation, which in turn can control chemical fluxes; (2) newly formed crystals, even at the high temperatures of the experiments, evolve continuously in chemistry and are not of an equilibrium composition; (3) the activation energy of the overall process (~70 kJ/mol) is lower than that for diffusion and provides a more effective means of chemical transformation, even though diffusion plays a central role in the process; and (4) it underscores the role of surface/interface free energy in the evolution of the transformation process. These results have important consequences for the reading of the petrological and geochemical signatures of the rock record - notably, in addition to knowing when a phase becomes chemically stable, it is also important to know when a particular crystal of the phase begins to exist. Some possible implications for thermobarometry, isotopic dating, and geospeedometry/ diffusion chronometry are discussed. © 2021 Mineralogical Society of America.
    view abstract10.2138/am-2021-7588
  • Novel approach to study diffusion of hydrogen bearing species in silicate glasses at low temperatures
    Bissbort, T. and Becker, H.-W. and Fanara, S. and Chakraborty, S.
    Chemical Geology 562 (2021)
    Diffusion of hydrogen bearing species in glasses plays a significant role in numerous applications in commercial as well as scientific domains. The investigation of diffusion of water in glasses at low temperatures led to experimental and analytical difficulties in the past. We present a new approach that lets us overcome these complications. Diffusion couples of An50Di50 glass (mol %, NBO/T = 0.67) were produced by coating anhydrous glass substrates with thin films of hydrated glass (~200 nm, ~2 wt% H2O) using pulsed laser deposition (PLD). Bonding the diffusant to the glass matrix of the thin film instead of using free water at the interface during experiments precludes other glass altering processes such as dissolution and precipitation. This allows us to confidently interpret the measured profiles to be a result of diffusion only. Nanoscale concentration profiles that result from diffusion at low temperatures on experimentally feasible time scales were measured with the Nuclear Resonance Reaction Analysis (NRRA, 1H(15N,αγ)12C). The non-destructive nature of NRRA enables us to observe and better understand the evolution of diffusion profiles with time within one sample. Evaluation of the sample quality by EPMA, SEM, optical microscopy, Rutherford backscattering spectroscopy (RBS), and NRRA was performed and confirmed the suitability of the samples for diffusion studies. Experiments at 1 atm in a box furnace and at 2 kbar in a CSPV (pressure medium = water) and an IHPV (pressure medium = Argon) prove that the diffusion couples can be used under various experimental conditions. We present diffusion profiles that were measured in experiments carried out in these devices and discuss the distinct features of each that result from different boundary conditions in the experiments. © 2020 Elsevier B.V.
    view abstract10.1016/j.chemgeo.2020.120037
  • Presentation of the Dana Medal of the Mineralogical Society of America for 2020 to Daniela Rubatto
    Chakraborty, S.
    American Mineralogist 106 (2021)
    view abstract10.2138/am-2021-AP10653
  • Time will tell: Secular change in metamorphic timescales and the tectonic implications
    Chowdhury, P. and Chakraborty, S. and Gerya, T.V.
    Gondwana Research 93 (2021)
    The pressure-temperature-time (P-T-t) evolution of metamorphic rocks is directly related to geodynamics as different tectonic settings vary in their thermal architecture. The shapes of P-T paths and thermobaric ratios (T/P) of metamorphic rocks have been extensively used to distinguish different tectonic domains. However, the role of metamorphic timescales in constraining tectonic settings remains underutilized. This is because of the poorly understood relationship between them, and the difficulty in accurately constraining the onset and end of a particular metamorphic event. Here, we show why and how the intrinsic relationship between thermal regime, rheology and rate of motion controlled by the heat, mass and momentum conservation laws translate to differences in heating, cooling, burial, exhumation rates of metamorphic rocks and thereby, to the duration of metamorphism. We compare the P-T-t paths of the orogenic metamorphic rocks of different ages and in particular, analyse their retrograde cooling rates and durations. The results show that cooling rates of the metamorphic rocks are variable but are dominantly <50 °C/Ma during most of the Precambrian before increasing by an order of magnitude (>100 °C/Ma) during the late Neoproterozoic to Phanerozoic. To seek what controlled this secular change in metamorphic cooling rates, we use thermomechanical modelling to calculate the P-T-t paths of crustal rocks in different types of continental orogenic settings and compare them with the rock record. The modelled P-T-t paths show that lithospheric peel-back driven orogenic settings, which are postulated as an orogenic mode operating under the hotter mantle conditions of late Archean to early Proterozoic, are characterised by longer durations of metamorphism and slower cooling rates (a few 10s of °C/Ma) as compared to the modern orogenic settings (a few 100s of °C/Ma) operating under relatively colder mantle conditions. This is because peel-back orogens feature: (1) hot lithospheres with very high crustal geotherms being sustained by high mantle heat-flow and profuse magmatism, and (2) distributed deformation patterns that limit vertical extrusion (exhumation) of the metamorphic rocks along localized deformation zones and instead, trap them in the orogenic core for a long time. In contrast, modern orogens mostly involve colder lithospheres and allow rapid exhumation through localized deformation, which facilitates faster cooling of hot, exhumed metamorphic rocks in a colder ambience. Thus, we propose that the secular change in metamorphic cooling rates indicates a changing regime of orogenesis and thereby, of plate tectonics through time. Predominance of the slower metamorphic cooling rates before the Neoproterozoic indicate the occurrences of peel-back orogenesis and truncated hot (collisional) orogenesis during that time, while the appearance of faster cooling rates since the late Neoproterozoic indicates the transition to modern style of orogenesis. A transition between these orogenic styles also accounts for the prolonged longevity (>100 million years) of many Precambrian orogenic belts as compared to the Phanerozoic ones. This study underscores the strength of timescales in combination with P-T paths to distinguish tectonic settings of different styles and ages. © 2021 International Association for Gondwana Research
    view abstract10.1016/
  • Multicomponent diffusion of F, Cl and OH in apatite with application to magma ascent rates
    Li, W. and Chakraborty, S. and Nagashima, K. and Costa, F.
    Earth and Planetary Science Letters 550 (2020)
    Chemical zoning in igneous minerals is a potential record of the time, processes, and thermal evolution during the lifetime of a given magma reservoir. Abundances of volatiles (OH, Cl and F) in apatite from terrestrial and extraterrestrial plutonic and volcanic rocks have been used to study volatile behavior in magmas, however, volatile diffusivities in apatite are poorly constrained. Here we report new experimental results on Cl, F and OH diffusivities in apatite and apply them to estimate magma ascent times and rates. The experiments were carried out on oriented natural Durango fluorapatite crystals at 800–1100 °C, 1-atm, and oxygen fugacity at the wüstite-magnetite buffer. Experimental charges and chemical profiles were investigated with a variety of methods, including scanning electron microscopy, transmission electron microscopy, electron probe microanalysis, secondary ion mass spectrometry, and nuclear reaction analysis. We find that the concentration profiles of Cl show evidence of uphill diffusion that is likely related to the co-existence of three monovalent anions, i.e., OH−, Cl−, F−, at the same site of the apatite structure. Chemical gradients of OH, Cl and F were reproduced using a multicomponent diffusion model to extract the tracer diffusion coefficient (Di⁎) of each component (i). The calculated values of Di⁎ parallel to the c-axis show a general relation of DF⁎&gt;DCl⁎&gt;DOH⁎, and define the following Arrhenius relations (parallel to the c-axis, at 1 bar) as: [Formula presented] [Formula presented] [Formula presented] The activation energy for Cl diffusion that we determined (294 kJ⋅mol−1) is within the range of that reported by Brenan (1994), but the pre-exponential factor is smaller and thus we obtain in general slower diffusivities than Brenan (1994). DCl⁎ and DOH⁎ parallel to the a-axis are 1 to 2 orders of magnitude slower than those parallel to the c-axis, indicating anisotropic diffusion of Cl and OH. Preliminary results on S diffusivity (parallel to the c-axis) at 800–900 °C show values between those of Cl and OH. The diffusion coefficients and model proposed in this study can be used to estimate the timescales of volatile re-equilibration in apatite in a variety of contexts from plutonic rocks and layered intrusions, to volcanic rocks and meteorites. We show that, for example, magma ascent rates can be determined by modelling Cl zoning in volcanic apatite. These applications provide new opportunities for understanding the influence of magma ascent rates on the eruption styles of volcanoes, thus having potential contributions to improving volcano forecasting and hazard assessments. © 2020
    view abstract10.1016/j.epsl.2020.116545
  • Peel-back controlled lithospheric convergence explains the secular transitions in Archean metamorphism and magmatism
    Chowdhury, P. and Chakraborty, S. and Gerya, T.V. and Cawood, P.A. and Capitanio, F.A.
    Earth and Planetary Science Letters 538 (2020)
    The preserved archive of continental crust suggests that various secular geologic and geochemical transitions took place during the Mesoarchean and Neoarchean (∼3.2–2.5 Ga). These transitions, which are imprinted in the metamorphic and magmatic rock-record, include the emergence of paired metamorphism, an increasing depth of tonalite-trondhjemite-granodiorite (TTG) formation, the widespread appearance of granites and changes in the composition of mafic rock. Previous studies have argued that these secular transitions reflect secular cooling of the mantle and herald a gradual transition from pre-plate tectonic to plate tectonic regimes. However, their tectonic driver remains elusive, in part due to a lack of detailed understanding of lithospheric dynamics prevalent during this transitional period when the mantle was warmer compared to the present-day. Here, we demonstrate that lithospheric convergence driven by peel-back (/peeling) process under warmer mantle conditions – termed as peel-back convergence – may explain the late Archean secular transitions. This tectonic phenomena features large-scale peeling (a form of delamination, but not dripping) of the mantle lithosphere with or without lower crustal rocks during convergence. We simulated numerical models of peel-back convergence under Archean crust-mantle conditions and predicted the pressure-temperature (P-T) conditions of crustal metamorphism and melting. The evolution of peel-back convergent setting features juxtaposition of a colder, compressional regime with thickened crust that forms at the site of peeling, and a warmer, extensional regime with thinned crust forming behind it. The metamorphic and magmatic P-T conditions prevalent in these two tectonothermal sites replicate the features of the late Archean rock record, including the: appearance of coeval high-T/P and intermediate-T/P metamorphic rocks; greater abundance of high-T/P relative to intermediate-T/P metamorphic rocks; dominant formation of TTGs at higher pressures where garnet ± rutile is stable in the residue; reworking of pre-existing felsic rocks/sediments to produce potassic granites; and interaction of crustal melts and mantle to form hybrid granitoids. Thus, peel-back convergence can explain the thermobaric bimodality of late Archean rocks within an asymmetric tectono-thermal framework. However, unlike modern convergent plate boundaries, the asymmetry is controlled by lithospheric peeling. © 2020 Elsevier B.V.
    view abstract10.1016/j.epsl.2020.116224
  • Lead diffusion in CaTiO3: A combined study using Rutherford backscattering and TOF-SIMS for depth profiling to reveal the role of lattice strain in diffusion processes
    Beyer, C. and Dohmen, R. and Rogalla, D. and Becker, H.-W. and Marquardt, K. and Vollmer, C. and Hagemann, U. and Hartmann, N. and Chakraborty, S.
    American Mineralogist 104 (2019)
    We present experimental data on the diffusivity of Pb in CaTiO3 perovskite, which is commonly used for dating kimberlites and carbonatites. Experiments were performed on oriented synthetic and natural CaTiO3 single crystals. The Pb-source was either a laser deposited (Ca0.83Pb0.07)Ti1.05O3 thin film or a (Ca0.9Pb0.1)TiO3 powder reservoir. The crystals were annealed in a high-temperature furnace between 736 and 1135 °C and for durations from 2 to 283 h. The diffusion profiles were measured with Rutherford backscattering and time-of-flight secondary ion mass spectrometry in the depth-profiling mode. The concentration profiles measured on the same samples with the two analytical methods are in agreement. The measured concentration profiles show two regions - a steep gradient at the diffusion interface that transitions sharply (at ~50 to 150 nm from the surface) to a low concentration tail that penetrates deeper into the crystal. This diffusion behavior could be modeled best using diffusion coefficients that are a function of the Pb concentration, with a different set of diffusion coefficient for the high and the low concentration region of the profile, respectively. The diffusion coefficients extracted from the thin film and powder source experiments are similar within uncertainties. Pb diffuses slower at concentrations between 8.5 and 0.6 wt% and 1.6 to 2.6 log units faster below ~0.5 wt% Pb. Temperature dependency for each region is discussed in the text, and the Arrhenius relation for the fast diffusion regime that is most relevant for natural samples is DPbfast=2.5×10-13×exp(-158(24)kJ/mol/RT)m2/s. $$begin array D- text Pb- text fast=2.5 times 10- -13times exp Big(-158big(24big)text kJ/mol/RTBig)m-2/s. end array $$ We found a distinct change in the structure of CaTiO3 in the surface region of the single crystal that is coincidental with the change in diffusivity. This initial region is dominated by planar defects. We propose that Pb is trapped in planar defects that have formed due to the high strain introduced into the perovskite structure caused by the mismatch in the ionic radius between Ca2+ and Pb2+. The activation energies obtained here yield closure temperature for Pb in CaTiO3 between 300 and 400 °C for a range of different cooling scenarios if diffusive resetting of Pb in CaTiO3 occurs at all. At typical cooling rates of hours to days for ascending kimberlite, the age of crystal growth is preserved, with closure temperatures similar to the magma temperature. © 2019 Walter de Gruyter GmbH, Berlin/Boston 2019.
    view abstract10.2138/am-2019-6730
  • Major shear zone within the Greater Himalayan Sequence and sequential evolution of the metamorphic core in Sikkim, India
    Chakraborty, S. and Mukul, M. and Mathew, G. and Pande, K.
    Tectonophysics 770 (2019)
    The Greater Himalayan Sequence (GHS), bounded by the South Tibetan detachment system, and the Main Central thrust (MCT), constitutes majority of the Himalayan Metamorphic Core. Recent studies identified several shear zones within the GHS, which played significant role in its evolution. Here, we report the presence of a major shear zone, the Chungthang-Thambi thrust (CTT), which is situated within the GHS in Sikkim, eastern India. The CTT exhibits “zone-type” geometry, with mylonite core flanked by protomylonite zones, and its thickness varies from ∼1 km to ∼5.6 km along the transport direction. New 40Ar/39Ar mica ages, and apatite fission track dates from this study, when integrated with published U[sbnd]Th[sbnd]Pb monazite, and Lu[sbnd]Hf garnet ages indicate that the CTT, the MCT, and the Munsiari thrust (MT), at base of the crystalline Lesser Himalayan Sequence (LHCS), developed in-sequence between ∼27 Ma and ∼12 Ma, and controlled the sequential evolution of the metamorphic core (GHS + LHCS) in Sikkim. Our imbricate model suggests that the upper-middle GHS was initially underthrusted below the Tethyan unit, and underwent prograde metamorphism from ∼37 Ma to ∼28 Ma. Thrusting on the CTT exhumed the middle GHS, and underthrusted the lower GHS between ∼27 Ma and ∼22.5 Ma. The MCT exhumed the lower GHS between ∼22.5 Ma and ∼12.1 Ma, and underthrusted the LHCS, which was later exhumed by the MT between ∼12 Ma and ∼10 Ma. The thrust sheets exhumed to near-surface conditions between ∼2.6 and ∼0.5 Ma, during growth of the underlying Lesser Himalayan Duplex. The MCT showed protracted activity till ∼7.2 Ma, and leading edge of the CTT was later reactivated at ∼13.5 Ma, and at ∼1.8 Ma. The CTT is equivalent to the High Himalayan Discontinuity, and forms part of a regional structure that, spans ∼800 km arc-length of the Himalaya from Sikkim to west-central Nepal. © 2019 Elsevier B.V.
    view abstract10.1016/j.tecto.2019.228183
  • New insights into the position and geometry of the main central thrust from Sikkim, Eastern Himalaya
    Chakraborty, S. and Mukul, M.
    Journal of Geology 127 (2019)
    The Main Central Thrust (MCT) features prominently in the Cenozoic evolution of the Himalaya, but no consensus exists on its definition and position. The MCT is best defined by a protolith boundary-structural definition: a high-strain zone with thrust-sense transporting the Greater Himalayan Sequence (GHS) rocks over the Lesser Himalayan Sequence (LHS) rocks. Protolith signatures have proved useful in distinguishing the GHS and LHS, but delineating the structural break of the MCT is still challenging. We have used the conceptual framework of shear zones to delineate the structural break of the MCT at different structural levels in Sikkim Himalaya, India, and identified rock units on either of its sides by available protolith signatures. Previous workers placed the MCT at different locations in Sikkim, varying up to ∼12 km structural distance, without providing any insights on its geometry. Our study shows that thickness and geometry of the MCT vary spatially along and across strike. In the relatively thicker exposures (∼2.5– 5.4 km), the MCT shows “island-channel geometry” with mylonites anastomosing around relatively undeformed rocks, transporting the GHS over the LHS and straddling both units. In the thinner exposures (∼1 km), the MCT shows three-dimensional zone-type geometry with a core of highly deformed mylonites flanked by relatively less-deformed proto-mylonite zones and has a minor portion of the GHS in its footwall. We define the MCT in Sikkim as a mappable shear zone that transported the GHS over the LHS, straddling both units in the thicker exposures, but has a minor part of the GHS in the footwall of the thinner exposures. Our shear zone framework–based approach can be used with protolith signatures along the Himalayan arc to map and study the MCT in detail. © 2019 by The University of Chicago.
    view abstract10.1086/702566
  • Slow Cooling at Higher Temperatures Recorded within High- P Mafic Granulites from the Southern Granulite Terrain, India: Implications for the Presence and Style of Plate Tectonics near the Archean-Proterozoic Boundary
    Chowdhury, P. and Chakraborty, S.
    Journal of Petrology 60 (2019)
    High-pressure metamorphism at relatively low to moderate temperature is considered to reflect the presence of plate margin processes. However, evidence of such metamorphism is scarce in the preserved archive of the Archean-early Proterozoic rock record. Extant geochronological studies show that parts of the Southern Granulite Terrain (SGT) of India experienced regional metamorphism from ∼2·49 to 2·44 Ga and thus provide an excellent natural laboratory to study the geodynamics that was prevalent within the first ∼50 Myr from the Archean-Proterozoic boundary. Here, we have constrained the pressureerature-time (P-T-t) evolution of a suite of mafic granulites from the Cauvery Shear System - a part of the SGT - that underwent the early Proterozoic regional metamorphism. Integrated results from mineral reaction histories, kinetically constrained thermobarometry and pseudosection analyses suggest that the studied mafic granulites, with a peak assemblage of garnet + aluminous clinopyroxene + plagioclase + rutile + quartz, were metamorphosed under high-P granulite (HPG) facies conditions of ∼800°C, 12-14 kbar. Subsequently, the rocks underwent simultaneous cooling and decompression, which is recorded by the formation of clinopyroxene + plagioclase coronae at ∼710°C, ∼10-11 kbar, and stabilization of amphibole in various modes at >580-620°C, ∼6-8 kbar. The constrained peak P-T values suggest that the studied rocks were buried significantly deeply below the Earth's surface at moderate temperatures and, at present, such metamorphic conditions are attained in orogenic plate margins. Consequently, the studied rocks suggest the presence of plate tectonics at the Archean-Proterozoic boundary. Furthermore, diffusion modeling of the preserved major element compositional zonations within garnet-clinopyroxene pairs shows that the mafic granulites cooled continuously, but relatively slowly, from peak-T to ∼650°C at rates varying between 5 and 30°C Ma -1. Such a cooling history (during exhumation) at high temperatures indicates that high, perturbed crustal temperatures >600°C were sustained for tens of millions of years, which is uncommon in modern collisional settings. However, the recently proposed model of peeling-off (planar delamination) orogenesis involving hotter mantle during the Archean is consistent with the maintenance of high temperatures for longer durations. Therefore, we propose that the studied rocks preserve evidence for plate motions at the Archean-Proterozoic transition, but possibly with a style that is different from that operating at modern orogens. © 2019 The Author(s).
    view abstract10.1093/petrology/egz001
  • Comment on ‘Formation of fast-spreading lower oceanic crust as revealed by a new Mg–REE coupled geospeedometer’ by Sun and Lissenberg
    Faak, K. and Chakraborty, S. and Coogan, L.A. and Dohmen, R.
    Earth and Planetary Science Letters 502 (2018)
    view abstract10.1016/j.epsl.2018.08.036
  • Channel flow and localized fault bounded slice tectonics (LFBST): Insights from petrological, structural, geochronological and geospeedometric studies in the Sikkim Himalaya, NE India
    Chakraborty, S. and Mukhopadhyay, D.K. and Chowdhury, P. and Rubatto, D. and Anczkiewicz, R. and Trepmann, C. and Gaidies, F. and Sorcar, N. and Dasgupta, S.
    Lithos 282-283 (2017)
    One of the enduring debates in the study of the Himalayan orogen (and continental collision zones in general) is whether the salient observed features are explained (a) by localized deformation along discrete, narrow fault zones/ductile shear zones separating individual blocks or slices (e.g. critical taper or wedge tectonic models), or (b) by distributed deformation dominated by wide zones of visco-plastic flow in the solid or a partially molten state (e.g. channel flow models). A balanced cross-section from Sikkim in the eastern Himalaya that is based on structural data and is drawn to satisfy petrological and geophysical constraints as well, is used in combination with information from petrology, geochronology, geospeedometry and microstructural data to address this question. We discuss that any tectonic model needs to be thermally, rheologically, geometrically and temporally viable in order to qualify as a suitable description of a system; models such as channel flow and critical taper are considered in this context. It is shown that channel flow models may operate with or without an erosional porthole (channel with tunnel and funnel mode vs. channels with only the tunnel mode) and that the predicted features differ significantly between the two. Subsequently, we consider a large body of data from Sikkim to show that a channel flow type model (in the tunneling without funneling mode), such as the ones of Faccenda et al. (2008), describes features formed at high temperatures very well, while features formed at lower temperatures are more consistent with the operation of localized, fault-bounded, slice tectonics, (LFBST, be it in the form of critical taper, wedge tectonics, or something else). Thus, the two modes are not competing, but collaborating, processes and both affect a given rock unit at different points of time during burial, metamorphism and exhumation. A transitional stage separates the two end-member styles of tectonic evolution. The proposed models bear similarities to those suggested by Mallet (1875) and Auden (1935) and mechanisms proposed by Beaumont and Jamieson (2010). We conclude by discussing some of the implications of such a model for motion on the major Himalayan faults, and by considering which features of any given rock are likely to record signatures of a particular style of tectonic evolution. Some directions for future research are suggested in the end. © 2017 Elsevier B.V.
    view abstract10.1016/j.lithos.2017.01.024
  • Diffusion across the glass transition in silicate melts: Systematic correlations, new experimental data for Sr and Ba in calcium-aluminosilicate glasses and general mechanisms of ionic transport
    Fanara, S. and Sengupta, P. and Becker, H.-W. and Rogalla, D. and Chakraborty, S.
    Journal of Non-Crystalline Solids 455 (2017)
    Viscosity and diffusivity of silicate melts and glasses are related to each other through relaxation timescales. The systematic is explored based on published data. Diffusion coefficients for Sr and Ba were measured in calcium aluminosilicate glasses at conditions near the glass/supercooled liquid boundary in temperature – time space making use of thin film technology and Rutherford Backscattering Spectroscopy (RBS) to measure concentration profiles on nanoscales. These data extend the range of published diffusion coefficients and combined with the systematic noted above allow the nature of change of diffusion coefficients across the glass transition region to be studied. Activation energies for diffusion in the glassy state (~ 360 kJ/mol) are higher than in the molten liquid (~ 213 kJ/mol). A defect based model of glass transition derived by Ojovan and coworkers, where attainment of a percolation threshold of configuron-type defects accounts for the glass – liquid transition, can explain the observed diffusion behaviour. Data treatment using this model yields a defect formation enthalpy of ~ 146 kJ/mol and a migration enthalpy of ~ 213 kJ/mol. The results of this study provide generalized expressions for the prediction of diffusion coefficients of cations in silicate melts for any composition at any temperature. © 2016
    view abstract10.1016/j.jnoncrysol.2016.10.013
  • Photoactive Zinc Ferrites Fabricated via Conventional CVD Approach
    Peeters, D. and Taffa, D.H. and Kerrigan, M.M. and Ney, A. and Jöns, N. and Rogalla, D. and Cwik, S. and Becker, H.-W. and Grafen, M. and Ostendorf, A. and Winter, C.H. and Chakraborty, S. and Wark, M. and Devi, A.
    ACS Sustainable Chemistry and Engineering 5 (2017)
    Owing to its narrow band gap and promising magnetic and photocatalytic properties, thin films of zinc ferrite (ZFO, ZnFe2O4) are appealing for fabrication of devices in magnetic recording media and photoelectrochemical cells. Herein we report for the first time the fabrication of photactive zinc ferrites via a solvent free, conventional CVD approach, and the resulting ZFO layers show promise as a photocatalyst in PEC water-splitting. For large scale applications, chemical vapor deposition (CVD) routes are appealing for thin film deposition; however, very little is known about ZFO synthesis following CVD processes. The challenge in precisely controlling the composition for multicomponent material systems, such as ZFO, via conventional thermal CVD is an issue that is caused mainly by the mismatch in thermal properties of the precursors. The approach of using two different classes of precursors for zinc and iron with a close match in thermal windows led to the formation of polycrystalline spinel type ZFO. Under the optimized process conditions, it was possible to fabricate solely ZFO in the desired phase. This work demonstrates the potential of employing CVD to obtain photoactive ternary material systems in the right composition. For the first time, the application of CVD grown ZFO films for photoelectrochemical applications is being demonstrated, showing a direct band gap of 2.3 eV and exhibiting activity for visible light driven photoelectrochemical water splitting. © 2017 American Chemical Society.
    view abstract10.1021/acssuschemeng.6b02233
  • Sequential kinetic modelling: A new tool decodes pulsed tectonic patterns in early hot orogens of Earth
    Bhowmik, S.K. and Chakraborty, S.
    Earth and Planetary Science Letters 460 (2017)
    Tectonic styles in an early hot Earth were different from the present-day situation governed by plate tectonics. Processes in such hot settings remain poorly understood because they often occur on timescales that are below the resolution of conventional isotopic clocks, the rock records are fragmentary, and these have been superposed by later high-temperature events. We have developed a tool based on diffusion kinetics to overcome these difficulties and reconstruct sequences of short-lived episodes. Application of the method to a rock from the ultra-hot c.1.6 Ga orogenic domain of the Central Indian Tectonic Zone, where additional data are available to verify the results, shows that pulses of approach and roll-back of colliding plates preceded the final closure and collision. We demonstrate that cooling from ultra-high temperature metamorphic conditions in the orogen took place in multiple pulses that occurred with a periodicity of about 10 Myr at rates that vary between 100's to 10's °C/Myr, and burial-/exhumation-rates that vary between 30 and 2 km/Myr, respectively. Such details of tectonic processes in the Precambrian, with quantification of variable heating-, cooling-, burial-, and exhumation-rates of individual stages, have not been accessible until now. Application of this method to other regions would provide a means of exploring the thermal viability of the inferred long durations (>100 Myr) for some ultra-high temperature orogenies. © 2016 Elsevier B.V.
    view abstract10.1016/j.epsl.2016.12.018
  • The nature and evolution of the Main Central Thrust: Structural and geochronological constraints from the Sikkim Himalaya, NE India
    Mukhopadhyay, D.K. and Chakraborty, S. and Trepmann, C. and Rubatto, D. and Anczkiewicz, R. and Gaidies, F. and Dasgupta, S. and Chowdhury, P.
    Lithos 282-283 (2017)
    The Main Central Thrust (MCT) is a prominent continental-scale fault within the Himalaya. Its definition has been the topic of some debate in the literature. After a brief consideration of the state of discussion to clarify the definition we use in this work, we report features from the field- to the microstructural- scale of a particularly well-exposed section in Sikkim, NE India. The nature of the protoliths as well as the overlying and underlying rocks is characterized in terms of ε-Nd. The dates of motion on the fault are constrained using U–Pb geochronology of zircon and monazite from pegmatitic dikes that cross-cut the deformation fabric. It is found that the mechanism of deformation recorded in the fault zone rocks is different compared to that found in the overlying Greater Himalayan (GH) or the underlying Lesser Himalayan (LH) rocks. The GH and LH have different protolith characteristics as well. Combined with existing data on P–T history, dates of metamorphism, and cooling- and exhumation-rates of the GH and the LH, our measurements show that major motion on this fault occurred before 20 Ma at 450–700 °C but after peak metamorphism of rocks (750–800 °C) in this zone. Isolated events occurred in this zone as late as 11 Ma, possibly in the brittle domain. This underscores the pulsed nature of movement over an extended period on such major faults, and the related difficulties in dating fault movement, determination of the rates of movement, and designating a fault plane as in- or out-of-sequence within a propagating deformation front. © 2017 Elsevier B.V.
    view abstract10.1016/j.lithos.2017.01.015
  • A review of thermal history and timescales of tectonometamorphic processes in Sikkim Himalaya (NE India) and implications for rates of metamorphic processes
    Chakraborty, S. and Anczkiewicz, R. and Gaidies, F. and Rubatto, D. and Sorcar, N. and Faak, K. and Mukhopadhyay, D.K. and Dasgupta, S.
    Journal of Metamorphic Geology 34 (2016)
    The lesser and greater Himalayan sequences (LH and GH) in Sikkim have been studied in detail recently. The LH consists mainly of pelitic lithologies with detrital zircon ages &gt;1800 Ma and εNd(0) of −27.7 to −23.4 (with intercalated calcsilicates, metabasites and a sheared granite gneiss – the Lingtse gneiss). The metamorphic sequence is characterized by (i) a remarkably continuous and systematic metamorphic field gradient (~60–70 °C kbar−1), (ii) peak pressures as well as temperatures of metamorphism increasing at higher metamorphic grades (~5 kbar, 500 °C at the garnet zone to ~8 kbar, 700 °C at the sillimanite–muscovite-zone), (iii) clockwise P–T paths of a tight ‘hairpin’ shape with a prograde slope of 20–30°C kbar−1, (iv) initiation of garnet growth (Lu–Hf chronometry) at systematically older ages at higher grades (11–10.5 Ma at garnet zone to 16.8 Ma at sillimanite–K-feldspar zone), (v) peak of metamorphism simultaneously at all grades (13–11 Ma), (vi) deformed and folded metapelitic rocks but with little evidence of pervasive shearing and (vii) duration of melt formation and segregation on the order of a few 100,000 years. The degree of melting was low, as indicated by the presence of muscovite all through the sequence. This evidence indicates that the LH behaved as a coherent block, and thermomechanical models quantitatively reproduce many of these features as well as explain the formation of inverted metamorphic sequence in such a coherent block. The greater Himalayan sequence (GH), also largely metapelitic, has different protoliths (with components as young as 800 Ma) and a different metamorphic history (extensive migmatization at higher grades ~8–12 kbar, 750–850 °C at an older age of largely &gt;20 Ma along a different clockwise P–T path characterized by sharp, isothermal decompression followed by near isobaric cooling). Geochronological data (monazite, zircon, garnet) as well as cooling rate calculations (geospeedometry) reveal that the GH is made up of at least two blocks with distinct, complex and multistage P–T–t histories. Different stages of the high-T cooling were rapid (100's °C Ma−1 to 10's °C Ma−1 at different stages) and at least some of it was unrelated to exhumation; synthesis of these results with low-T thermochronometry allows the detailed multistage cooling and exhumation history of this block to be reconstructed. The reconstructed cooling and exhumation histories show that a number of interlinked processes operating on a hierarchy of timescales leads to the overall evolution. The P–T history with rapid cooling can be reproduced by thermomechanical models that consider conductive as well as advective heat transport and there is no need to invoke additional processes (such as the role of fluids). Metabasic inclusions within both the LH and the GH appear to record a different (? older in part) history. The role of these in the overall evolution, as well as the difference in metamorphic ages of the LH and GH remain open questions that need to be addressed in future research. © 2016 John Wiley & Sons Ltd
    view abstract10.1111/jmg.12200
  • Development of Protective Coating of MoSi2 over TZM Alloy Substrate by Slurry Coating Technique
    Chakraborty, S.P.
    Materials Today: Proceedings 3 (2016)
    TZM alloy possesses high strength at elevated temperature but due to high volatility of its oxide, MoO3 the alloy undergoes catastrophic oxidation in air above 700°C. In the present study, development of high temperature oxidation resistant MoSi2 coating over TZM alloy was investigated by slurry coating technique. As-coated TZM alloy by Si powder was exposed to thermal treatment at 1200°C to develop MoSi2 coating. Characterization studies like surface, cross section morphologies, phase identification, evaluation of hardness, adhesive strength, oxidation resistance etc. were carried out to determine the quality of coatings. A double layered coating structure was obtained with thick outer surface layer of MoSi2 (thickness ∼ 60 μm) and thin interfacial layer of Mo5Si3 (thickness ∼ 5 μm). The coating showed good adhesion strength (∼ 25 MPa) and stable oxidation resistance with negligible mass gain at 1000°C in air. © 2016 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.matpr.2016.09.022
  • Fe-Mg interdiffusion in orthopyroxene
    Dohmen, R. and Ter Heege, J. H. and Becker, H. W. and Chakraborty, S.
    American Mineralogist 101 (2016)
    We have measured Fe-Mg interdiffusion coefficients, DFe-Mg, parallel to the three main crystallographic axes in two natural orthopyroxene single crystals [approximately En(98)Fs(1) (X-Fs = X-Fe = 0.01) and En(91)Fs(9)] using diffusion couples consisting of a 20-90 nm thick silicate thin film deposited under vacuum on polished and oriented pyroxene single crystals. The thin films were prepared using pulsed laser ablation of polycrystalline olivine pellets (composition: Fo(30)Fa(70)). Samples were annealed for 4-337 h at 870-1100 degrees C under atmospheric pressure in a continuous flow of CO + CO2 to control the oxygen fugacity,f(o2), between 10(-11) and 10(-7) Pa within the stability field of pyroxene. Film thickness and compositional profiles were measured using Rutherford backscattering spectroscopy (RBS) on reference and annealed samples, and Fe concentration depth profiles were extracted from the RBS spectra and fitted numerically considering a compositional dependence of DFe_mg in orthopyroxene. We obtain an Arrhenius relationship for both types of crystals, but only for the more Fe-rich composition a dependence on f(o2) could be clearly identified. For diffusion along [001] in the composition Fs(9), least-squares regression of the log DFe-mg vs. reciprocal temperature yields the following Arrhenius equation: D(Fe MG [)m(2)/s] = 1.12 x 10(-6) (f(o2) {Pa])(0.053 +/- 0.027) exp[-308 +/- 23[ kJ/mol]/(RT)]. DFe-Mg in Opx with X-Fe = 0.01 obeys a relationship that does not depend on f(o2): DFe-Mg [m(2)/s] = 1.66 x 10(-4) exp[-377 +/- 30 [kJ/mol]/(RT)]. Diffusion along [001] is faster than diffusion along [100] by a factor of 3.5, while diffusion along [010] is similar to that along [001]. Comparison of D-Fe.Mg and rates of order-disorder kinetics indicates that for foe around the IW buffer and lower, diffusion in natural orthopyroxene becomes insensitive to f(o2), which could be related to a transition in the diffusion mechanism from a transition metal extrinsic (TaMED) domain to a pure extrinsic (PED) domain. This behavior is analogous to that observed for Fe-Mg diffusion in olivine and this complexity precludes the formulation of a closed form expression for the composition and f(o2) dependence of D-Fe.Mg in orthopyroxene at present. We were not able to quantitatively constrain the dependence of DFe-Mg on the XE, content from the profile shapes, but consideration of the experimentally measured diffusion coefficients along with the data for order-disorder kinetics suggests that the compositional dependence is weaker than previously estimated, at least for orthopyroxene with X-Fe < 0.5. For the major element compositional T and f(o2) range of available experimental data, Fe-Mg interdiffusion in orthopyroxene is slower than in olivine and aluminous spinel, comparable to garnet, and faster than in clinopyroxene.
    view abstract10.2138/am-2016-5815
  • Constraining the conditions of Barrovian metamorphism in Sikkim, India: P-T-t paths of garnet crystallization in the Lesser Himalayan Belt
    Gaidies, F. and Petley-Ragan, A. and Chakraborty, S. and Dasgupta, S. and Jones, P.
    Journal of Metamorphic Geology 33 (2015)
    Garnet crystallization in metapelites from the Barrovian garnet and staurolite zones of the Lesser Himalayan Belt in Sikkim is modelled utilizing Gibbs free energy minimization, multi-component diffusion theory and a simple nucleation and growth algorithm. The predicted mineral assemblages and garnet-growth zoning match observations remarkably well for relatively tight, clockwise metamorphic P-T paths that are characterized by prograde gradients of approximate to 30 degrees Ckbar(-1) for garnet-zone rocks and approximate to 20 degrees Ckbar(-1) for rocks from the staurolite zone. Estimates for peak metamorphic temperature increase up-structure toward the Main Central Thrust. According to our calculations, garnet stopped growing at peak pressures, and protracted heating after peak pressure was absent or insignificant. Almost identical P-T paths for the samples studied and the metamorphic continuity of the Lesser Himalayan Belt support thermo-mechanical models that favour tectonic inversion of a coherent package of Barrovian metamorphic rocks. Time-scales associated with the metamorphism were too short for chemical diffusion to substantially modify garnet-growth zoning in rocks from the garnet and staurolite zones. In general, the pressure of initial garnet growth decreases, and the temperature required for initial garnet growth was reached earlier, for rocks buried closer toward the MCT. Deviations from this overall trend can be explained by variations in bulk-rock chemistry.
    view abstract10.1111/jmg.12108
  • Fe-Mg diffusion in spinel: New experimental data and a point defect model
    Vogt, K. and Dohmen, R. and Chakraborty, S.
    American Mineralogist 100 (2015)
    We have measured Fe-Mg interdiffusion rates (DFe-Mg) in synthetic Mg-Al spinel and a natural (Mg,Fe) aluminous spinet from Sri Lanka (X-Fe similar to 0.07) at atmospheric pressure over a range of different oxygen fiigacities {1og(10) (f(O2)[Pa]) = 14 to -10} and temperatures (750-900 degrees C). Diffusion couples made of single-crystal spinet and thin films of hercynitic composition (X-Fe similar to 0.5) were used for the diffusion anneals. The experimentally induced diffusion profiles were analyzed by Rutherford backscattering Spectroscopy to retrieve true depth concentration profiles for Fe. These were fitted numerically by an explicit finite difference scheme that allows compositionally dependent interdiffusion processes to be modeled by relating self- and interdiffusion coefficients. Synthesis of data from the two diffusion couples indicate that: (1) DFe-Mg depends on X-Mg of spinet, with increasing diffusion rates with increasing X-Mg. This behavior is opposite of that found in silicates. (2) Self-diffusion coefficients could not be determined from these experiments, but the results indicate that D-Fe/D-Mg > 100. (3) DFe-Mg in Mg-spinel is independent of oxygen fugacity, whereas it depends strongly and nonlinearly on oxygen fugacity for the natural spinel. This observation indicates that the mechanisms of diffusion are different in the two kinds of spinet (Fe-bearing vs. Fe-free), which is also seen in the difference in activation energy obtained for these. Moreover, the nonlinear dependence on oxygen fugacity indicates that diffusion occurs by an interstitial mechanism at low-oxygen fugacities and by a vacancy mechanism at high-oxygen fugacities in natural, Fe-bearing spinet. (4) Simple Arrhenius relations that describe the data within the range of experimental conditions are: Synthetic magnesium spinet: Q(Fe-Mg) = 219 +/- 19 kJ/mol, log(10)D(Fe-Mg) = -7.76 +/- 0.90 [m(2)/s]. Natural Fe-bearing spinet for log(10) (f(O2) [Pa]) = -12}: Q(Fe-Mg) = 139 +/- 18 kJ/mol, log(10)D(Fe-Mg) = 12.33 +/- 0.85 [m(2)/s]. A model based on point defect considerations that describes the temperature as well as oxygen fugacity dependence of DFe-Mg in Fe-bearing spinel is: D [m(2) s] = D-v [m(2)/s]f(O2) [Pa](m)exp {-Q(v) [J/mol]/RT [K]} + D-i [m(2)/s]f(O2) [Pa](-m)exp(-Q(i) [J/mol]/RT [K]), with D-v = 1.07 x 10(-9) +/- 1.55 x 10(-9) m(2)/s, Q(v) = 131 +/- 66 kJ/mot, D-i = 1.03 x 10(-17) +/- 7.32 x 10(-17) m(2)/s, Q(v) = 130 +/- 80 kJ/mol and m = 0.34 +/- 0.18. Poor coverage of T-f(O2) space by available experimental data results in large uncertainties in the fit parameters. As a result, these expressions are useful for understanding the diffusion behavior in spinels, but not for extrapolation and calculation of diffusion coefficients for cooling rate or other related calculations. Until the parameters can be better constrained through the availability of more data, we recommend that for such calculations, the parameters noted above for Fe-bearing spinels be used for compositions and f(O2) conditions that are close to those of the experiments. (5)DFe-Mg in spinet is faster than DFe-Mg in olivines, pyroxenes, and garnets at most conditions.
    view abstract10.2138/am-2015-5109
  • Near conductive cooling rates in the upper-plutonic section of crust formed at the East Pacific Rise
    Faak, K. and Coogan, L.A. and Chakraborty, S.
    Earth and Planetary Science Letters 423 (2015)
    A new geospeedometer, based on diffusion modeling of Mg in plagioclase, is used to determine cooling rates of the upper section of the lower oceanic crust formed at fast-spreading mid-ocean ridges. The investigated natural sample suites include gabbroic rocks formed at three different locations along the fast-spreading East Pacific Rise. These samples cover a depth interval of 0-840 m below the sheeted dike/gabbro boundary and therefore allow the variation of cooling rate as a function of depth within the upper plutonic sequence to be determined. We demonstrate that the cooling rates we obtained are robust (reproducible and consistent across different vertical sections at fast spreading ridges) and decrease significantly with increasing sample depth (covering almost 4 orders of magnitude, ranging from ~1 °C y-1 for the shallowest samples to 0.0003 °C y-1 for the deepest samples). Both the absolute cooling rates, and the rate of decrease of cooling rate with depth, are consistent with conductive thermal models. In contrast, the absolute cooling rates determined from the deeper samples (>300 m below DGB), and the large decrease in cooling rate with depth are inconsistent with thermal models that include substantial cooling by off-axis hydrothermal circulation within the upper plutonic section of the crust. © 2015 Elsevier B.V.
    view abstract10.1016/j.epsl.2015.04.025
  • A hypothetical model based on effectiveness of combined alkali and polymer latex modified jute fibre in controlling the setting and hydration behaviour of cement
    Jo, B.-W. and Chakraborty, S. and Yoon, K.W.
    Construction and Building Materials 68 (2014)
    Present investigation deals with the effectiveness of combined dilute alkali and polymer modified jute in controlling the setting and hydration behaviour of cement. Modification of jute fibre by combined alkali and polymer emulsion is a selective pathway to recover the delayed setting and hydration behaviour of jute cement paste. Setting time measurement, hydration test, free lime content estimation, SEM and FTIR were the selective choice to evaluate the targeted importance. Based on these analyses a model has been proposed to explain the hydration delaying effect of jute is recovered by the application of appropriate polymer content (0.0513%) in emulsion. © 2014 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.conbuildmat.2014.06.043
  • A molecular iron catalyst for the acceptorless dehydrogenation and hydrogenation of N-heterocycles
    Chakraborty, S. and Brennessel, W.W. and Jones, W.D.
    Journal of the American Chemical Society 136 (2014)
    A well-defined iron complex (3) supported by a bis(phosphino)amine pincer ligand efficiently catalyzes both acceptorless dehydrogenation and hydrogenation of N-heterocycles. The products from these reactions are isolated in good yields. Complex 3, the active catalytic species in the dehydrogenation reaction, is independently synthesized and characterized, and its structure is confirmed by X-ray crystallography. A trans-dihydride intermediate (4) is proposed to be involved in the hydrogenation reaction, and its existence is verified by NMR and trapping experiments. © 2014 American Chemical Society.
    view abstract10.1021/ja504523b
  • A new Mg-in-plagioclase geospeedometer for the determination of cooling rates of mafic rocks
    Faak, K. and Coogan, L.A. and Chakraborty, S.
    Geochimica et Cosmochimica Acta 140 (2014)
    We present a new Mg-in-plagioclase geospeedometer based on the diffusive exchange of Mg between plagioclase and clinopyroxene that allows cooling rates of a wide range of mafic rocks to be determined. The major element composition of plagioclase is shown to play an important role in driving the flux of Mg as well as the partitioning of Mg between plagioclase and clinopyroxene. Therefore, commonly used analytical solutions to the diffusion equation are not applicable in this system and an alternate approach is developed. Practical aspects of the method such as the role of anorthite zoning patterns and influence of shapes of cooling paths and grain geometries in controlling the Mg concentration profile shapes are discussed. Propagation of uncertainties in the knowledge of diffusion and partition coefficients shows that the method is capable of resolving small differences in cooling rates (e.g., log[dT/dt°Cyr-1]=-3±0.3 in one example). The approach is illustrated by application to two samples from the lower oceanic crust. © 2014 Elsevier Ltd.
    view abstract10.1016/j.gca.2014.06.005
  • Constraints on the nature and evolution of the magma plumbing system of Mt. Etna volcano (1991-2008) from a combined thermodynamic and kinetic modelling of the compositional record of minerals
    Kahl, M. and Chakraborty, S. and Pompilio, M. and Costa, F.
    Journal of Petrology 56 (2014)
    Deciphering the evolution of the internal dynamics of magmatic plumbing systems and identifying the key parameters that drive such dynamics are major goals of modern volcanology. Here we present a novel petrological approach that combines kinetic modelling of the diffusive relaxation of chemical zoning patterns in olivine crystals with thermodynamic modelling (MELTS) to constrain the nature and evolution of the plumbing system of Mt. Etna and the processes governing its internal dynamics. We investigated the compositional and temporal record preserved in 180 olivine crystals that were erupted between 1991 and 2008. Detailed systemization of the information stored in the sequential zoning record of the olivines reveals the existence of at least five compositionally different magmatic environments (MEs), characterized by different olivine compositions: M0 (Fo79-83), M1 (Fo75-78), M2 (Fo70-72), M3 (Fo65-69) and mm1 (Fo73-75). Several routes of magma transfer connect these environments. We identified three prominent magma passageways between the environments M0:M1, M2:mm1 and M1:M2 that were active during the entire period of observation between 1991 and 2008. Modelling the diffusive relaxation of the olivine zoning patterns reveals that the transfer of magma along such routes can occur over fairly heterogeneous timescales ranging from days to 2 years. Although some of the passageways have been sporadically active in the months and sometimes years before an eruption, the magma migration activity increases clearly in the weeks and days prior to an eruptive event. In this context, major transfer routes such as M2:mm1 might represent temporary passageways that are activated only shortly before eruptive events. A forward modelling approach was developed using thermodynamic calculations with the MELTS software to identify the key intensive variables associated with the different magmatic environments. In this approach the observed populations of mineral compositions (e.g. Fo79-83), rather than single compositions, are associated with thermodynamic parameters [pressure, temperature, water content, oxygen fugacity (fO2) and bulk composition of the melt] to identify the most plausible set corresponding to each ME. We found that temperature, water content and possibly oxidation state are the main distinguishing features of the different magmatic environments. The most primitive olivine population M0 (Fo79-83) and some of its associated clinopyroxenes formed at high melt water contents (3·5-5·2 wt %), at fO2 conditions buffered at quartz-fayalite-magnetite (QFM) or Ni-NiO (NNO), at temperatures ≥ 1110°C and at pressures ranging between 1·5 and 3·0 kbar (or higher). The intermediate population M1 (Fo75-78) can be produced over a broad spectrum of conditions, but all require similar temperatures and lower water contents (0·1-1·4 wt %). The most evolved, more Fe-rich olivines of M2 and M3 are products of melts with much lower water contants (0·2-1·1 wt % H2O for M2, < 0·5 wt % H2O for M3), and at probably somewhat more reducing (QFM) conditions at somewhat lower temperatures (~1080°C). The M3 environment characterized by very low water contents and reducing conditions could be related to an enhanced flux of CO2. Combination of the characteristics of the various magmatic environments with temporal information (residence time in different environments and timing of magma transfer between these environments) allows a dynamic model of the plumbing system beneath Mt. Etna to be constructed. © The Author 2015.
    view abstract10.1093/petrology/egv063
  • Effectiveness of the top-down nanotechnology in the production of ultrafine cement ( 220 nm)
    Jo, B.-W. and Chakraborty, S. and Kim, K.H. and Lee, Y.S.
    Journal of Nanomaterials 2014 (2014)
    The present investigation is dealing with the communition of the cement particle to the ultrafine level (220 nm) utilizing the bead milling process, which is considered as a top-down nanotechnology. During the grinding of the cement particle, the effect of various parameters such as grinding time (1-6 h) and grinding agent (methanol and ethanol) on the production of the ultrafine cement has also been investigated. Performance of newly produced ultrafine cement is elucidated by the chemical composition, particle size distribution, and SEM and XRD analyses. Based on the particle size distribution of the newly produced ultrafine cement, it was assessed that the size of the cement particle decreases efficiently with increase in grinding time. Additionally, it is optimized that the bead milling process is able to produce 90% of the cement particle <350 nm and 50% of the cement particle < 220 nm, respectively, after 6.3 h milling without affecting the chemical phases. Production of the ultrafine cement utilizing this method will promote the construction industries towards the development of smart and sustainable construction materials. © 2014 Byung-Wan Jo et al.
    view abstract10.1155/2014/131627
  • Fabrication of a Mo based high temperature TZM alloy by non-consumable arc melting technique
    Chakraborty, S.P. and Krishnamurthy, N.
    Proceedings - International Symposium on Discharges and Electrical Insulation in Vacuum, ISDEIV (2014)
    High temperature structural materials are in great demand for power, chemical and nuclear industries which can perform beyond 1000°C as super alloys usually fail. In this regard, Mo based TZM alloy is capable of retaining strength up to 1500°C with excellent corrosion compatibility against molten alkali metals. Hence, currently this alloy is considered an important candidate material for high temperature compact nuclear and fusion reactors. Due to reactive nature of Mo and having high melting point, manufacturing this alloy by conventional process is unsuitable. Powder metallurgy technique has limited success due to restriction in quantity and purity. This paper deals with fabrication of TZM alloy by non-consumable tungsten arc melting technique. Initially a ternary master alloy of Mo-Ti-Zr was prepared which subsequently by dilution method, was converted into TZM alloy gradually by external addition of Mo and C in various proportions. A number of melting trials were conducted to optimize the process parameters like current, voltage and time to achieve desired alloy composition. The alloy was characterized with respect to composition, elemental distribution profile, microstructure, hardness profile and phase analysis. Well consolidated alloy button was obtained having desired composition, negligible material loss and having microstructure as comparable to standard TZM alloy. © 2014 IEEE.
    view abstract10.1109/DEIV.2014.6961791
  • High-temperature cooling histories of migmatites from the High Himalayan Crystallines in Sikkim, India: Rapid cooling unrelated to exhumation?
    Sorcar, N. and Hoppe, U. and Dasgupta, S. and Chakraborty, S.
    Contributions to Mineralogy and Petrology 167 (2014)
    The High Himalayan Crystallines (HHCs) provide an excellent natural laboratory to study processes related to crustal melting, crustal differentiation, and the tectonic evolution of mountain belts because partial melting in these rocks occurred under well-defined tectonic boundary conditions (N-S collision of the Indian and the Eurasian plates) and the rocks have not been modified by subsequent metamorphic overprinting. We have used petrogenetic grids, kinetically constrained individual thermobarometry, pseudosection calculations, and reaction histories constrained by textural evidence to determine that the migmatites in the HHC of Sikkim attained peak P-T conditions of 750-800 °C, 9-12 kbar, followed by steep isothermal decompression to 3-5 kbar, and then isobaric cooling to ~600 °C. There may be a trend where rocks to the north [closer to the South Tibetan detachment system (STDS)] attained somewhat higher maximum pressures. The decompression may have been triggered by a reduction in density due to the production of melt (~20 vol%); minor amounts of additional melt may have been produced in individual packages of rock during decompression itself, depending on the exact geometry of the P-T path and the bulk composition of the rock. The stalling of rapid, isothermal exhumation at depths of 10-18 km (3-5 kbar) is related to metamorphic reactions that occur in these rocks. Geospeedometry indicates that at least a two-stage cooling history is required to describe the compositional zoning in all garnets. Both of these stages are rapid (several 100's °C/my between 800 and 600 °C, followed by several 10's °C/my between 600 and 500 °C), but there appears to be a spatial discontinuity in cooling history: Rocks to the south (closer to main central thrust) cooled more slowly than rocks to the north (closer to STDS). The boundary between these domains coincides with the discontinuity in age found in the same area by Rubatto et al. (Contrib Mineral Petrol 165:349-372, 2013). Combined with the information on petrologic phase relations, the data reveal the remarkable aspect that the rapid cooling and change of cooling rates all occurred after, rather than during, the rapid exhumation. This result underscores that high-temperature (e.g., >550 °C) cooling is a result of several processes in addition to exhumation and a one-to-one correlation of cooling and exhumation may sometimes be misplaced. Moreover, average cooling rates inferred from the closure temperatures of two isotopic systems should be interpreted judiciously in such nonlinearly cooling systems. While many aspects (e.g., isothermal decompression, isobaric cooling, duration of metamorphism, and cooling rates) of the pressure-temperature history inferred by us are consistent with the predictions of thermomechanical models that produce midcrustal channel flow, the occurrence of blocks with two different cooling histories within the HHC is not explained by currently available models. It is found that while exhumation may be initiated by surface processes such as erosion, the course of exhumation and its rate, at least below depths of ~15 km, is mostly controlled by a coupling between mechanical (density gain/loss) and chemical (metamorphic reactions) processes at depth. © 2014 Springer-Verlag Berlin Heidelberg.
    view abstract10.1007/s00410-013-0957-3
  • Investigation on the effectiveness of chemically synthesized nano cement in controlling the physical and mechanical performances of concrete
    Jo, B.W. and Chakraborty, S. and Kim, K.H.
    Construction and Building Materials 70 (2014)
    Present investigation deals with the effectiveness of the chemically synthesized nano cement in controlling physical and mechanical performances of concrete. In this investigation, concrete samples were fabricated using variable amounts of aggregates and alkali activator content w.r.t. weight of nano cement. Based on the mechanical properties analyses, it is assessed that chemically synthesized cement is able to produce 43 MPa compressive strength of concrete after 14 days curing instead of 28 days at an optimized amount of aggregates content as well as alkali activator content. Finally, a model has been proposed to explain the overall performances of nano cement based concrete. © 2014 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.conbuildmat.2014.07.090
  • Iron-based catalysts for the hydrogenation of esters to alcohols
    Chakraborty, S. and Dai, H. and Bhattacharya, P. and Fairweather, N.T. and Gibson, M.S. and Krause, J.A. and Guan, H.
    Journal of the American Chemical Society 136 (2014)
    Hydrogenation of esters is vital to the chemical industry for the production of alcohols, especially fatty alcohols that find broad applications in consumer products. Current technologies for ester hydrogenation rely on either heterogeneous catalysts operating under extreme temperatures and pressures or homogeneous catalysts containing precious metals such as ruthenium and osmium. Here, we report the hydrogenation of esters under relatively mild conditions by employing an iron-based catalyst bearing a PNP-pincer ligand. This catalytic system is also effective for the conversion of coconut oil derived fatty acid methyl esters to detergent alcohols without adding any solvent. © 2014 American Chemical Society.
    view abstract10.1021/ja504034q
  • Synthesis of a cementitious material nanocement using bottom-up nanotechnology concept: An alternative approach to avoid COemission during production of cement
    Jo, B.W. and Chakraborty, S. and Yoon, K.W.
    Journal of Nanomaterials 2014 (2014)
    The world's increasing need is to develop smart and sustainable construction material, which will generate minimal climate changing gas during their production. The bottom-up nanotechnology has established itself as a promising alternative technique for the production of the cementitious material. The present investigation deals with the chemical synthesis of cementitious material using nanosilica, sodium aluminate, sodium hydroxide, and calcium nitrate as reacting phases. The characteristic properties of the chemically synthesized nanocement were verified by the chemical composition analysis, setting time measurement, particle size distribution, fineness analysis, and SEM and XRD analyses. Finally, the performance of the nanocement was ensured by the fabrication and characterization of the nanocement based mortar. Comparing the results with the commercially available cement product, it is demonstrated that the chemically synthesized nanocement not only shows better physical and mechanical performance, but also brings several encouraging impacts to the society, including the reduction of COemission and the development of sustainable construction material. A plausible reaction scheme has been proposed to explain the synthesis and the overall performances of the nanocement. © 2014 Byung Wan Jo et al.
    view abstract10.1155/2014/409380
  • Timescales of disequilibrium melting in the crust: Constraints from modelling the distribution of multiple trace elements and a case study from the Lesser Himalayan rocks of Sikkim
    Neogi, S. and Bolton, E.W. and Chakraborty, S.
    Contributions to Mineralogy and Petrology 168 (2014)
    A numerical code has been developed to track the distribution of trace elements in crustal rocks undergoing melting. The model handles diffusion with moving boundaries and accounts for the processes of diffusion, dissolution and precipitation in a partially molten system. Among the various input parameters for modelling, source composition (i.e. modal abundance) variations, diffusion coefficients and partition coefficients are found to exert a significant control on the melt chemistry. The other inputs such as melt reaction stoichiometry, kinetics of melting and grain size of protolith have lesser influence. Exploration of the general behaviour indicates that for systems in which disequilibrium melting of the kind considered in this paper occurs, trace element concentrations may be used to constrain the composition of the protolith or the timescales of melting, depending on the specific circumstances. After exploring some general features of melting in a pelitic system, the model is applied to calculate trace element distributions in migmatites from the Lesser Himalayan rocks in Sikkim, India. We focus on the distribution of trace elements during the initial stages of melt formation. These partially molten rocks show disequilibrium distribution of trace elements, and the numerical code is capable of quantitatively reproducing many of the observed patterns. The results of the modelling indicate that the observed melts in this zone were formed within 50,000 years and that segregation of melts (into leucosome and restite) was complete between 50,000 and 250,000 years. These short timescales may point to deformation-enhanced melt segregation at least on a hand specimen scale. It is important to distinguish between timescales of segregation over these scales and timescales of removal of melt on an outcrop scale to form plutons-the latter, requiring higher degrees of melting and larger distances of migration, take longer. © 2014 Springer-Verlag Berlin Heidelberg.
    view abstract10.1007/s00410-014-1020-8
  • Timing, duration and inversion of prograde Barrovian metamorphism constrained by high resolution Lu-Hf garnet dating: A case study from the Sikkim Himalaya, NE India
    Anczkiewicz, R. and Chakraborty, S. and Dasgupta, S. and Mukhopadhyay, D. and Kołtonik, K.
    Earth and Planetary Science Letters 407 (2014)
    We investigated Lu-Hf isotopic systematics in garnets from gradually higher grade metamorphic rocks from the first appearance of garnet at c. 500 °C to biotite dehydration melting at c. 800 °C in the Sikkim Himalaya, India. Exceptionally precise Lu-Hf ages obtained for the Barrovian metasedimentary sequence in the Lesser Himalaya (LH) correspond to the time of early garnet formation on a prograde path and show remarkable correlation with increasing metamorphic grade and decreasing structural depth. The youngest age is reported for the garnet zone (10.6. ±. 0.2Ma) and then the ages become progressively older in the staurolite (12.8. ±. 0.3Ma), kyanite (13.7. ±. 0.2Ma) and sillimanite (14.6. ±. 0.1Ma) zones. The oldest age of 16.8. ±. 0.1Ma was recorded at the top of the sequence in the zone marking the onset of muscovite dehydration melting, directly below the Main Central Thrust (MCT). These ages provide a tight constraint on the timing and duration of the Barrovian sequence formation which lasted about 6 Ma. The age pattern is clearly inverted with respect to structural depth but shows "normal" correlation with the metamorphic grade, i.e. earlier garnet growth in higher grade rocks.The timing of the peak metamorphic conditions estimated for the garnet and kyanite zones suggests that thermal climax occurred in all zones within a relatively short time span about 10-13 Ma ago. The metamorphic inversion in the Sikkim Himalaya must therefore have occurred after the metamorphic peak was attained in all grades.Lu-Hf garnet ages in the overthrust Higher Himalaya (HH) continue to be older but do not show a clear progression. There is about 6 Ma jump to 22.6. ±. 0.1Ma within the MCT zone and even older ages of 26-28 Ma were obtained for migmatites from the lower part of the HH. The Lu-Hf system in garnets from the HH records the time of melting at or near the thermal peak rather than dates initial garnet growth as in the LH.The age pattern obtained in this study provides precise time constraints on the continental collision models which must take into account early and prolonged anatexis in the HH, progressive delay in initial garnet growth in different Barrow zones in the LH, nearly contemporaneous metamorphic peak in all grades and post peak inversion of the coherent LH block during exhumation. © 2014 Elsevier B.V.
    view abstract10.1016/j.epsl.2014.09.035
  • A genetic strategy to identify targets for the development of drugs that prevent bacterial persistence
    Kim, J.-H. and O'Brien, K.M. and Sharma, R. and Boshoff, H.I.M. and Rehren, G. and Chakraborty, S. and Wallach, J.B. and Monteleone, M. and Wilson, D.J. and Aldrich, C.C. and Barry, C.E. and Rhee, K.Y. and Ehrt, S. and Schnappinger, D.
    Proceedings of the National Academy of Sciences of the United States of America 110 (2013)
    Antibacterial drug development suffers from a paucity of targets whose inhibition kills replicating and nonreplicating bacteria. The latter include phenotypically dormant cells, known as persisters, which are tolerant to many antibiotics and often contribute to failure in the treatment of chronic infections. This is nowhere more apparent than in tuberculosis caused by Mycobacterium tuberculosis, a pathogen that tolerates many antibiotics once it ceases to replicate. We developed a strategy to identify proteins that Mycobacterium tuberculosis requires to both grow and persist and whose inhibition has the potential to prevent drug tolerance and persister formation. This strategy is based on a tunable dualcontrol genetic switch that provides a regulatory range spanning three orders of magnitude, quickly depletes proteins in both replicating and nonreplicating mycobacteria, and exhibits increased robustness to phenotypic reversion. Using this switch, we demonstrated that depletion of the nicotinamide adenine dinucleotide synthetase (NadE) rapidly killed Mycobacterium tuberculosis under conditions of standard growth and nonreplicative persistence induced by oxygen and nutrient limitation as well as during the acute and chronic phases of infection in mice. These findings establish the dual-control switch as a robust tool with which to probe the essentiality of Mycobacterium tuberculosis proteins under different conditions, including those that induce antibiotic tolerance, and NadE as a target with the potential to shorten current tuberculosis chemotherapies.
    view abstract10.1073/pnas.1315860110
  • A robust nickel catalyst for cyanomethylation of aldehydes: Activation of acetonitrile under base-free conditions
    Chakraborty, S. and Patel, Y.J. and Krause, J.A. and Guan, H.
    Angewandte Chemie - International Edition 52 (2013)
    Nick of time: The nickel cyanomethyl complex 1 catalyzes the room temperature coupling of aldehydes with acetonitrile under base-free conditions. The catalytic system is long-lived and remarkably efficient with high turnover numbers (TONs) and turnover frequencies (TOFs) achieved. The mild reaction conditions allow a wide variety of aldehydes, including base-sensitive ones, to catalytically react with acetonitrile. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/anie.201302613
  • Compositionally zoned crystals and real-time degassing data reveal changes in magma transfer dynamics during the 2006 summit eruptive episodes of Mt. Etna
    Kahl, M. and Chakraborty, S. and Costa, F. and Pompilio, M. and Liuzzo, M. and Viccaro, M.
    Bulletin of Volcanology 75 (2013)
    One of the major objectives of volcanology remains relating variations in surface monitoring signals to the magmatic processes at depth that cause these variations. We present a method that enables compositional and temporal information stored in zoning of minerals (olivine in this case) to be linked to observations of real-time degassing data. The integrated record may reveal details of the dynamics of gradual evolution of a plumbing system during eruption. We illustrate our approach using the 2006 summit eruptive episodes of Mt. Etna. We find that the history tracked by olivine crystals, and hence, most likely the magma pathways within the shallow plumbing system of Mt. Etna, differed considerably between the July and October eruptions. The compositional and temporal record preserved in the olivine zoning patterns reveal two mafic recharge events within months of each other (June and September 2006), and each of these magma supplies may have triggered the initiation of different eruptive cycles (July 14-24 and August 31-December 14). Correlation of these observations with gas monitoring data shows that the systematic rise of the CO2/SO2 gas values is associated with the gradual (pre- and syn-eruptive) supply of batches of gas-rich mafic magma into segments of Etna's shallow plumbing system, where mixing with pre-existing and more evolved magma occurred. © 2013 Springer-Verlag Berlin Heidelberg.
    view abstract10.1007/s00445-013-0692-7
  • Effect of high pressure and high temperature on the microstructural evolution of a single crystal Ni-based superalloy
    Lopez-Galilea, I. and Huth, S. and Theisen, W. and Fockenberg, T. and Chakraborty, S.
    Journal of Materials Science 48 (2013)
    The application of high nearly hydrostatic pressures at elevated temperatures on the LEK94 single crystal (SX) nickel-based superalloy directly affects its microstructure. This is due to a combination of the effect of pressure on the Gibbs free energy, on the diffusion coefficients of the alloying elements, on the internal coherent stresses, and on the porosity distribution. The last effect depends at least on the first three. Therefore, based on the theoretical influences of the pressure, the main objective of this work is to understand, by means of an experimental work, the effect of high pressure at elevated temperature during annealing on the evolution of the phases morphology, and porosity of the high-temperature material LEK94. Specifically, pressures up to 4 GPa, temperatures up to 1180 C, and holding times up to 100 h were investigated. The main findings are that, porosity can be considerably reduced without affecting significantly the γ/γ′ microstructure by high pressure annealing and the verification that increasing the external pressure stabilizes the γ′-phase. © 2012 Springer Science+Business Media, LLC.
    view abstract10.1007/s10853-012-6752-0
  • Effect of jute as fiber reinforcement controlling the hydration characteristics of cement matrix
    Chakraborty, S. and Kundu, S.P. and Roy, A. and Adhikari, B. and Majumder, S.B.
    Industrial and Engineering Chemistry Research 52 (2013)
    The present investigation deals with the effect of jute as a natural fiber reinforcement on the setting and hydration behavior of cement. The addition of jute fiber in cement matrix increases the setting time and standard water consistency value. The hydration characteristics of fiber reinforced cement were investigated using a variety of analytical techniques including thermal, infrared spectroscopy, X-ray diffraction, and free lime estimation by titration. Through these analyses it was demonstrated that the hydration kinetics of cement is retarded with the increase in jute contents in cement matrix. A model has been proposed to explain the retarded hydration kinetics of jute fiber reinforced cement composites. The prolonged setting of these fiber reinforced cement composites would be beneficial for applications where the premixed cement aggregates are required to be transported from a distant place to the construction site. © 2012 American Chemical Society.
    view abstract10.1021/ie300607r
  • Fe-Mg interdiffusion rates in clinopyroxene: Experimental data and implications for Fe-Mg exchange geothermometers
    Müller, T. and Dohmen, R. and Becker, H.W. and ter Heege, J.H. and Chakraborty, S.
    Contributions to Mineralogy and Petrology 166 (2013)
    Chemical interdiffusion of Fe-Mg along the c-axis [001] in natural diopside crystals (XDi = 0.93) was experimentally studied at ambient pressure, at temperatures ranging from 800 to 1,200 °C and oxygen fugacities from 10-11 to 10-17 bar. Diffusion couples were prepared by ablating an olivine (XFo = 0.3) target to deposit a thin film (20-100 nm) onto a polished surface of a natural, oriented diopside crystal using the pulsed laser deposition technique. After diffusion anneals, compositional depth profiles at the near surface region (~400 nm) were measured using Rutherford backscattering spectroscopy. In the experimental temperature and compositional range, no strong dependence of DFe-Mg on composition of clinopyroxene (Fe/Mg ratio between Di93-Di65) or oxygen fugacity could be detected within the resolution of the study. The lack of fO2-dependence may be related to the relatively high Al content of the crystals used in this study. Diffusion coefficients, DFe-Mg, can be described by a single Arrhenius relation with (Formula presented). DFe-Mg in clinopyroxene appears to be faster than diffusion involving Ca-species (e.g., DCa-Mg) while it is slower than DFe-Mg in other common mafic minerals (spinel, olivine, garnet, and orthopyroxene). As a consequence, diffusion in clinopyroxene may be the rate-limiting process for the freezing of many geothermometers, and compositional zoning in clinopyroxene may preserve records of a higher (compared to that preserved in other coexisting mafic minerals) temperature segment of the thermal history of a rock. In the absence of pervasive recrystallization, clinopyroxene grains will retain compositions from peak temperatures at their cores in most geological and planetary settings where peak temperatures did not exceed ~1,100 °C (e.g., resetting may be expected in slowly cooled mantle rocks, many plutonic mafic rocks, or ultra-high temperature metamorphic rocks). © 2013 Springer-Verlag Berlin Heidelberg.
    view abstract10.1007/s00410-013-0941-y
  • Formation of Silicide Based Oxidation Resistant Coating Over Mo-30 wt% W Alloy
    Paul, B. and Chakraborty, S.P. and Suri, A.K.
    Transactions of the Indian Ceramic Society 72 (2013)
    Studies were carried out to develop silicide based oxidation resistant coatings over Mo-30W alloy substrate employing halide activated pack cementation coating process. Effect of activator content and temperature on coating was studied. Coated samples were characterized for phase and microstructure evaluation by SEM and EDS. Cyclic oxidation tests on coated alloy were performed at 1000°C up to 50 h. The coating provided enough protection from oxidation. © 2013 Copyright The Indian Ceramic Society.
    view abstract10.1080/0371750X.2013.793993
  • H-chondrite parent asteroid: A multistage cooling, fragmentation and re-accretion history constrained by thermometric studies, diffusion kinetic modeling and geochronological data
    Ganguly, J. and Tirone, M. and Chakraborty, S. and Domanik, K.
    Geochimica et Cosmochimica Acta 105 (2013)
    We present a detailed thermometric study and cooling history analysis of selected H-chondrites from the petrologic types 4-6 on the basis of their mineralogical properties, and integrate these data with other available constraints on the cooling rates to develop a comprehensive model for the cooling, fragmentation and re-accretion history of the parent asteroid. Temperatures have been determined on the basis of two-pyroxene (2-Px) and spinel (Spnl)-orthopyroxene (Opx)/olivine (Ol) thermometers using the average of line scans and distributed spot analysis of coexisting pairs in each set. All of these minerals have been found to be compositionally homogeneous from ∼1 to 2. μm from the interface within the resolution of microprobe analysis. The thermometric results for the H5 (Allegan and Richardton) and H6 (Guarena and Kernouvé) samples are very similar. Also, while the 2-Px temperature increases by ∼90. °C from H4 to H5/6, a reverse trend is observed for the Spnl-Opx/Ol temperatures implying compositional resetting of these pairs during cooling. For the H4 sample (Forest Vale) all thermometric results are similar. The cooling rates calculated from numerical modeling of the compositional profiles in Opx-Cpx pairs in H5 and H6, corrected for the spatial averaging or convolution effect in microprobe analysis, are ∼25-100. °C/ky, which are 3-4 orders of magnitude higher than the cooling rates implied by in situ cooling in an onion-shell parent body model. Similar numerical simulation of the compositional profile in Opx-Spnl pair in H4 yields a cooling rate ∼50. °C/ky, which is in very good agreement with recent metallographic cooling rate of this sample and geochronological constraints on the cooling T-t path. Numerical simulation suggests that the slow cooling of the H5/6 samples at a rate of ∼15. °C/My, as deduced by recent metallographic study, could not have commenced at a temperature above ∼700. °C since, otherwise, the simulated compositional profile fails to match the observed profile. For the H5 samples, the T-t path constructed on the basis of Hf-W age of peak metamorphism and two stage cooling model satisfies the constraints imposed by the Pb-Pb ages of the phosphates and Ar-Ar ages of the feldspars vs. their respective closure temperatures, whereas that for H6 samples constructed using the same approach fails to satisfy these geochronological data. A second stage of even slower cooling at ∼3. °C/My needs to be invoked to satisfy the geochronological age vs. closure temperature relations. We present a model of fragmentation and re-accretion history of the parent body that could lead to the reconstructed T-t paths of the H-chondrite samples studied in this work, and discuss some of their broader implications. The cooling rates retrieved from the available data on the Fe-Mg ordering states of orthopyroxenes of some other H5 and H6 samples are orders of magnitude faster than the metallographic cooling rates that are applicable to similar low temperature domain. It is, thus, likely that all samples of the same petrologic type did not share a common cooling history. © 2012 Elsevier Ltd.
    view abstract10.1016/j.gca.2012.11.024
  • Improvement of the mechanical properties of jute fibre reinforced cement mortar: A statistical approach
    Chakraborty, S. and Kundu, S.P. and Roy, A. and Basak, R.K. and Adhikari, B. and Majumder, S.B.
    Construction and Building Materials 38 (2013)
    We have demonstrated that the physical characteristics and mechanical properties of cement mortar are significantly improved by the jute fibre reinforcement. Three different processes methodologies were adopted to mix the jute fibre homogeneously in the mortar matrix. By optimising the processing conditions and fibre loading; the cold crushing strength and flexural strength, flexural toughness and the toughness index of the mortar has significantly been increased. Based on the Fourier transformed infrared spectroscopy and thermo-gravimetric analyses a plausible mechanism of the effect of jute reinforcement controlling the physical and mechanical properties of cement mortar have been proposed. © 2012 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.conbuildmat.2012.09.067
  • Lignocellulosic jute fiber as a bioadsorbent for the removal of azo dye from its aqueous solution: Batch and column studies
    Roy, A. and Chakraborty, S. and Kundu, S.P. and Adhikari, B. and Majumder, S.B.
    Journal of Applied Polymer Science 129 (2013)
    The feasibility of the use of jute fiber for the adsorption of azo dye from an aqueous solution was evaluated with batch and fixed-bed column studies. The batch studies illustrated that dye uptake was highly dependent on different process variables, namely, the pH, initial dye concentration of the solution, adsorbent dosage, contact time, ionic strength, and temperature. The exothermic and spontaneous nature of adsorption was revealed from thermodynamic study. The equilibrium adsorption data were highly consistent with the Langmuir isotherm and yielded an R2 value of 0.999. Kinetic studies divulged that the adsorption followed a pseudo-second-order model with regard to the intraparticle diffusion. In the column studies, the total amount of adsorbed dye and the adsorption capacity decreased with increasing flow rate and increased with increasing bed height and initial dye concentration. Also, the breakthrough time and exhaustion time increased with increasing bed depth but decreased with increasing flow rate and influent dye concentration. The column performances were predicted by the application of the bed-depth service time model and Thomas model to the experimental data. The virgin and dye-adsorbed jute fiber was characterized by Fourier transform infrared spectroscopy and scanning electron microscopy analyses. The investigation suggested that jute fiber could be applied as a promising low-cost adsorbent for dye removal. Copyright © 2012 Wiley Periodicals, Inc.
    view abstract10.1002/app.38222
  • Mg in plagioclase: Experimental calibration of a new geothermometer and diffusion coefficients
    Faak, K. and Chakraborty, S. and Coogan, L.A.
    Geochimica et Cosmochimica Acta 123 (2013)
    The temperature-sensitive exchange of Mg between plagioclase (Pl) and clinopyroxene (Cpx) has been studied experimentally, accounting for different anorthite-contents in plagioclase (XAn) and various silica activities (aSiO2) in the system. The partitioning of Mg between plagioclase and clinopyroxene was determined over a temperature range of 1100-1200°C, using plagioclase single crystals of different compositions (XAn=0.5-0.8), surrounded by different clinopyroxene-bearing matrix powders to account for different silica activities from 0.55 to 1.0. The experimental design also allows the diffusivity (DMgPl) of Mg in plagioclase under these conditions to be determined. Both KMgPl/Cpx (defined as KMgPl/Cpx=CMgPl/CMgCpx) and DMgPl decrease with temperature and increase with aSiO2. Isothermal data for different XAn in plagioclase show a linear increase of lnKMgPl/Cpx with increasing XAn, but DMgPl appears to be insensitive to XAn. The partitioning data allow a new geothermometer to be calibrated, which may be widely applicable to terrestrial and extraterrestrial rocks where plagioclase and clinopyroxene coexist:T[K]=(-9219+2034XAn)/(lnKMgPl/Cpx-1.6-lnaSiO2).Application of this geothermometer to experimental data from this study reproduces the experimental temperatures within ±20. °C. Diffusion of Mg in plagioclase is described by. DMgPl[m2s-1]=1.25×10-4[m2s-1]exp(-320,924[J mol-1]/(RT))(aSiO2)2.6. © 2013 Elsevier Ltd.
    view abstract10.1016/j.gca.2013.05.009
  • Monitoring bound HA1(H1N1) and HA1(H5N1) on freely suspended graphene over plasmonic platforms with infrared spectroscopy
    Banerjee, A. and Chakraborty, S. and Altan-Bonnet, N. and Grebel, H.
    Chemical Physics Letters 582 (2013)
    Infrared (IR) spectroscopy provides fingerprinting of the energy and orientation of molecular bonds. The IR signals are generally weak and require amplification. Here we present a new plasmonic platform, made of freely suspended graphene, which was coating periodic metal structures. Only monolayer thick films were needed for a fast signal recording. We demonstrated unique IR absorption signals of bound proteins: these were the hemagglutinin area (HA1) of swine influenza (H1N1) and the avian influenza (H5N1) viruses bound to their respective tri-saccharides ligand receptors. The simplicity and sensitivity of such approach may find applications in fast monitoring of binding events. © 2013 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.cplett.2013.07.010
  • Para-aminosalicylic acid acts as an alternative substrate of folate metabolism in Mycobacterium tuberculosis
    Chakraborty, S. and Gruber, T. and Barry III, C.E. and Boshoff, H.I. and Rhee, K.Y.
    Science 339 (2013)
    Folate biosynthesis is an established anti-infective target, and the antifolate para-aminosalicylic acid (PAS) was one of the first anti-infectives introduced into clinical practice on the basis of target-based drug discovery. Fifty years later, PAS continues to be used to treat tuberculosis. PAS is assumed to inhibit dihydropteroate synthase (DHPS) in Mycobacterium tuberculosis by mimicking the substrate p-aminobenzoate (PABA). However, we found that sulfonamide inhibitors of DHPS inhibited growth of M. tuberculosis only weakly because of their intracellular metabolism. In contrast, PAS served as a replacement substrate for DHPS. Products of PAS metabolism at this and subsequent steps in folate metabolism inhibited those enzymes, competing with their substrates. PAS is thus a prodrug that blocks growth of M. tuberculosis when its active forms are generated by enzymes in the pathway they poison.
    view abstract10.1126/science.1228980
  • Pincer-ligated nickel hydridoborate complexes: The dormant species in catalytic reduction of carbon dioxide with boranes
    Chakraborty, S. and Zhang, J. and Patel, Y.J. and Krause, J.A. and Guan, H.
    Inorganic Chemistry 52 (2013)
    Nickel pincer complexes of the type [2,6-(R2PO) 2C6H3]NiH (R = tBu, 1a; R = iPr, 1b; R = cPe, 1c) react with BH3·THF to produce borohydride complexes [2,6-(R2PO)2C 6H3]Ni(η2-BH4) (2a-c), as confirmed by NMR and IR spectroscopy, X-ray crystallography, and elemental analysis. The reactions are irreversible at room temperature but reversible at 60 C. Compound 1a exchanges its hydrogen on the nickel with the borane hydrogen of 9-BBN or HBcat, but does not form any observable adduct. The less bulky hydride complexes 1b and 1c, however, yield nickel dihydridoborate complexes reversibly at room temperature when mixed with 9-BBN and HBcat. The dihydridoborate ligand in these complexes adopts an η2- coordination mode, as suggested by IR spectroscopy and X-ray crystallography. Under the catalytic influence of 1a-c, reduction of CO2 leads to the methoxide level when 9-BBN or HBcat is employed as the reducing agent. The best catalyst, 1a, involves bulky substituents on the phosphorus donor atoms. Catalytic reactions involving 1b and 1c are less efficient because of the formation of dihydridoborate complexes as the dormant species as well as partial decomposition of the catalysts by the boranes. © 2012 American Chemical Society.
    view abstract10.1021/ic300587b
  • Polymer modified jute fibre as reinforcing agent controlling the physical and mechanical characteristics of cement mortar
    Chakraborty, S. and Kundu, S.P. and Roy, A. and Adhikari, B. and Majumder, S.B.
    Construction and Building Materials 49 (2013)
    Polymer modified alkali treated jute fibre as a reinforcing agent, substantially improves the physical and mechanical properties of cement mortar with a mix design cement:sand:fibre:water::1:3:0.01:0.6. The workability of the mortar is found to increase systematically from 155 ± 5 mm (control mortar) to 167 ± 8 mm (0.2050% polymer modified mortar). The density of the mortar is increased from 2092 kg/m3 to 2136 kg/m3 with a concomitant reduction of both water absorption and apparent porosity. Optimal polymer content in emulsion (0.0513%) is found to increase the compressive strength, modulus of rupture and flexural toughness 25%, 28%, 387% respectively as compared to control mortar. Based on the X-ray diffraction and infra-red spectroscopy analyses of the mortar samples a plausible mechanism of the effect of modified jute fibre controlling the physical and mechanical properties of cement mortar has been proposed. © 2013 Elsevier Inc. All rights reserved.
    view abstract10.1016/j.conbuildmat.2013.08.025
  • Surface grafting of Corchorus olitorius fibre: A green approach for the development of activated bioadsorbent
    Roy, A. and Chakraborty, S. and Kundu, S.P. and Majumder, S.B. and Adhikari, B.
    Carbohydrate Polymers 92 (2013)
    The present work is an endeavor to prepare lignocellulosic biomass based adsorbent, suitable for removal of organic and inorganic pollutants from industrial effluents. Lignocellulosic Corchorus olitorius fibre (jute fibre) surface was grafted with naturally available polyphenol, tannin, preceded by the epoxy-activation of fibre surface with epichlorohydrin under mild condition in an aqueous suspension. The reaction parameters for the modification, viz., concentration of epichlorohydrin and tannin, time, and temperature were optimized. The successful occurrence of surface modification of jute fibre (JF) was characterized and estimated from weight gain percent, elemental analysis, Fourier transform infrared spectroscopy (FTIR), X-ray diffraction, scanning electron and atomic force microscopy, and thermogravimetric analysis. An extensive analysis of deconvoluted FTIR spectra using the Voigt model was utilized to ensure the surface grafting. The microbiological susceptibility study revealed high persistency of JF towards biodegradation after efficient grafting with tannin. © 2012 Elsevier Ltd.
    view abstract10.1016/j.carbpol.2012.11.039
  • Thermal analysis on conversion of MoO3 to MoO2 and its silicothermic reduction
    Paul, B. and Kishor, J. and Chakraborty, S.P. and Suri, A.K.
    Journal of Thermal Analysis and Calorimetry 112 (2013)
    The present paper deals with differential thermal analysis studies conducted to find out the onset temperature for silicothermic reduction of MoO2 to Mo. The reaction kinetics of Si-MoO2 system has been analyzed by a model-free Kissinger method. X-ray diffraction analysis has confirmed the formation of Mo metal and SiO2 as the slag phase after silicothermic reduction of MoO2. The activation energy for silicothermic reduction of MoO2 to Mo was evaluated to be 309 kJ mol-1. © 2012 Akadémiai Kiadó, Budapest, Hungary.
    view abstract10.1007/s10973-012-2828-y
  • Timescales of crustal melting in the Higher Himalayan Crystallines (Sikkim, Eastern Himalaya) inferred from trace element-constrained monazite and zircon chronology
    Rubatto, D. and Chakraborty, S. and Dasgupta, S.
    Contributions to Mineralogy and Petrology 165 (2013)
    The petrology and timing of crustal melting has been investigated in the migmatites of the Higher Himalayan Crystalline (HHC) exposed in Sikkim, India. The metapelites underwent pervasive partial melting through hydrous as well as dehydration melting reactions involving muscovite and biotite to produce a main assemblage of quartz, K-feldspar, plagioclase, biotite, garnet ± sillimanite. Peak metamorphic conditions were 8-9 kbar and ~800 °C. Monazite and zircon crystals in several migmatites collected along a N-S transect show multiple growth domains. The domains were analyzed by microbeam techniques for age (SHRIMP) and trace element composition (LA-ICP-MS) to relate ages to conditions of formation. Monazite preserves the best record of metamorphism with domains that have different zoning pattern, composition and age. Zircon was generally less reactive than monazite, with metamorphic growth zones preserved in only a few samples. The growth of accessory minerals in the presence of melt was episodic in the interval between 31 and 17 Ma, but widespread and diachronous across samples. Systematic variations in the chemical composition of the dated mineral zones (HREE content and negative Eu anomaly) are related to the variation in garnet and K-feldspar abundances, respectively, and thus to metamorphic reactions and P-T stages. In turn, this allows prograde versus decompressional and retrograde melt production to be timed. A hierarchy of timescales characterizes melting which occurred over a period of ~15 Ma (31-17 Ma): a given block within this region traversed the field of melting in 5-7 Ma, whereas individual melting reactions lasted for time durations below, or approaching, the resolution of microbeam dating techniques (~0.6 Ma). An older ~36 Ma high-grade event is recorded in an allocthonous relict related to mafic lenses. We identify two sections of the HHC in Sikkim that traversed similar P-T conditions at different times, separated by a tectonic discontinuity. The higher structural levels reached melting and peak conditions later (~26-23 Ma) than the lower structural levels (~31-27 Ma). Diachronicity across the HHC cannot be reconciled with channel flow models in their simplest form, as it requires two similar high-grade sections to move independently during collision. © 2012 Springer-Verlag.
    view abstract10.1007/s00410-012-0812-y
  • Adsorption of anionic-azo dye from aqueous solution by lignocellulose- biomass jute fiber: Equilibrium, kinetics, and thermodynamics study
    Roy, A. and Chakraborty, S. and Kundu, S.P. and Adhikari, B. and Majumder, S.B.
    Industrial and Engineering Chemistry Research 51 (2012)
    The present investigation describes the evaluation of feasibility of lignocellulosic-biomass jute fiber (JF) toward adsorptive removal of anionic-azo dye from aqueous solution. Batch studies illustrated that dye uptake was highly dependent on different process variables, pH, initial dye concentration of solution, adsorbent dosage, and temperature. Further, an attempt has been taken to correlate these process variables with dye absorption and was optimized through a full-factorial central composite design (CCD) in response surface methodology (RSM). Maximum adsorption capacity (29.697 mg/g) under optimum conditions of variables (pH 3.91, adsorbent dose 2.04 g/L, adsorbate concentration 244.05 mg/L, and temperature 30 °C), as predicted by RSM, was found to be very close to the experimentally determined value (28.940 mg/g). Exothermic and spontaneous nature of adsorption was revealed from thermodynamic study. Equilibrium adsorption data were highly consistent with Langmuir isotherm yielding R 2 = 0.999. Kinetic studies revealed that adsorption followed pseudo second-order model regarding the intraparticle diffusion. Activation parameters for the adsorption process were computed using Arrhenius and Eyring equations. Maximum desorption efficiency of spent adsorbent was achieved using sodium hydroxide solution (0.1 M). © 2012 American Chemical Society.
    view abstract10.1021/ie301708e
  • Catalytic properties of nickel bis(phosphinite) pincer complexes in the reduction of CO 2 to methanol derivatives
    Chakraborty, S. and Patel, Y.J. and Krause, J.A. and Guan, H.
    Polyhedron 32 (2012)
    A new nickel bis(phosphinite) pincer complex [2,6-(R 2PO) 2C 6H 3]NiCl (L RNiCl, R = cyclopentyl) has been prepared in one pot from resorcinol, ClP(C 5H 9) 2, NiCl 2, and 4-dimethylaminopyridine. The reaction of this pincer compound with LiAlH 4 produces a nickel hydride complex, which is capable of reducing CO 2 rapidly at room temperature to give a nickel formate complex. X-ray structures of two related nickel formate complexes L RNiOCHO (R = cyclopentyl and isopropyl) have shown an "in plane" conformation of the formato group with respect to the coordination plane. The stoichiometric reaction of nickel formate complexes L RNiOCHO (R = cyclopentyl, isopropyl, and tert-butyl) with catecholborane has suggested that the reaction is favored by a bulky R group. L RNiOCHO (R = tert-butyl) does not react with PhSiH 3 at room temperature; however, it reacts with 9-borabicyclo[3.3.1]nonane and pinacolborane to generate a methanol derivative and a boryl formate species, respectively. The catalytic reduction of CO 2 with catecholborane is more effectively catalyzed by a more sterically hindered nickel pincer hydride complex with bulky R groups on the phosphorus donor atoms. The nickel pincer hydride complexes are inactive catalysts for the hydrosilylation of CO 2 with PhSiH 3. © 2011 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.poly.2011.04.030
  • Chemically modified jute fibre reinforced non-pressure (NP) concrete pipes with improved mechanical properties
    Kundu, S.P. and Chakraborty, S. and Roy, A. and Adhikari, B. and Majumder, S.B.
    Construction and Building Materials 37 (2012)
    To improve the load bearing capacity of underground sewage pipe, we have formulated a concrete mix using chemically-modified jute fibre (reinforcing agent), polymer latex (surface modifier both for fibre and matrix) and tannin (water reducing admixture). As compared to commercial non-pressure grade pipes (NP3 type), significant strength improvement, under three-edge-bearing test (∼129.4%), is achieved in the pipes made using the modified concrete mix. NP3 pipes made using this modified concrete exhibit similar mechanical properties to that of NP4 pipes resulting an effective reduction of 31.6 wt% of steel used for reinforcement in NP4 pipes. © 2012 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.conbuildmat.2012.07.082
  • Determination of pre-steady-state rate constants on the escherichia coli pyruvate dehydrogenase complex reveals that loop movement controls the rate-limiting step
    Balakrishnan, A. and Nemeria, N.S. and Chakraborty, S. and Kakalis, L. and Jordan, F.
    Journal of the American Chemical Society 134 (2012)
    Spectroscopic identification and characterization of covalent and noncovalent intermediates on large enzyme complexes is an exciting and challenging area of modern enzymology. The Escherichia coli pyruvate dehydrogenase multienzyme complex (PDHc), consisting of multiple copies of enzymic components and coenzymes, performs the oxidative decarboxylation of pyruvate to acetyl-CoA and is central to carbon metabolism linking glycolysis to the Krebs cycle. On the basis of earlier studies, we hypothesized that the dynamic regions of the E1p component, which undergo a disorder-order transition upon substrate binding to thiamin diphosphate (ThDP), play a critical role in modulation of the catalytic cycle of PDHc. To test our hypothesis, we kinetically characterized ThDP-bound covalent intermediates on the E1p component, and the lipoamide-bound covalent intermediate on the E2p component in PDHc and in its variants with disrupted active-site loops. Our results suggest that formation of the first covalent predecarboxylation intermediate, C2α-lactylthiamin diphosphate (LThDP), is rate limiting for the series of steps culminating in acetyl-CoA formation. Substitutions in the active center loops produced variants with up to 900-fold lower rates of formation of the LThDP, demonstrating that these perturbations directly affected covalent catalysis. This rate was rescued by up to 5-fold upon assembly to PDHc of the E401K variant. The E1p loop dynamics control covalent catalysis with ThDP and are modulated by PDHc assembly, presumably by selection of catalytically competent loop conformations. This mechanism could be a general feature of 2-oxoacid dehydrogenase complexes because such interfacial dynamic regions are highly conserved. © 2012 American Chemical Society.
    view abstract10.1021/ja3062375
  • Improvement in mechanical properties of jute fibres through mild alkali treatment as demonstrated by utilisation of the Weibull distribution model
    Roy, A. and Chakraborty, S. and Kundu, S.P. and Basak, R.K. and Basu Majumder, S. and Adhikari, B.
    Bioresource Technology 107 (2012)
    Chemically modified jute fibres are potentially useful as natural reinforcement in composite materials. Jute fibres were treated with 0.25%-1.0% sodium hydroxide (NaOH) solution for 0.5-48. h. The hydrophilicity, surface morphology, crystallinity index, thermal and mechanical characteristics of untreated and alkali treated fibres were studied.The two-parameter Weibull distribution model was applied to deal with the variation in mechanical properties of the natural fibres. Alkali treatment enhanced the tensile strength and elongation at break by 82% and 45%, respectively but decreased the hydrophilicity by 50.5% and the diameter of the fibres by 37%. © 2011 Elsevier Ltd.
    view abstract10.1016/j.biortech.2011.11.073
  • Multicomponent diffusion in garnets I: General theoretical considerations and experimental data for Fe-Mg systems
    Borinski, S.A. and Hoppe, U. and Chakraborty, S. and Ganguly, J. and Bhowmik, S.K.
    Contributions to Mineralogy and Petrology 164 (2012)
    We have carried out a combined theoretical and experimental study of multicomponent diffusion in garnets to address some unresolved issues and to better constrain the diffusion behavior of Fe and Mg in almandine-pyrope-rich garnets. We have (1) improved the convolution correction of concentration profiles measured using electron microprobes, (2) studied the effect of thermodynamic non-ideality on diffusion and (3) explored the use of a mathematical error minimization routine (the Nelder-Mead downhill simplex method) compared to the visual fitting of concentration profiles used in earlier studies. We conclude that incorporation of thermodynamic non-ideality alters the shapes of calculated profiles, resulting in better fits to measured shapes, but retrieved diffusion coefficients do not differ from those retrieved using ideal models by more than a factor of 1.2 for most natural garnet compositions. Diffusion coefficients retrieved using the two kinds of models differ only significantly for some unusual Mg-Mn-Ca-rich garnets. We found that when one of the diffusion coefficients becomes much faster or slower than the rest, or when the diffusion couple has a composition that is dominated by one component (&gt;75 %), then profile shapes become insensitive to one or more tracer diffusion coefficients. Visual fitting and numerical fitting using the Nelder-Mead algorithm give identical results for idealized profile shapes, but for data with strong analytical noise or asymmetric profile shapes, visual fitting returns values closer to the known inputs. Finally, we have carried out four additional diffusion couple experiments (25-35 kbar, 1,260-1,400 °C) in a piston-cylinder apparatus using natural pyrope- and almandine-rich garnets. We have combined our results with a reanalysis of the profiles from Ganguly et al. (1998) using the tools developed in this work to obtain the following Arrhenius parameters in D = D 0 exp{-[Q 1bar + (P-1)ΔV +]/RT} for D Mg * and D Fe *: Mg: Q 1bar = 228.3 ± 20.3 kJ/mol, D 0 = 2.72 (±4.52) × 10 -10 m 2/s, Fe: Q 1bar = 226.9 ± 18.6 kJ/mol, D 0 = 1.64 (±2.54) × 10 -10 m 2/s. ΔV + values were assumed to be the same as those obtained by Chakraborty and Ganguly (1992). © 2012 Springer-Verlag.
    view abstract10.1007/s00410-012-0758-0
  • Petrology and tectonic significance of metabasite slivers in the Lesser and Higher Himalayan domains of Sikkim, India
    Faak, K. and Chakraborty, S. and Dasgupta, S.
    Journal of Metamorphic Geology 30 (2012)
    The metamorphic history of the Himalayas has been constrained mostly through studies of the ubiquitous metapelitic rocks. Non-eclogitic metabasite rock lenses that occur intercalated with the metapelites have received little attention and it is not clear whether they share a common metamorphic history. This study reports the results of a petrological study of the metabasite lenses (dm 3-m 3) from the Lesser Himalayan (LH) and the Higher Himalayan (HH) domains in Sikkim. These have similar bulk chemical compositions and chemical affinities (sub-alkaline tholeiitic basalts), with plagioclase and amphibole as the dominant mineralogical constituents. Garnet and clinopyroxene occur in some samples depending on small variations in bulk chemistry; and orthopyroxene is developed as a retrograde phase in some rocks. Minor phases are ilmenite, chlorite, titanite and rutile. The rocks were metamorphosed at similar conditions (~9-12kbar, 800°C). Minor differences in bulk chemical composition lead to different phase assemblages and mineral chemistry in adjacent metabasite lenses, a feature that is used to demonstrate that metamorphic conditions (peak P-T as well as retrograde P-T path) can be reliably retrieved through a combination of pseudosection analysis and kinetically constrained individual thermobarometry. The peak P-T conditions of the metabasites from this region are independent of the present geographic or tectonic (i.e. within the LH or HH) location of the samples and they differ from the conditions at which the regional metapelites (i.e. metapelites not immediately adjacent to the metabasite lenses) were metamorphosed. Metapelites that are immediately adjacent to the metabasite lenses differ in their appearance, phase assemblage and recorded P-T history from those of the regional metapelites, either because they were emplaced as slivers along with the metabasites, or because they were modified when they came in contact with the metabasites. The retrograde P-T paths of the LH and HH metabasites are different: the HH samples underwent steep decompression whereas the LH followed a more gentle exhumation path. The P-T conditions of peak metamorphism (9-12kbar, 800°C) are commensurate with a thermal perturbation at the base of a crust of average thickness and may be the signature of a widespread (samples found across different regions in the Himalaya) and long-lasting (e.g. homogeneous garnet compositions) crustal underplating event that occurred during the early stages (?subduction) of the Himalayan orogeny, or earlier if the metamorphism was pre-Himalayan. © 2012 Blackwell Publishing Ltd.
    view abstract10.1111/j.1525-1314.2012.00987.x
  • Solid-state nuclear magnetic resonance studies delineate the role of the protein in activation of both aromatic rings of thiamin
    Balakrishnan, A. and Paramasivam, S. and Chakraborty, S. and Polenova, T. and Jordan, F.
    Journal of the American Chemical Society 134 (2012)
    Knowledge of the state of ionization and tautomerization of heteroaromatic cofactors when enzyme-bound is essential for formulating a detailed stepwise mechanism via proton transfers, the most commonly observed contribution to enzyme catalysis. In the bifunctional coenzyme, thiamin diphosphate (ThDP), both aromatic rings participate in catalysis, the thiazolium ring as an electrophilic covalent catalyst and the 4″-aminopyrimidine as acid-base catalyst involving its 1″,4″-iminopyrimidine tautomeric form. Two of four ionization and tautomeric states of ThDP are well characterized via circular dichroism spectral signatures on several ThDP superfamily members. Yet, the method is incapable of providing information about specific proton locations, which in principle may be accessible via NMR studies. To determine the precise ionization/tautomerization states of ThDP during various stages of the catalytic cycle, we report the first application of solid-state NMR spectroscopy to ThDP enzymes, whose large mass (160,000-250,000 Da) precludes solution NMR approaches. Three de novo synthesized analogues, [C2,C6″- 13C 2]ThDP, [C2- 13C]ThDP, and [N4″- 15N]ThDP used with three enzymes revealed that (a) binding to the enzymes activates both the 4″-aminopyrimidine (via pK a elevation) and the thiazolium rings (pK a suppression); (b) detection of a pre-decarboxylation intermediate analogue using [C2,C6″- 13C 2]ThDP, enables both confirmation of covalent bond formation and response in 4″-aminopyrimidine ring's tautomeric state to intermediate formation, supporting the mechanism we postulate; and (c) the chemical shift of bound [N4″- 15N]ThDP provides plausible models for the participation of the 1″,4″-iminopyrimidine tautomer in the mechanism. Unprecedented detail is achieved about proton positions on this bifunctional coenzyme on large enzymes in their active states. © 2011 American Chemical Society.
    view abstract10.1021/ja209856x
  • Development of graded Ni-YSZ composite coating on Alloy 690 by Pulsed Laser Deposition technique to reduce hazardous metallic nuclear waste inventory
    Sengupta, P. and Rogalla, D. and Becker, H.W. and Dey, G.K. and Chakraborty, S.
    Journal of Hazardous Materials 192 (2011)
    Alloy 690 based 'nuclear waste vitrification furnace' components degrade prematurely due to molten glass-alloy interactions at high temperatures and thereby increase the volume of metallic nuclear waste. In order to reduce the waste inventory, compositionally graded Ni-YSZ (Y 2O 3 stabilized ZrO 2) composite coating has been developed on Alloy 690 using Pulsed Laser Deposition technique. Five different thin-films starting with Ni80YSZ20 (Ni 80wt%+YSZ 20wt%), through Ni60YSZ40 (Ni 60wt%+YSZ 40wt%), Ni40YSZ60 (Ni 40wt%+YSZ 60wt%), Ni20YSZ80 (Ni 20wt%+YSZ 80wt%) and Ni0YSZ100 (Ni 0wt%+YSZ 100wt%), were deposited successively on Alloy 690 coupons. Detailed analyses of the thin-films identify them as homogeneous, uniform, pore free and crystalline in nature. A comparative study of coated and uncoated Alloy 690 coupons, exposed to sodium borosilicate melt at 1000°C for 1-6h suggests that the graded composite coating could substantially reduced the chemical interactions between Alloy 690 and borosilicate melt. © 2011 Elsevier B.V.
    view abstract10.1016/j.jhazmat.2011.05.006
  • Dynamic plumbing system beneath volcanoes revealed by kinetic modeling, and the connection to monitoring data: An example from Mt. Etna
    Kahl, M. and Chakraborty, S. and Costa, F. and Pompilio, M.
    Earth and Planetary Science Letters 308 (2011)
    Our ability to monitor volcanoes (using seismic signals, ground deformation, gas fluxes, or other ground and satellite based observations) as well as our understanding of melt reservoirs that feed eruptions have evolved tremendously in recent years. The complex plumbing systems that are thought to feed eruptions are, however, difficult to relate to the monitoring signals. Here we show that the record preserved in compositional zoning of erupted minerals may be used to reconstruct sections of the plumbing system. Kinetic modeling of such zoning can yield information on the residence time of magma in different segments of the plumbing systems. This allows a more nuanced evaluation of the link between observed monitoring signals or eruption styles and the magmatic processes and movement of batches of melts at depth. The approach is illustrated through a study of the compositional zoning recorded in olivine crystals from the 1991-1993 SE-flank eruption products of Mt. Etna (Sicily). The zoning patterns in crystals reveal that the plumbing system of the volcano consisted of at least three different magmatic environments between which magma was transported and mixed in the year or two preceding the start of eruption. Quantification of this history indicates that two main pathways of melt migration and three timescales dominated the dynamics of the system. Combination of this information with the timing of observation of various monitoring signals allows a reconstruction of the dynamic evolution of this section of the plumbing system during the early stages of the 1991-1993 eruption. It is seen, for example, how the migration of melt through the same sections of the plumbing system can cause pre-eruptive triggering, enhance Strombolian activity, and through the ensuing eruption cleanse and flush the plumbing system. Different kinds of mixing occur simultaneously at different sections of the plumbing system on different timescales (a few days up to two years). © 2011 Elsevier B.V.
    view abstract10.1016/j.epsl.2011.05.008
  • Graphenated IR screen for detection of human and avian flu viruses
    Banerjee, A. and Chakraborty, S. and Grebel, H.
    Proceedings of the IEEE Conference on Nanotechnology (2011)
    Targeted drug delivery and screening of drugs with specific affinity to membrane proteins require a good monitoring method. Here we monitor the binding of hemagglutinin (HA) to a model membrane- lipid bilayer- by using Infrared (IR) spectroscopy. IR spectroscopy is a useful spectroscopic tool to assess bio-molecular vibrations and provides with molecular fingerprinting. Yet, in order to achieve such goal, one needs to devise a bio-compatible platform that enhances the respective IR signals. We use graphene-coated IR (G-IR) screens as substrates and demonstrate their usefulness in monitoring the attachment of HA from influenza virus to proper receptors. © 2011 IEEE.
    view abstract10.1109/NANO.2011.6144595
  • Preliminary study on calcium aluminosilicate glass as a potential host matrix for radioactive 90Sr-An approach based on natural analogue study
    Sengupta, P. and Fanara, S. and Chakraborty, S.
    Journal of Hazardous Materials 190 (2011)
    Given the environmental-, safety- and security risks associated with sealed radioactive sources it is important to identify suitable host matrices for 90Sr that is used for various peaceful applications. As SrO promotes phase separation within borosilicate melt, aluminosilicate bulk compositions belonging to anorthite-wollastonite-gehlenite stability field are studied in this work. Tests for their homogeneity, microstructural characteristics and resistance to phase separation narrowed the choice down to the composition CAS11 (CaO=35wt%, Al2O3=20wt%, SiO2=45wt%). We find that up to 30wt% SrO can be loaded in this glass without phase separation (into Ca, Sr-rich and Sr-poor, Si-rich domains). Leaching behaviour of the glasses differs depending on the content and distribution of Sr. In general, the elemental leach rates determined from conventional PCT experimental procedure yield values better than 10-7gcm-2day-1 for both CAS11 base glass as well as SrO doped glass. It was noted that leach rates calculated on the basis of Ca2+ and Sr2+ were of the same order and bit higher compared to those calculated on the basis of Si4+ and Al3+. During accelerated leaching tests, zeolite and zeolite+epidote were found to have developed on CAS11 base glass and SrO doped glasses respectively. The Sr bulk diffusion coefficients is found to vary from ~10-15 to 10-13cm2/s at temperature intervals as high as 725-850°C. Based on the experimental observations, it is suggested that CAS11 glass can be used as host matrix of 90Sr for various applications of radioactive Sr-pencils. © 2011 Elsevier B.V.
    view abstract10.1016/j.jhazmat.2011.03.031
  • Silicon and magnesium diffusion in a single crystal of MgSiO3 perovskite
    Xu, J. and Yamazaki, D. and Katsura, T. and Wu, X. and Remmert, P. and Yurimoto, H. and Chakraborty, S.
    Journal of Geophysical Research: Solid Earth 116 (2011)
    Si and Mg self-diffusion coefficients were measured simultaneously in single crystals of MgSiO<inf>3</inf> perovskite under lower mantle conditions. There is little difference in Si volume diffusivity measured directly using single crystals (this study) and those retrieved from experiments with polycrystals (earlier studies). This agreement between studies establishes the reliability of Si diffusion coefficients measured in perovskite. Within the uncertainties of our measurements, no anisotropy in the diffusion of either Si or Mg could be resolved. Diffusion of Si and Mg in perovskite are described by an Arrhenius equation, D=D<inf>0</inf> exp (-H/RT) at 25GPa, with D <inf>0</inf>=5.10×10-11m2/s for Si and 4.99×10-11m2/s for Mg, H=308kJ/mol for Si, and 305kJ/mol for Mg. Mg diffusivity in MgSiO<inf>3</inf> perovskite is distinctly lower than those measured in olivine, wadsleyite, and ringwoodite. We find that Mg has very similar diffusivity to Si in perovskite. As a consequence, the rheological properties of the lower mantle may be controlled by the coupled motion of Si and Mg. A point defect-based model is discussed that may account for the diffusion behavior of Si and Mg in MgSiO<inf>3</inf> perovskite. Our data indicate that, within realistic ranges of temperature, grain size, and state of stress, both diffusion creep as well as dislocation creep may be observed in the lower mantle. Copyright 2011 by the American Geophysical Union.
    view abstract10.1029/2011JB008444
  • Sintering kinetics study of mechanically alloyed nanocrystalline Mo-30 wt.% W
    Paul, B. and Jain, D. and Chakraborty, S.P. and Sharma, I.G. and Pillai, C.G.S. and Suri, A.K.
    Thermochimica Acta 512 (2011)
    The paper details the results of sintering kinetics studies conducted on nanocrystalline Mo-30 wt.% W alloy powders prepared through mechanical alloying route. Both, constant rate of heating method as well as Stepwise Isothermal Dilatometry (SID) technique were used for studying the sintering kinetics. Measured step isothermal shrinkage data were analyzed by Mekipritti-Meng method. The shrinkage data was found to fit well with the rate equation proposed in this method and its validity was established for mechanically alloyed systems. Kinetic parameters were evaluated and sintering was found to occur through two major mechanisms operative successively, which are grain boundary diffusion and lattice diffusion with corresponding energies of activation as 230 kJ/mol and 480 kJ/mol, respectively. The results have been well supported by micro structural evaluation of specimens at different stages of sintering. © 2010 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.tca.2010.09.015
  • Studies on synthesis and characterization of mo based in situ composite by silicothermy co-reduction process
    Paul, B. and Chakraborty, S.P. and Kishor, J. and Sharma, I.G. and Suri, A.K.
    Metallurgical and Materials Transactions B: Process Metallurgy and Materials Processing Science 42 (2011)
    The results of an in situ synthesis of refractory metal-intermetallic composite (RMIC), Mo-16Cr-4Si (wt pct) multiphase alloy and its characterization, are presented in this study. The alloy was prepared from the oxides of molybdenum and chromium by their co-reduction with Si metal powder as a reductant. The exothermic nature of these reactions resulted in the formation of consolidated composite as a product in a single step. The thermodynamic aspects of exothermic reactions were studied by thermogravimetry/differential thermal analyzer. As-reduced alloys were remelted by arc melting and heat treated to obtain a homogenous microstructure. The evolution of phases and microstructures qA studied by X-ray diffraction, scanning electron microscopy, and energy-dispersive spectrum analysis. The multiphase alloy consisted of Mo3Si and discontinuous (Mo, Cr) (ss) phase with a volume percentage of 28 pct. The synthesized alloys were characterized with respect to composition, phases, microstructure, hardness, and oxidation behavior. © The Minerals Metals & Materials Society and ASM International 2011.
    view abstract10.1007/s11663-011-9513-6
  • Studies on the development of TZM alloy by aluminothermic coreduction process and formation of protective coating over the alloy by plasma spray technique
    Chakraborty, S.P.
    International Journal of Refractory Metals and Hard Materials 29 (2011)
    TZM alloy is a potential candidate for high temperature structural applications. However, in the preparation of this alloy by conventional melt-casting route, difficulties are encountered in achieving homogenized alloy composition in view of high melting temperature of the alloy and presence of minor alloying components. Therefore, an alternative technique of aluminothermic co-reduction was adopted to prepare TZM alloy of composition, Mo-0.5Ti-0.1Zr-0.02 °C, wt.% by simultaneous reduction of uniformly premixed oxides of MoO2, TiO2 and ZrO2 by aluminium in presence of requisite amount of carbon. The as-reduced alloy was further arc melted for consolidation. Since, TZM alloy is by nature highly susceptible to oxidation at elevated temperature in air or oxygen, therefore feasibility of development of silicide type of coating over the synthesized alloy by plasma coating technique was also examined. Silicon powder coated on TZM alloy surface by plasma spray technique was finally converted into MoSi2 coating by sintering at 1350 °C for 2-4 h duration under argon. A double layer coating structure was formed with two distinct phases. The inner thin layer was consisted of Mo2Si5 phase (~ 10 μm) followed by thick outer layer of MoSi2 (~ 150 μm). The coating showed good adhesion strength and stable oxidation with negligible mass gain (10 g/m2) at 1000 °C in air. © 2011 Elsevier Ltd.
    view abstract10.1016/j.ijrmhm.2011.04.010
  • An efficient nickel catalyst for the reduction of carbon dioxide with a borane
    Chakraborty, S. and Zhang, J. and Krause, J.A. and Guan, H.
    Journal of the American Chemical Society 132 (2010)
    Nickel hydride with a diphosphinite-based ligand catalyzes the highly efficient reduction of CO2 with catecholborane, and the hydrolysis of the resulting methoxyboryl species produces CH3OH in good yield. The mechanism involves a nickel formate, formaldehyde, and a nickel methoxide as different reduced stages for CO2. The reaction may also be catalyzed by an air-stable nickel formate. © 2010 American Chemical Society.
    view abstract10.1021/ja103982t
  • Diffusion coefficients in olivine, wadsleyite and ringwoodite
    Chakraborty, S.
    Reviews in Mineralogy and Geochemistry 72 (2010)
    view abstract10.2138/rmg.2010.72.13
  • Diffusion of Li in olivine. Part I: Experimental observations and a multi species diffusion model
    Dohmen, R. and Kasemann, S.A. and Coogan, L. and Chakraborty, S.
    Geochimica et Cosmochimica Acta 74 (2010)
    There are an increasing number of studies that focus on the systematics of the distribution of Li and its isotopes among different geochemical reservoirs. These studies have found that Li is relatively mobile compared to many other elements (e.g., Fe, Mg), and diffusion has been considered as a mechanism to generate large isotopic fractionations even at high temperatures. In order to quantify some of these aspects, we have measured Li diffusion rates experimentally along [0 0 1] of single crystals of olivines from San Carlos, Arizona and Pakistan, at 800-1200 °C at a total pressure of 100 kPa and fO2 ≈ WM buffer. A complex diffusion behavior of Li is observed, indicating that two mechanisms of diffusion (a fast and a slower one) operate simultaneously. The behavior is well described by a model that partitions Li between two different sites in olivine - an octahedral site (LiMe) and an interstitial site (Lii). Transport of Li is a combination of hopping within and between each of these kinds of sites involving also vacancies on the octahedral site (VMe). It is assumed that the homogeneous reaction (LiMe = VMe + Lii) that maintains equilibrium distribution of Li between the sites is instantaneous compared to the timescales of all other processes associated with diffusive transport. One consequence of this mode of transport of Li in olivine is that the shape and length of diffusion profiles depend on the boundary conditions imposed at the surface of a crystal; i.e., the chemical environment (e.g., fO2, aLi4SiO4), in addition to temperature and pressure. Our model describes the variable experimentally determined Li-profile shapes produced at different temperatures and with different boundary conditions, as well as their time evolution, quantitatively. Modeling the observed isotopic fractionation shows that 6Li diffuses about 5% faster than 7Li on the interstitial site. Inspection of published data on Li distribution in natural olivines that are available until now indicates that the fast (interstitial) mechanism of Li diffusion is unlikely to be dominant in most natural systems; Li rich, oxidizing environments (e.g., fluids?) may be exceptions. However, when it operates it can decouple the equilibration of Li isotopic gradients from the time scale of equilibration of overall Li concentrations. Diffusion dominated by the slower mechanism will occur on the average at a rate that is about an order of magnitude faster than diffusion of Fe, Mg and most other divalent cations in olivine; such diffusion of Li in olivine will be much slower than the rates of diffusion in clinopyroxene and plagioclase crystals at the same conditions. Fractionation of isotopes of Li by diffusion is likely to be a transient phenomenon and is more likely to be observed in crystals showing zoning of Li concentrations. © 2009 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.gca.2009.10.016
  • First-row transition metal catalyzed reduction of carbonyl functionalities: A mechanistic perspective
    Chakraborty, S. and Guan, H.
    Dalton Transactions 39 (2010)
    The use of first-row transition metals for the catalytic reduction of carbonyl functionalities has become increasingly important in homogeneous catalysis. This Perspective examines the mechanistic aspects of these reduction reactions, with a focus on various interactions between metal complexes and substrates. Four different types of catalytic pathways, namely catalysis with dihydride (or dihydrogen) complexes, catalysis with monohydride complexes, metal-ligand bifunctional catalysis, and catalysis involving ionic mechanisms, are discussed with recent examples highlighted. © 2010 The Royal Society of Chemistry.
    view abstract10.1039/c002942d
  • The time scales of magma mixing and mingling involving primitive melts and melt-mush interaction at mid-ocean ridges
    Costa, F. and Coogan, L.A. and Chakraborty, S.
    Contributions to Mineralogy and Petrology 159 (2010)
    We have studied the chemical zoning of plagioclase phenocrysts from the slow-spreading Mid-Atlantic Ridge and the intermediate-spreading rate Costa Rica Rift to obtain the time scales of magmatic processes beneath these ridges. The anorthite content, Mg, and Sr in plagioclase phenocrysts from the Mid-Atlantic Ridge can be interpreted as recording initial crystallisation from a primitive magma (~ 11 wt% MgO) in an open system. This was followed by crystal accumulation in a mush zone and later entrainment of crystals into the erupted magma. The initial magma crystallised plagioclase more anorthitic than those in equilibrium with any erupted basalt. Evidence that the crystals accumulated in a mush zone comes from both: (1) plagioclase rims that were in equilibrium with a Sr-poor melt requiring extreme differentiation; and (2) different crystals found in the same thin section having different histories. Diffusion modelling shows that crystal residence times in the mush were < 140 years, whereas the interval between mush disaggregation and eruption was ≤1.5 years. Zoning of anorthite content and Mg in plagioclase phenocrysts from the Costa Rica Rift show that they partially or completely equilibrated with a MgO-rich melt (>11 wt%). Partial equilibration in some crystals can be modelled as starting <1 year prior to eruption but for others longer times are required for complete equilibration. This variety of times is most readily explained if the mixing occurred in a mush zone. None of the plagioclase phenocrysts from the Costa Rica Rift that we studied have Mg contents in equilibrium with their host basalt even at their rims, requiring mixing into a much more evolved magma within days of eruption. In combination these observations suggest that at both intermediate- and slowspreading ridges: (1) the chemical environment to which crystals are exposed changes on annual to decadal time scales; (2) plagioclase crystals record the existence of melts unlike those erupted; and (3) disaggregation of crystal mush zones appears to precede eruption, providing an efficient mechanism by which evolved interstitial melt can be mixed into erupted basalts. © Springer-Verlag 2009.
    view abstract10.1007/s00410-009-0432-3
  • kinetics

  • petrology

  • scanning electron microscopy

  • thermodynamics

  • thin films

  • x-ray diffraction

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