High-temperature cooling histories of migmatites from the High Himalayan Crystallines in Sikkim, India: rapid cooling unrelated to exhumation?   总被引：1，自引：0，他引：1  

Nilanjana Sorcar  Ulrich Hoppe  Somnath Dasgupta  Sumit Chakraborty 《Contributions to Mineralogy and Petrology》2014,167(2):1-34

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.  相似文献   

Interactions of mercury with different molecular weight fractions of humic substances in aquatic systems     

Parthasarathi Chakraborty  Koffi Marcellin Yao  Kartheek Chennuri  Krishna Vudamala  P. V. Raghunadh Babu 《Environmental Earth Sciences》2014,72(3):931-939

Interactions of mercury (Hg) with different molecular weight fractions of humic substances (HS) play an important role in controlling distribution, diffusion, speciation, and bioavailability of Hg in natural systems. This study suggests that Hg prefers to associate with higher molecular weight fractions of HS and this association particularly predominates at low pH and high ionic strengths of the medium. The concentrations of aggregated HS (with higher molecular weight) become high at lower pH (acidic condition) and high ionic strength. Molecular weight of HS gradually decreases with the increasing pH (basic condition) and low ionic strength of the medium. The disaggregation property of HS which involves the release of monomers from the surface of the aggregates produces HSs of different intermediate molecular weight with different Hg complexing capacity. Distribution of Hg in different molecular weight fractions of HS is dependent on aggregational and disaggregational properties of HS in aquatic medium. Association of Hg with high molecular weight fraction of HS may alter distribution and bioavailability of Hg in a system as the bioreactivity of organic matter decrease along a continuum of size in aquatic medium.  相似文献   

Higher dimensional inhomogeneous cosmological models in Lyra geometry     

F. Rahaman  S. Chakraborty  S. Das  R. Mukherjee  M. Hossain  N. Begam 《Astrophysics and Space Science》2003,288(4):379-387

We have presented in homogeneous cosmological models within the framework of Lyra geometry. We have considered an inhomogeneous spherically symmetric higher dimensional model in presence of a mass less scalar field whose potential has a flat part. The scalar field is considered to be inhomogeneous. Also an inhomogeneous cosmological model is derived in a Kaluza-Klein type of space time. The matter field is taken as an inhomogeneous distribution of fluid. It is observed that there is no singularity at finite past in our model and the desirable feature of dimensional reduction is also possible for the extra space. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

Additional Evidence Supporting a Model of Shallow,High-Speed Supergranulation     

T. L. Duvall Jr.  S. M. Hanasoge  S. Chakraborty 《Solar physics》2014,289(9):3421-3433

Recently, Duvall and Hanasoge (Solar Phys. 287, 71, 2013) found that large-distance separation [Δ] travel-time differences from a center to an annulus [δt _oi] implied a model of the average supergranular cell that has a peak upflow of 240 m?s^?1 at a depth of 2.3 Mm and a corresponding peak outward horizontal flow of 700 m?s^?1 at a depth of 1.6 Mm. In the present work, this effect is further studied by measuring and modeling center-to-quadrant travel-time differences [δt _qu], which roughly agree with this model. Simulations are analyzed that show that such a model flow would lead to the expected travel-time differences. As a check for possible systematic errors, the center-to-annulus travel-time differences [δt _oi] are found not to vary with heliocentric angle. A consistency check finds an increase of δt _oi with the temporal frequency [ν] by a factor of two, which is not predicted by the ray theory.  相似文献   

A study of stability of rotating fluid disks using a variational principle     

K.N. Mishra  D.K. Chakraborty 《Astrophysics and Space Science》2002,282(3):527-535

Stability of non-self gravitating, non-accreting fluid disk rotating around a Schwarzschild black hole is studied by using a variational principle. We find a set of solutions which depending on the value of a parameter, either a stable or an unstable specific angular momentum distribution (Rayleigh's criterion). Using these solutions we find that the calculations based on the variational principle yield results which agree with the Rayleigh criteria when a suitable choice of the trial function is made.  相似文献   

Fe–Mg interdiffusion rates in clinopyroxene: experimental data and implications for Fe–Mg exchange geothermometers     

T. Müller  R. Dohmen  H. W. Becker  Jan H. ter Heege  S. Chakraborty 《Contributions to Mineralogy and Petrology》2013,166(6):1563-1576

Chemical interdiffusion of Fe–Mg along the c-axis [001] in natural diopside crystals (X _Di = 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 (X _Fo = 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 D ^Fe–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, D ^Fe–Mg, can be described by a single Arrhenius relation with $$D^{{{\text{Fe}} - {\text{Mg}}}} = 2. 7 7\pm 4. 2 7\times 10^{ - 7} {\text{exp(}}-3 20. 7\pm 1 6.0{\text{ kJ}}/{\text{mol}}/{\text{RT)m}}^{ 2} /{\text{s}}.$$ D ^Fe–Mg in clinopyroxene appears to be faster than diffusion involving Ca-species (e.g., D ^Ca–Mg) while it is slower than D ^Fe–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).  相似文献   

Universe bounded by apparent horizon: an irreversible thermodynamic prescription     

Subenoy Chakraborty  Atreyee Biswas 《Astrophysics and Space Science》2013,343(1):395-399

In this work, we study universe bounded by apparent horizon as an irreversible thermodynamical system. Using the non equilibrium thermodynamical technique, the modified entropy variation on the apparent horizon has been evaluated in general. Two dark energy models are presented and results are analyzed.  相似文献   

Observational study of higher dimensional magnetic universe in non-linear electrodynamics     

Chayan Ranjit  Shuvendu Chakraborty  Ujjal Debnath 《Astrophysics and Space Science》2013,346(1):291-299

In this work, we have considered the flat FRW model of the universe in (n+2)-dimensions filled with the dark matter and the magnetic field. We present the Hubble parameter in terms of the observable parameters Ω _m0 and H ₀ with the redshift z and the other parameters like B ₀, ω, μ ₀, δ, n, w _m . The natures of magnetic field B, deceleration parameter q and $\operatorname{Om}$ diagnostic have also been analyzed for accelerating expansion of the universe. From Stern data set (12 points), we have obtained the bounds of the arbitrary parameters by minimizing the χ ² test. The best-fit values of the parameters are obtained by 66 %, 90 % and 99 % confidence levels. Now to find the bounds of the parameters (B ₀,ω) and to draw the statistical confidence contour, we fixed four parameters μ ₀, δ, n, w _m . Here the parameter n determines the higher dimensions and we perform comparative study between three cases: 4D (n=2), 5D (n=3) and 6D (n=4) respectively. Next due to joint analysis with BAO observation, we have also obtained the bounds of the parameters (B ₀,ω) by fixing other parameters μ ₀, δ, n, w _m for 4D, 5D and 6D. The best fit of distance modulus for our theoretical model and the Supernova Type Ia Union2 sample are drawn for different dimensions.  相似文献   

Land,Labor, and Rural Development: Analyzing Participation in India's Village Dairy Cooperatives   总被引：1，自引：0，他引：1  

Pratyusha Basu  Jayajit Chakraborty 《The Professional geographer》2013,65(3):299-313

Livestock-based livelihoods are currently being promoted by international development agencies as part of global efforts to combat poverty. India's dairy development program, organized around village cooperatives, has become an important model for such efforts. This article aims to identify household characteristics that influence membership in India's rural dairy cooperatives by comparing two villages representing different degrees of success. Utilizing logistic regression methods, data collected through a comprehensive survey of all households in the two villages are analyzed to examine (1) how variables describing animal ownership, agricultural attributes, and household labor availability contribute to explaining membership in the dairy cooperative; and (2) whether factors influencing membership differ across the two villages. Our results indicate that although agricultural property ownership influences cooperative membership in both villages, the kind of dairy animal used and labor utilized for dairying work also have a significant and context-specific effect on household participation.  相似文献   

Environmental Health Injustice: Exposure to Air Toxics and Children's Respiratory Hospital Admissions in El Paso,Texas     

Sara E. Grineski  Timothy W. Collins  Jayajit Chakraborty  Yolanda J. McDonald 《The Professional geographer》2013,65(1):31-46

Although much environmental justice research tacitly assumes that unequal environmental exposures produce geographic disparities in adverse health outcomes, very few empirical environmental justice studies have tested that assumption. This article does so by using estimates of exposure to air toxics disaggregated by emission source (point and mobile) to predict children's hospitalization rates for both asthma and respiratory infections in El Paso, Texas. Air toxics emissions from most source categories were found to be significant predictors of children's respiratory infection hospitalization rates, but not asthma hospitalization rates, at the census tract level. Findings suggest that sociospatial disparities in respiratory infection rates might be linked to environmental inequalities.  相似文献