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61.
Abhishek Saha Sohini Ganguly Jyotisankar Ray Avik Dhang 《Journal of the Geological Society of India》2010,76(1):26-32
A new occurrence of (syenite-hosted) Vanadium bearing titaniferous magnetite ore body has been reported from Ganjang (26°09′35″
N: 93°20′ E), Karbi-Anglong, Northeastern India. The magnetite ore bodies have lumpy and sporadic occurrences within the host
syenite pluton intrusive into gneissic country rocks. Ore microscopic studies reveal that magnetite is often associated with
haematite and ilmenite depicting different textural patterns. Critical consideration of several elemental patterns suggests
magmatic differentiation to be main ore-forming process. The ore body is suggested to have been formed as late stage segregation
from a differentiating alkaline magma in a fluid enriched milieu. 相似文献
62.
Massimiliano Tirone Jibamitra Ganguly Ralf Dohmen Richard Hervig 《Geochimica et cosmochimica acta》2005,69(9):2385-2398
We determined the diffusion coefficient of Sm in almandine garnet as function of temperature at 1 bar and fO2 corresponding to that of wüstite-iron buffer, and to a limited extent, that of a few other selected rare earth elements in almandine and pyrope garnets. Both garnets were demonstrated to have metastably survived the diffusion annealing at conditions beyond their stability fields. The experimental diffusion profiles were analyzed by secondary ion mass spectrometry, and in addition, by Rutherford back scattering spectroscopy for two samples. Transmission electron microscopic study of an almandine crystal that was diffusion-annealed did not reveal any near-surface fast diffusion path. Using reasonable approximations and theoretical analysis of vacancy diffusion, the experimental data were used to develop an expression of rare earth element (REE) diffusion coefficient in garnet as a function of temperature, pressure, fO2, ionic radius, and matrix composition. Calculation of the closure temperature for the Sm-Nd decay system in almandine garnet in a metamorphic terrain shows very good agreement with that constrained independently. Modeling of the REE evolution in melt and residual garnet suggests that for dry melting condition, the REE pattern in the melt should commonly conform closely to that expected for equilibrium melting. However, for much lower solidus temperatures that would prevail in the presence of a H2O-CO2 fluid, the concentration of light REE in the melt could be significantly lower than that under equilibrium melting condition. A reported core and rim differences in the REE content of a garnet crystal in a mantle xenolith in kimberlite have been reproduced by assuming that the REE zoning was a consequence of entrapment in a magma derived from an external source for ∼32,000 yr before the eruption. 相似文献
63.
We have determined Cr diffusion coefficients (D) in orthopyroxene parallel to the a-, b-, and c-axial directions as a function temperature at f(O2) corresponding to those of the wüstite-iron (WI) buffer. Diffusion is found to be significantly anisotropic with D(//c) > D(//b) > D(//a), conforming to an earlier theoretical prediction. Increase of f(O2) from WI buffer conditions to 4.5 log unit above the buffer at 950 and 1050 °C leads to decrease of D(Cr) by a factor of two to three, possibly suggesting significant contribution from an interstitial diffusion mechanism. We have used the diffusion data to calculate the closure temperatures (Tc) of the Mn-Cr decay system in orthopyroxene as a function of initial temperature (T0), grain size (a) and cooling rate for spherical and plane sheet geometries. We also present graphical relations that permit retrieval of cooling rates from knowledge of the resetting of Mn-Cr ages in orthopyroxene during cooling, T0 and a. Application of these relations to the Mn-Cr age data of the cumulate eucrite Serra de Magé yields a Tc of 830-980 °C, and cooling rates of 2-27 °C/Myr at Tc and ∼1-13 °C/Myr at 500 °C. It is shown that the cooling of Serra de Magé to the closure temperature of the Mn-Cr system took place at its original site in the parent body, and thus implies a thickness for the eucrite crust in the commonly accepted HED parent body, Vesta, of greater than 30 km. This thickness of the eucrite crust is compatible only with a model of relatively olivine-poor bulk mineralogy in which olivine constitutes 19.7% of the total asteroidal mass. 相似文献
64.
Somenath Chakrabarty 《Journal of Astrophysics and Astronomy》1995,16(3-4):399-405
The effect of intense magnetic field on the cosmic quark-hadron phase transition and also on the baryon number inhomogeneity
has been investigated using phenomenological MIT bag model for the quark sector. For the sake of simplicity an ideal gas equation
of state has been considered for the hadronic phase. 相似文献
65.
Ravi Shanker S. Nag A. Ganguly A. Absar B. P. Rawat G. S. Singh 《Journal of Earth System Science》2001,110(1):63-76
The diamond bearing pipe rocks in Majhgawan-Hinota (more than four pipes) occur as intrusives in sandstones of Kaimur Group.
These Proterozoic (974 ±30-1170 ±20 Ma) intrusive rocks, occupying the southeastern margin of Aravalli craton, were called
as ‘micaceous kimberlite’ in tune with the reported kimberlite occurrences from other parts of the world. Judging from the
definition of kimberlite, as approved by the IUGS Subcommission on Systematics of Igneous Rocks, it is not justified to call
these rocks as ‘micaceous kimberlite’. Rather the mineralogical assemblages such as absence of typomorphic mineral monticellite
(primary), abundance of phlogopite cognate, frequent presence of barite and primary carbonate mostly as calcite coupled with
ultrapotassic and volatile-rich (dominantly H2O) nature and high concentration of incompatible elements (such as Ba, Zr, Th, U), low Th/U ratios, low REE and no Eu-anomaly
clearly indicate a close similarity with that of South African orangeites. Thus orangeites of Proterozoic age occur outside
the Kaapvaal craton of South Africa which are much younger (200 Ma to 110 Ma) in age. 相似文献
66.
Jibamitra Ganguly 《Earth and Planetary Science Letters》1973,19(2):145-153
Theoretical considerations suggest that the activity coefficient of jadeite in natural multicomponent omphacite pyroxene can be approximated by an expression of the form RTlnγjd ? W12 (XDi + XHed) (I ? XJd), where W12 is an adjustable interchange parameter in the jadeite-diopside join, and X stands for the mole fraction of the specified component in the pyroxene solid solution. Synthesis of the available experimental and observational data yields W12 ? ? 1400 (± 1400) + 1.16 (± 1.16) cal/mole, suggesting a nearly ideal solution around 1100°C, and negative departure from ideality at low temperature of the jadeite-diopside solid solution. 相似文献
67.
Shawn G. Benner Matthew L. Polizzotto Benjamin D. Kocar Somenath Ganguly Kongkea Phan Kagna Ouch Michael Sampson Scott Fendorf 《Applied Geochemistry》2008
To advance understanding of hydrological influences on As concentrations within groundwaters of Southeast Asia, the flow system of an As-rich aquifer on the Mekong Delta in Cambodia where flow patterns have not been disturbed by irrigation well pumping was examined. Monitoring of water levels in a network of installed wells, extending over a 50 km2 area, indicates that groundwater flow is dominated by seasonally-variable gradients developed between the river and the inland wetland basins. While the gradient inverts annually, net groundwater flow is from the wetlands to the river. Hydraulic parameters of the aquifer (K ≈ 10−4 ms−1) and overlying clay aquitard (K ≈ 10−8 ms−1) were determined using grain size, permeameter and slug test analyses; when coupled with observed gradients, they indicate a net groundwater flow velocity of 0.04–0.4 ma−1 downward through the clay and 1–13 ma−1 horizontally within the sand aquifer, producing aquifer residence times on the order 100–1000 a. The results of numerical modeling support this conceptual model of the flow system and, when integrated with observed spatial trends in dissolved As concentrations, reveal that the shallow sediments (upper 2–10 m of fine-grained material) are an important source of As to the underlying aquifer. 相似文献
68.
We study the linear stability of nondissipative flow of an electrically conducting fluid subject to non-axisymmetric disturbances in the following cases: (i) the radial flow of an incompressible fluid between two concentric porous circular cylinders in the presence of a radial magnetic field and (ii) axial flow of a compressible fluid between two concentric circular cylinders permeated by a helical magnetic field (0,B
0(r),B
0z) in a cylindrical coordinate system. It is shown that in case (i), the flow is stable if the Alfvén velocity based on the undisturbed radial magnetic field exceeds the radial velocity due to suction or injection at the cylinder surfaces. In case (ii), it is found that under certain conditions the complex wave speed for an unstable mode lies within a circle of diameterW
max-W
min, whereW
max andW
min are the maximum and minimum values of the axial velocity in the flow region. In the presence of a purely axial magnetic field, however, the complex wave speed for an unstable mode always lies within the above circle. 相似文献
69.
70.
The granite-greenstone terranes nested in Archean cratonic nuclei of continents over the world are composed of variably metamorphosed igneous and sedimentary remnants of ancient ocean basins.These rocks preserve distinct geological and geochemical imprints of mantle evolution and differentiation of primordial crust,varying 相似文献