Parthasarathy G.,National Geophysical Research Institute
American Mineralogist | Year: 2011
Electrical resistivity of synthetic nanocrystalline (30-40 nm crystallite size) and a crystalline natural sample of geikielite MgTiO3 has been measured at simultaneous high pressure and high temperature up to 6 GPa and 800 K, respectively. The temperature dependence of the electrical resistivity of both the synthetic and natural sample obeys the Arrehenius behavior in the temperature range between 3 and 800 K and pressure range up to 6.0 GPa. The activation volume of the electrical conduction for coarse crystalline natural sample of geikielite is almost twice that of the synthetic nanocrystalline geikielite indicating the increase of activation volume with the crystallite size. The activation energy for the electronic conduction decreases from 0.39 eV at room pressure to 0.25 eV at 6.0 GPa for natural geikielite, and 0.68 to 0.225 eV in the same pressure range for synthetic geikielite. The pressure dependence of the activation energy of geikielite sample is found to obey the following expressions ΔE (eV) = 0.39 - 0.026(1) P + 0.0036 P2 for natural sample, and ΔE (eV) = 0.68 - 0.080 (2) P + 0.0007 P 2 for synthetic sample, where P is pressure in GPa. We observe a crossover from extended state type conduction to hopping conduction at 4.0 GPa and 350 K for nano-crystalline geikielite. However, there is no such change of conduction mechanism observed for the natural geikielite at high pressures and high temperatures. The present study reveals the phase stability of nano-crystalline geikielite and natural geikielite up to mantle pressure and temperature conditions, viz. 6 GPa and 800 K, and no phase transition or decomposition is observed in the sample.
Behera L.,National Geophysical Research Institute
Earth and Planetary Science Letters | Year: 2011
The crustal structure toward southern part of SGT is poorly defined leaving an opportunity to understand the tectonic and geodynamic evolution of this high-grade granulite terrain surrounded by major shear and tectonically disturbed zones like Achankovil Shear Zone (AKSZ) and Palghat Cauvery Shear Zone (PCSZ). To develop a geologically plausible crustal tectonic model depicting major structural elements, a comprehensive tomographic image was derived using deep-seismic-sounding data corroborated by Bouguer gravity modeling, coincident-reflection-seismic, heat-flow and available geological/geochronological informations along the N-S trending Vattalkundu-Kanyakumari geotransect. The final tectonic model represents large compositional changes of subsurface rocks accompanied by velocity heterogeneities with crustal thinning (44-36. km) and Moho upwarping from north to south. This study also reveals and successfully imaged anomalous zone of exhumation near AKSZ having transpression of exhumed rocks at mid-to-lower crustal level (20-30. km) with significant underplating and mantle upwelling forming a complex metamorphic province. The presence of shear zones with high-grade charnockite massifs in the upper-crust exposed in several places reveal large scale exhumation of granulites during the Pan-African rifting (~. 550. Ma) and provide important insights of plume-continental lithosphere interaction with reconstruction of the Gondwanaland. © 2011 Elsevier B.V.
Roy S.,National Geophysical Research Institute |
Mareschal J.-C.,University of Quebec
Journal of Geophysical Research: Solid Earth | Year: 2011
We have used constraints from seismic shear wave vertical velocity profiles, geothermobarometry estimates on mantle xenoliths, and surface heat flux and heat production measurements to analyze the thermal regime of the deep lithosphere beneath India. In the Dharwar craton of southern India, the shear wave velocity gradient in the mantle, as well as xenolith geothermobarometry data, suggests a low mantle heat flux, 14-20 mW m-2, consistent with surface heat flux measurements. However, for standard cratonic mantle composition, seismic velocities require Moho and mantle temperatures to be about 300 K higher than inferred from heat flux and xenolith data. This discrepancy can be only resolved by changing the mantle composition, specifically by increasing the Fe number. The shear wave velocities are highest beneath north central India, where calculated S wave travel times are 2 s shorter than in the Dharwar craton. These differences in traveltime and the very steep gradient in the shear wave velocity profiles in north central India cannot be explained by variations in mantle temperature but require differences in mantle composition. Copyright © 2011 by the American Geophysical Union.
Manikyamba C.,National Geophysical Research Institute |
Kerrich R.,University of Saskatchewan
Precambrian Research | Year: 2011
Well-preserved alkaline basalts, bearing relict aegirine, leucite and nepheline mineralogy, are stratigraphically associated with high-Mg basalts in the Neoarchean Penakacherla greenstone belt, eastern Dharwar craton, India. Alkaline basalts (Mg #∼0.70-0.58) are enriched in alkalies (K2O+Na2O∼7wt.%), and TiO2 (2.3-2.1wt.%), and exhibit fractionated REE patterns with (La/Yb)N ranging from 23 to 29. On primitive mantle normalized diagrams they record a downturn from Ce to Th, small negative Nb anomalies relative to La, and Zr/Hf ratios higher than the primitive mantle value, in common with compositional characteristics of Phanerozoic alkaline ocean island basalts (OIB). Some interelement ratios are intermediate between EM1- and HIMU-OIB. Associated high-Mg basalts (MgO 17.2-9.2wt.%) have comparatively lower TiO2 (1.2-0.50wt.%), and flat HREE patterns with slight depletion in LREE, and small positive Nb anomalies. These basalts are compositionally similar to tholeiitic basalts associated with komatiites in many Neoarchean greenstone terranes. Two samples have the conjunction of Nb/Th<8 and fractionated LREE [(La/Yb)N 2.60-2.63] consistent with crustal contamination. On the global array of Phanerozoic to Recent ocean island basalts, in SiO2 versus Nb/Y coordinates, alkaline basalts plot with counterparts from Aitutaki and Heard, whereas high-Mg basalts plot near Iceland tholeiites. Alkaline basalts plot with OIB, and high-Mg basalts near N-MORB, on the Th/Yb versus Nb/Yb MORB-OIB array of Phanerozoic intraplate basalts; accordingly the mantle components of that array were established in the 2.7Ga mantle asthenosphere. Alkaline basalts are rare in Archean volcanic sequences. This occurrence of alkaline basalts indicates subduction, recycling, and incubation of Mesoarchaean oceanic and continental crust in the mantle, and generation of high-Mg and alkaline basalts from a mantle plume at 2.7. Ga, possibly analogous to counterparts of Iceland. The mantle plume likely erupted at a thin craton margin, given flat HREE of most high-Mg basalts. © 2011 Elsevier B.V.
Shankar U.,National Geophysical Research Institute |
Riedel M.,Geological Survey of Canada
Marine and Petroleum Geology | Year: 2011
During the Indian National Gas Hydrate Program (NGHP) Expedition 01, a series of well logs were acquired at several sites across the Krishna-Godavari (KG) Basin. Electrical resistivity logs were used for gas hydrate saturation estimates using Archie's method. The measured in situ pore-water salinity, seafloor temperature and geothermal gradients were used to determine the baseline pore-water resistivity. In the absence of core data, Arp's law was used to estimate in situ pore-water resistivity. Uncertainties in the Archie's approach are related to the calibration of Archie coefficient (a), cementation factor (m) and saturation exponent (n) values. We also have estimated gas hydrate saturation from sonic P-wave velocity logs considering the gas hydrate in-frame effective medium rock-physics model. Uncertainties in the effective medium modeling stem from the choice of mineral assemblage used in the model. In both methods we assume that gas hydrate forms in sediment pore space. Combined observations from these analyses show that gas hydrate saturations are relatively low (<5% of the pore space) at the sites of the KG Basin. However, several intervals of increased saturations were observed e.g. at Site NGHP-01-03 (S h = 15-20%, in two zones between 168 and 198 mbsf), Site NGHP-01-05 (S h = 35-38% in two discrete zone between 70 and 90 mbsf), and Site NGHP-01-07 shows the gas hydrate saturation more than 25% in two zones between 75 and 155 mbsf. A total of 10 drill sites and associated log data, regional occurrences of bottom-simulating reflectors from 2D and 3D seismic data, and thermal modeling of the gas hydrate stability zone, were used to estimate the total amount of gas hydrate within the KG Basin. Average gas hydrate saturations for the entire gas hydrate stability zone (seafloor to base of gas hydrate stability), sediment porosities, and statistically derived extreme values for these parameters were defined from the logs. The total area considered based on the BSR seismic data covers ~720 km 2. Using the statistical ranges in all parameters involved in the calculation, the total amount of gas from gas hydrate in the KG Basin study area varies from a minimum of ~5.7 trillion-cubic feet (TCF) to ~32.1 TCF. © 2010.