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Fluid-absent Melting of High-grade Semi-pelites: P-T Constraints on Orthopyroxene Formation and Implications for Granulite Genesis 总被引:4,自引:0,他引:4
Rocks of semi-pelitic composition are common in high-grade terranes.The first appearance of orthopyroxene in these rocks marks thetransition from amphibolite- to granulite-facies conditions,and is commonly attributed to the process of fluid-absent partialmelting. We have conducted fluid-absent melting experimentson two natural semi-pelitic rocks (quartz, plagioclase, alkalifeldspar, biotite and garnet) with the specific objective ofdetermining the pressuretemperature conditions necessaryto produce orthopyroxene. In contrast to previous experimentalstudies, our starting materials were obtained from a transitionalamphibolitegranulite terrane. Importantly, the high TiO2(>5 wt %) and F (>1 wt %) contents of biotite in our experimentsare more representative of biotite found in rocks on the vergeof granulite-facies conditions than those used in earlier studies.Experiments were conducted in a piston-cylinder apparatus at8001050°C and 715 kbar. We reversed the firstappearance of orthopyroxene in two-stage experiments at 7 and10 kbar. Fluid-absent melting of biotite began at 相似文献
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Spatial and temporal variation of uranium in a shallow weathered rock aquifer in southern India 总被引:2,自引:0,他引:2
Uranium occurs naturally in groundwater and surface water. The objective of this study is to understand the causes for the
occurrence of uranium and its spatio-temporal variation in groundwater in a part of Nalgonda district, Andhra Pradesh, south
India. Uranium deposits occur in the southeastern part of this area. Groundwater samples were collected from 44 wells every
two months from March 2008 to January 2009. The samples were analyzed for pH, ORP and uranium concentration. The uranium concentration
in groundwater varies from 0.2 ppb to a maximum of 68 ppb with a mean of 18.5 ppb. About 21.6% of the samples were above the
drinking water limit of 30 ppb set by USEPA. The uranium concentration varied with fluctuation in groundwater level, pH and
ORP. Uranium concentration in groundwater changes depending on lithology, degree of weathering and rainfall recharge. 相似文献
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B AJAYKUMAR K SREEDHARAN MAHESH MOHAN JOBY PAUL A P THOMAS P K K NAIR 《Journal of Earth System Science》2012,121(4):1093-1103
The climatic variability and the influence of temperature and sea level fluctuations on the earth??s surface configuration during the Holocene are being discussed all over the world. The present study evaluates the palaeo-environmental conditions of western coast of India during this epoch through the analysis of pollen grains embedded in a carbonaceous clay formation (? 0.4?C0.6?m) found sandwiched within the palaeodeposit of sand of Meenachil River basin. The carbon dating revealed that the clay formation has an age of 5786 ± 94 14C yr BP, while the embedded wood samples have the age varying in between 2888 ± 78 and 5780 ± 95 14C yr BP. The overall analysis suggests that the southwestern margin of India had experienced high intensity rainfall during the earlier part of the Atlantic chronozone due to then strengthened Asiatic monsoon, while water stagnation and hydrological modifications were observed during the later part. The dominance of weeds and lesser number of tree elements suggested a drier climate during the end of the Atlantic period. Besides, the morphometric rearrangement of the Meenachil River contemporaneous to the geomorphological modifications of the southwestern coast of India shall be classified into three categories: (1) Pre-Vembanad Lake formation, (2) Contemporaneous to lake formation and (3) Post-Lake formation. 相似文献
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R T RATHEESH KUMAR TANMAY K MAJI SURESH CH KANDPAL D SENGUPTA RAJESH R NAIR 《Journal of Earth System Science》2011,120(3):447-458
Global estimates of the elastic thickness (Te) of the structure of passive continental margins show wide and varying results
owing to the use of different methodologies. Earlier estimates of the elastic thickness of the North Atlantic passive continental
margins that used flexural modelling yielded a Te value of ~20–100 km. Here, we compare these estimates with the Te value
obtained using orthonormalized Hermite multitaper recovered isostatic coherence functions. We discuss how Te is correlated
with heat flow distribution and depth of necking. The E–W segment in the southern study region comprising Nova Scotia and
the Southern Grand Banks show low Te values, while the zones comprising the NE–SW zones, viz., Western Greenland, Labrador,
Orphan Basin and the Northern Grand Bank show comparatively high Te values. As expected, Te broadly reflects the depth of
the 200–400°C isotherm below the weak surface sediment layer at the time of loading, and at the margins most of the loading
occurred during rifting. We infer that these low Te measurements indicate Te frozen into the lithosphere. This could be due
to the passive nature of the margin when the loads were emplaced during the continental break-up process at high temperature
gradients. 相似文献
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Variability of Summer Monsoon Rainfall in India on Inter-Annual and Decadal Time Scales 总被引:1,自引:0,他引:1 下载免费PDF全文
Porathur Vareed JOSEPH Bindu GOKULAPALAN Archana NAIR Shinu Sheela WILSON 《大气和海洋科学快报》2013,6(5):398-403
Indian Summer Monsoon Rainfall(ISMR)exhibits a prominent inter-annual variability known as troposphere biennial oscillation.A season of deficient June to September monsoon rainfall in India is followed by warm sea surface temperature(SST)anomalies over the tropical Indian Ocean and cold SST anomalies over the western Pacific Ocean.These anomalies persist until the following monsoon,which yields normal or excessive rainfall.Monsoon rainfall in India has shown decadal variability in the form of 30 year epochs of alternately occurring frequent and infrequent drought monsoons since1841,when rainfall measurements began in India.Decadal oscillations of monsoon rainfall and the well known decadal oscillations in SSTs of the Atlantic and Pacific oceans have the same period of approximately 60 years and nearly the same temporal phase.In both of these variabilities,anomalies in monsoon heat source,such as deep convection,and middle latitude westerlies of the upper troposphere over south Asia have prominent roles. 相似文献
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