• Alok K Gupta

      Articles written in Journal of Earth System Science

    • Preliminary phase equilibria of the nepheline-diopside system under variable pressures (up to 28 kb) and temperatures

      Puniti Singh Rajeev Chopra Alok K Gupta

      More Details Abstract Fulltext PDF

      The nepheline-diopside join defines the ultra-alkaline portion of the basalt tetrahedron and the bulk composition of nephelinitic rocks lie in this join. Schairer and others established that under atmospheric pressure, the join cuts through the primary phase volumes of oliviness, carnegieitess and nepheliness. Melilite coexists with nepheliness, oliviness and diopsidess below 1160±10°C and olivine reacts out at low temperature.

      Experimental studies on seven compositions show the presence of a pseudoeutectic at Ne70Di30 and 1420°C, where diopsidess, nepheliness and liquid are in equilibrium. Olivine and melilite do not appear in the system and the assemblage below 1225±20°C is diopsidess+nepheliness.

      Four compositions studied at 1000°C show the appearance of the assemblage diopsidess+nepheliness+melilite at 15kb, whereas diopsidess and nepheliness are the stable phases at 20 and 25kb. The appearance of melilite is therefore restricted to a pressure of 18±3kb. Diopsidess and nepheliness coexist without olivine and melilite in albite-nepheline-diopside and sanidine-nepheline-diopside system. However, this study shows that feldspar-free nephelinitic rocks, which are devoid of melilite, may have crystallized under mantle conditions, whereas their melilite-bearing counterparts equilibrated at a shallower depth within the crust.

    • High pressure-temperature studies on an olivine tholeiite and a tholeiitic picrite from the pavagarh region, Gujarat, India

      Snehmani Agrawal M M Bindal Alok K Gupta

      More Details Abstract Fulltext PDF

      Experimental studies have been performed on an olivine tholeiite and tholeiitic picrite at pressure and temperature ranges of 20–40 kb and 1200–1300°C. The lower and upper limits of basalt-eclogite transition zone for tholeiitic picrite are 23 kb and 31·67 kb at 1200°C, and 24·67 kb and 33·67 kb at 1300°C, whereas for olivine tholeiite, these are 27 kb and 32·33 kb at 1200°C, and 28·70 kb and 33·70 kb at 1300°C. While the assemblages for both samples below the transition region are Pl+Px+Mt, they are Pl+Gt+Px+Mt within it. The eclogite field has Gt+Px+Mt. The ratio of garnet to plagioclase increases from the transition zone to the eclogite field and with the disappearance of plagioclase, the percentage of garnet increases to 30 in the eclogite field.

      Comparison of our results with previous studies on basalt-eclogite transition shows that the transition zone found by us occurs at higher pressure-temperature conditions. Seismic studies of the region below the Deccan Traps show an increase in velocity (1–4%) at depth. It is suggested that after partial melting, during ascent of the basaltic liquid, a significant portion of it crystallizes within the upper mantle as pockets of eclogite. As eclogite is more dense than peridotite, their presence should cause a similar increase in the seismic velocity below the Deccan area.

  • Journal of Earth System Science | News

    • Editorial Note on Continuous Article Publication

      Posted on July 25, 2019

      Click here for Editorial Note on CAP Mode

© 2017-2019 Indian Academy of Sciences, Bengaluru.