Articles written in Journal of Earth System Science

    • Hydrothermal alteration studies of gabbros from Northern Central Indian Ridge and their geodynamic implications

      Dwijesh Ray Catherine Mevel Ranadip Banerjee

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      Mylonitic gabbro and altered gabbro were recovered from off-axis high and corner high locations at ridge-transform intersection, adjacent to Vityaz transform fault of the slow spreading (32–35mm/yr, full spreading) Northern Central Indian Ridge. Both the varieties show signatures of extensive alteration caused due to interaction with sea water. Mylonitic gabbro represents high temperature metamorphism (∼700–800° C) and comprised of hornblende mineral which exhibits well defined foliation/gneissic appearance along with dynamically recrystallised plagioclase grains frequently intercalated with magnetite-ilmenite. Altered gabbro from corner high generally includes low temperature greenschist grade (∼300° C) mineralogical assemblages: chlorite, albite, quartz and locally magnesio hornblende. Crystal plastic deformation resulted in mylonite formation and often porphyroclasts of plagioclase and clinopyroxene grains, while altered gabbro locally exhibits cataclastic texture. Presence of Vityaz transform fault and adjacent megamullion at the weakly magmatic ridge-transform intersection and off-axis high locations prompted the present scenario very much conducive for hydrothermal circulation and further facilitate the exhumation of present suite of gabbro.

    • Change of lithofacies in marine sediment core from Quaternary to Pre-Quaternary: A case study from the Central Indian Ocean Basin


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      A distinct change in lithofacies was observed from red clay to siliceous ooze in a core from Indian Ocean. Radiolarian index species were used to ascertain the Quaternary datum levels and an age of 2.0 Ma is determined up to 125 cm depth from surface. Below 185 cm depth, the sediment core lacked radiolariantest completely. At about 50 cm from core top, the sedimentation rate decreased drastically from 0.18 to 0.05 cm/kyr with a corresponding age of 500 kyr, marking Marine Isotope Stage 13. The changes in lithofacies and sedimentation rate are further accompanied by the change in clay mineralogy from smectite rich older sediments to smectite depleted younger sediments and presence of higher amount of volcanogenic materials in the older sediments. Higher values of chemical index of alteration (CIA) and lower $\rm{K_{2}O/Al_{2}O_{3}}$ in older sediments indicate dominance of chemical weathering in the older sediments which decreased gradually towards core top. Ti normalized concentrations of elements like $\rm{Co}$, $\rm{V}$, $\rm{Cu}$, $\rm{Ni}$, $\rm{Ce}$, $\rm{Y}$ and $\Sigma\rm{REEs}$ exhibit lowest values at the transition zone above which the ratios increased steadily towards the core top. All these observations indicate a marked change in the climatic regime from late Quaternary onwards, prior to which the environment was less bio-productive.

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