• S S Sawant

      Articles written in Journal of Earth System Science

    • 𝑛-Alkanes in surficial sediments of Visakhapatnam harbour, east coast of India

      V R Punyu R R Harji N B Bhosle S S Sawant K Venkat

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      Surface sediments collected from 19 stations along Visakhapatnam harbour were analysed for organic carbon (OC), 𝛿13CoC, total lipids (TL), total hydrocarbon (THC), 𝑛-alkane concentration and composition. OC, 𝛿13CoC, TL and THC ranged from 0.6% to 7.6%, -29.3 to -23.8‰, 300 to 14,948 𝜇 g g−1 dw, and 0.2 to 2,277 𝜇 g g−1 dw, respectively. Predominance of even carbon numbers 𝑛-alkanes C12–C21 with carbon preference index (CPI) of < 1 suggests major microbial influence. Fair abundance of odd carbon number 𝑛-alkanes in the range of C15–C22 and C23–C33 indicates some input from phytoplankton and terrestrial sources, respectively. Petrogenic input was evident from the presence of hopanes and steranes. The data suggest that organic matter (OM) sources varied spatially and were mostly derived from mixed source.

    • Geochemistry of PGE in mafic rocks of east Khasi Hills, Shillong Plateau, NE India

      Sampa Hazra Jyotisankar Ray C Manikyamba Abhishek Saha S S Sawant

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      The mafic rocks of east Khasi Hills of the Meghalaya Plateau, northeastern India, occur as an intrusive body which cut across the weakly metamorphosed Shillong Group of rocks. Other than Shillong Group of rocks, high grade Archaean gneissic rocks and younger porphyritic granites are also observed in the study area. The studied mafic rocks of east Khasi Hills cover an area of about 4 km2 and represent structurally controlled intrusion and varying grades of deformation. Structurally, these mafic rocks can be divided into massive type of mafic rocks, which are more or less deformation free and foliated type of mafic rocks that experienced deformation. Petrographically, this massive type can be classified as leuco-hornblende-gabbro whereas foliated type can be designated as amphibolite. On the basis of major oxide geochemistry, the investigated mafic rocks can be discriminated into high titanium (HT) (TiO2 > 2 wt%) and low titanium (LT) types (TiO2 < 2 wt%). Use of several geochemical variation diagrams, consideration of chondrite-normalized and mantle-normalized REE and PGE plots suggest role of magmatic differentiation (with almost no role of plagioclase fractionation) in a subduction controlled tectonic environment. The PGE trends of the studied rocks suggest relative enrichment of palladium group of PGE (PPGE) compared to iridium group PGE (IPGE). Critical consideration of Sm vs. La, Cu vs. La, Pd vs. La and Cu/Pd vs. La/Sm plots strongly favours generation of the parent magma at a columnar melting regime with batch melting of cylindrical column of the parent mantle to the tune of $\sim$25%. The characteristic PGE behaviours of the presently investigated mafic rocks of east Khasi Hills can be typically corroborated as ‘orogenic’ (discordant) type. These rocks have an enriched mantle affinity with a co-magmatic lineage and they have been generated by slab-dehydration, wedge-melting and assimilation fractional crystallization process at a continental margin arc setting.

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