Geochemical studies on oceanic basalts from the Indian Ocean

Abstract

Major and selected trace elements (large ion lithophile-LIL, transitional metal elements-TME, rare earth elements-REE) and Sr isotopic data for relatively less fractionated (Mg×100/Mg+Fe² =58-68) tholeiites from different tectonic environments, such as the spreading Mid-Indian Oceanic Ridge (MIOR) (including Carlsberg Ridge-CR), non-spreading Ninetyeast Ridge and from the Indian Ocean are presented in order to discuss their genesis. MIOR rocks show an inverse relationship of TiO₂ with Al₂O₃/TiO₂ and CaO/TiO₂ ratios suggesting that the degree of melting increases with decrease in TiO₂. Modelling of this data suggests that CR and MIOR basalts are probably generated by 15-25% melting. This modelling is consistent with REE abundances. MIOR basalts are distinctly more radiogenic (⁸⁷Sr/⁸⁶Sr 0.7028-0.7035), and less depleted in LIL and REE with an average (La/Sm)e.f. of ~1.0, characteristic of transitional ridge segments, than CR, which represents normal ridge segments [(La/Sm)_e.f. < 1, ⁸⁷Sr/⁸⁶Sr=0.7027-0.7030]. The non-spreading Ninetyeast Ridge comprise of pyroxene-picrite-basalts and oceanic andesites with a wide range of differentiation. These are characterized by the presence of higher Fe₂O₃+FeO (11%), TiO₂ (2-3%), K₂O (2-3%), LIL, enriched light REE (LREE) patterns, and more radiogenic Sr similar to plume/hot spot derived island basalts. It is suggested that LREE enriched plume type Ninetyeast Ridge patterns reflect the LREE enrichment of their metasomatized source regions. Increase of ⁸⁷Sr/⁸⁶Sr ratios for basalts away from the MIOR is probably due to systematic depletion of the mantle as a result of continuous differentiation of the mantle and favours the possible existence of ordered heterogeneities on a large scale of the order of even hundreds of kilometers.