Geochemistry of basalts from the Indian Ocean triple junction: implications for the generation and evolution of Indian Ocean ridge basalts

Basalts dredged from ridge axes within 70 km of the Indian Ocean triple junction in the western Indian Ocean have many geochemical and petrologic characteristics in common with depleted mid-ocean ridge basalts (MORBs) from the Atlantic and Pacific. For example there is overlap in major and trace element abundances, and in diagnostic ratios such as K/Rb (700–925) and La/Sm (less than chondritic). Also, glass inclusions in calcic plagioclase (An_89–90) provide evidence for a primitive high Mg/Fe, low TiO₂ melt. In contrast, basalts dredged from 250 to 400 km southwest of the triple junction on the Southwest Indian Ridge are compositionally distinct from depleted MORB. They are nepheline-normative or slightly hypersthene normative and have higher alkali metal and incompatible element abundances than depleted MORBs with similar MgO contents.All of these Indian Ocean basalts have Sr, Nd and Pb isotope ratios which corroborate previous studies showing that relative to depleted Atlantic and Pacific MORB, many Indian Ocean MORBs have low²⁰⁶Pb/²⁰⁴Pb and high⁸⁷Sr/⁸⁶Sr. However, individual Indian Ocean ridges have different radiogenic isotope characteristics, and basalts from the vicinity of the triple junction have unusually high⁸⁷Sr/⁸⁶Sr (～ 0.7032) at low²⁰⁶Pb/²⁰⁴Pb ratios (17.3–18.2). Moreover, the shallow axial region of the Central Indian Ridge from ～ 12°S to the triple junction (26°S) has high⁸⁷Sr/⁸⁶Sr (> 0.7030). Apparently, the depleted component of Indian Ocean MORBs has been contaminated by an isotopically unusual component which does not occur in Pacific and Atlantic MORBs, and is not dominant in basalts from many Indian Ocean islands. The degree of this contamination is not uniform in western Indian Ocean MORB; the most contaminated basalts occur from 12°S on the Central Indian Ridge to the triple junction (～ 26°S) and easterly along the Southeast Indian Ridge to ～ 72°E.