Isotopic and geochemical provinces of the western Indian Ocean Spreading Centers

Abstract

Basalt glasses from the Central Indian Ridge are distinct isotopically from mid‐ocean ridge basalts (MORB) of the Indian Ocean triple junction and western few hundred kilometers of the Southeast Indian Ridge. In particular, very low 206Pb/204Pb and high 87Sr/86Sr signatures, which characterize the latter region, are absent over most of the Central Indian Ridge. In turn, lavas from the unusually deep eastern 1100–1500 km of the Southwest Indian Ridge are different chemically and isotopically from those of the above areas. A rather abrupt eastern boundary to Southwest Indian Ridge‐type compositions occurs at or very near the geographic triple junction. This provinciality in western Indian Ocean ridge basalts partly mirrors fundamental regional differences in the underlying mantle but, at least between the eastern Southwest Indian Ridge and the western Southeast Indian Ridge and triple junction, also may reflect variations in extent and depth of melting in a vertically zoned upper mantle. A pronounced low εNd, high 206Pb/204Pb, high 87Sr/86Sr anomaly exists on the Central Indian Ridge at the Marie Celeste Fracture Zone and on the adjacent ridge segment to the south. Despite the great distance (>1100 km) of Réunion Island from the ridge, this zone appears to demark a region of mantle containing substantial Réunion hotspotlike material. Several old (35–60 m.y.) Deep Sea Drilling Project basalts which erupted on the ancestral Central Indian Ridge also record a significant Réunion hotspotlike influence, whereas a 46‐m.y.‐old sample that formed farther from the presumed locus of the hotspot possesses isotopic values identical to many present (non‐Marie Celeste area) Central Indian Ridge MORB. The variably expressed and/or heterogeneous low 206Pb/204Pb material partly responsible for the isotopic distinctiveness of Indian Ocean ridge basalts may have entered into the Indian MORB mantle as a result of continental lithospheric remobilization preceding the breakup of Gondwana, particularly from the portion that would eventually become Greater India.