Geochemical and Pb and Nd Isotopic Characteristics of the Tethyan Asthenosphere: Implications for the Origin of the Indian MORB-type Mantle

It is unclear why the Pb, Nd, and Sr isotopic composition of the modern mid-ocean ridge basalts (MORB) from the Indian Ocean is different from that of the North Atlantic and Pacific Oceans. A possible explanation for this is that the Indian MORB-type isotopic signature is a long-lived regional feature of the mantle, as evidently shown by the isotopic composition of the ~350 Ma MORB-like crust of the Tethys Ocean, which existed in the same region presently occupied by the Indian Ocean (Xu et al., Earth Planet. Sci. Lett. 198, 2002). However, this hypothesis is in conflict with the lack of Indian MORB-type isotopic signature in a number of ~150 Ma Tethyan and Indian Ocean crusts (Mahoney et al., J. Petrol. 39, 1998; Weis and Frey, J. Geophys. Res. 101, 1996). To further constrain the origin of the Indian MORB-type isotopic signature, we analyze the chemical and Pb, Nd, and Sr isotopic composition of representative mafic rocks from four Tethyan ophiolites ranging in age from 90 to 360 Ma. The Sr isotopic composition of the samples is unreliable due to alteration, but the alteration resistant trace element and age-corrected Nd and Pb isotopic results indicate that these Tethyan rocks were derived from a geochemically depleted asthenospheric source that had a clear Indian MORB-type isotopic signature. We therefore conclude that the bulk of the Indian suboceanic mantle was most probably inherited from the Tethyan asthenosphere. A few regions in both the Tethyan and Indian Oceans, however, are most probably underlain by North Atlantic and Pacific MORB-type mantle (and vice-versa) because of the flow of the asthenosphere in response tectonic plate reorganizations that lead to openings and closings of ocean basins (e.g., Flower et al., Tectonophys. 333, 2001).