Heterogeneous Fossil Réunion Plume Component in the Source Region of Enriched MORB Along the Central Indian Ridge Between 12° and 17°S

Abstract

Mid-ocean ridge basalts (MORB) from the Central Indian Ridge (CIR) between 12° and 17°S show a wide range of geochemical and isotopic variations. Particularly, MORB from a segment between 14° and 15°S are more enriched in incompatible trace elements with more radiogenic Sr and Pb isotope and unradiogenic Nd isotope values than the lavas between 15° and 16°S with geochemical features of normal MORB. However, the causes for the enrichment between 14° and 15°S are poorly constrained. In this study, we re-examined the CIR MORB from 12° to 17°S with new geochemical data obtained based on high spatial resolution sampling to better understand the nature of the enriched mantle source. Our new geochemical data show that the MORB between 14° and 15°S, with maximum values for (La/Sm)N = 1.95, 87Sr/86Sr = 0.703526 and 206Pb/204Pb = 18.7558, are more enriched than those from the southern segments (16° to 20°S) known to be influenced by the Réunion mantle component. The new trace element and isotopic compositions of MORB suggest that three mantle end-members are required to explain the geochemical variations shown between 14° and 16°S: the depleted Indian-type MORB mantle, Réunion Plume (RP), and Seychelles/Madagascar-like continental crust components. Moreover, our mixing model suggests that the differences in enriched MORB signature from 14° to 20°S are due to variable proportions of continental material previously mixed with the RP. Our study implies that a continental component interacted with the plume into the asthenosphere, possibly beneath Madagascar or below Mauritius Island and the Mascarene plateau.