Geochemical stratification in the upper mantle: evidence from Leg 115 basalts in the Indian Ocean

Abstract

Melting-phase relations at high pressures and Sr-Nd isotopic compositions are reported for the basalts. Based on the concentrations of high-field-strength elements, the basalts have been subdivided into eight groups. A tholeiitic primary magma estimated using an olivine maximum fractionation model is representative of depleted lavas. An alkaline primary magma is also present. Results imply that the tholeiitic magmas were segregated from mantle diapirs at shallower levels than the alkaline magmas. The highest 143Nd/144Nd value is obtained for the most depleted tholeiitic basalts, and the lowest value corresponds to the enriched alkaline basalt. The Sr isotopes of the basalts range from 0.70378 to 0.70449 and are inversely correlated with the Nd isotopic values. The present experimental and geochemical data suggest that depleted mantle material is underlain by the enriched material in the upper mantle beneath the region. -from Authors