Variations in Melt Productivity and Melting Conditions along SWIR (70°E–49°E): Evidence from Olivine-hosted and Plagioclase-hosted Melt Inclusions

Abstract

Melt inclusion and host glass compositions from the eastern end of the Southwest Indian Ridge show a progressive depletion in light rare earth elements (LREE), Na₈ and (La/Sm)_n, but an increase in Fe₈, from the NE (64°E) towards the SW (49°E). These changes indicate an increase in the degree of mantle melting towards the SW and correlate with a shallowing of the ridge axial depth and increase in crustal thickness. In addition, LREE enrichment in both melt inclusions and host glasses from the NE end of the ridge are compatible with re-fertilization of a depleted mantle source. The large compositional variations (e.g. P₂O₅ and K₂O) of the melt inclusions from the NE end of the ridge (64°E), coupled with low Fe₈ values, suggest that melts from the NE correspond to a variety of different batches of melts generated at shallow levels in the mantle melting column. In contrast, the progressively more depleted compositions and higher Fe₈ values of the olivine- and plagioclase-hosted melt inclusions at the SW end of the studied region (49°E), suggest that these melt inclusions represent batches of melt generated by higher degrees of melting at greater mean depths in the mantle melting column. Systematic differences in Fe₈ values between the plagioclase- and the olivine-hosted melt inclusions in the SW end (49°E) of the studied ridge area, suggest that the plagioclase-hosted melt inclusions represent final batches of melt generated at the top of the mantle melting column, whereas the olivine-hosted melt inclusions correspond to melts generated from less depleted, more fertile mantle at greater depths.