Mantle melting beneath the Southwest Indian Ridge: signals from clinopyroxene in abyssal peridotites

Abstract

The mineral chemistry and texture of clinopyroxenes in peridotite from the Kingkong tectonic zone of the Southwest Indian Ridge segment in an effort to constrain mantle melting beneath this slow-spreading ridge are reported. There are three types of clinopyroxenes in the abyssal peridotites: coarse-grained, intergranular and exsolved. The compositional variations among these three types suggest that the coarse-grained clinopyroxene is a mantle-derived source. The Al, Na and Ti contents and the Na/Ti ratio of the coarsegrained clinopyroxene may be used to monitor the degree of partial melting, combined with the contradistinction with Spinel Cr#, which is calculated to be between 7.9% and 14.9%, and may represent low degrees of melting in the global ocean ridge system. The along-axis compositional variations in the coarse-grained clinopyroxene suggest that the degree of partial melting is primarily controlled by the transform faults on both sides of the ridge. Nonetheless, the northwestern side of the ridge may be affected by a hypothesised detachment fault as documented by the calculated P-T conditions. Simultaneously high Na and low Ti contents in the coarse-grained clinopyroxene points to mantle heterogeneities along the ridge axis.