Recycling of subduction-modified refractory mantle beneath the Marion Rise, Southwest Indian Ridge

Abstract

The Marion Rise, located in the central portion of the Southwest Indian Ridge (SWIR), marks a relief high but is overall covered with a thin crust, and thus is inferred to be supported by depleted buoyant mantle. However, direct evidence of the regional mantle compositions from abyssal peridotites are still rare for such a hypothesis. This study presents whole rock and mineral compositions of 34 abyssal peridotites dredged from 7 sites between the Discovery and Indomed fracture zones on the Marion Rise. The samples are divided into low-Cr# (Cr# = 0.23–0.33) and high-Cr# (Cr# = 0.40–0.57) groups. The high-Cr# group samples display highly refractory characteristics (whole rock Al₂O₃ contents down to 0.52 wt%), which are reinforced by the depleted pyroxene compositions that indicate partial melting of up to > 18%. Nonetheless, the overall high extents of melting indicated by the peridotites are inconsistent with the regional thin crust, hence require an inherited origin of the melting signatures. Moreover, the Ti and Yb (Y) concentrations of clinopyroxenes (orthopyroxenes) in the high-Cr# group are too depleted to be residues of anhydrous melting at mid-ocean ridges, but instead suggest for a hydrous melting history near subduction zones. Collectively, we fill in a piece of the puzzle of mantle heterogeneity beneath the SWIR, by providing solid evidence for the existence of a highly refractory mantle beneath the Marion Rise. These mantle components carry subduction-modified characteristics, and very likely have a recycled mantle wedge origin.