Oceanic mantle beneath ultraslow spreading ridges metasomatized by variably evolved melts

As the melting residue of the asthenosphere at ocean ridges, the oceanic mantle lithosphere is initially depleted, which might be re-enriched by various types of melts. However, the metasomatic enrichments in the sub-ridge mantle lithosphere are generally assumed to be limited, because these melts have low abundances of incompatible elements. In this study, we report a case of extreme metasomatic enrichment in abyssal peridotites from the Atlantis Bank oceanic core complex on the Southwest Indian Ridge. We identify two types of metasomatized peridotites (Type 1 and 2) and a clinopyroxenite, which show different degrees of interaction with evolved melts. Type 1 peridotite exhibits strong incompatible element zonation and negative Eu anomalies in clinopyroxene, indicating a short-term percolation of a modestly evolved mid-ocean ridge basalt (MORB). Type 2 peridotite shows textural and trace element features consistent with the impregnation of a highly evolved, oxide-saturated MORB magma. The clinopyroxenite shows remarkable enrichments in iron and incompatible trace elements and negative Eu anomalies in clinopyroxene and contains abundant zircon and apatite, which imply reaction with an extremely evolved Fe–Zr–P-rich melt. We attribute such extreme mantle metasomatic enrichments to the lateral infiltration of variably evolved melts from a local magmatic center, which is now exposed as the Atlantis Bank gabbroic massif. As slow- and ultraslow-spreading ridges exhibit similarly discontinuous lower crust, the intervening shallow mantle between local magmatic centers may have been pervasively metasomatized by the evolved melts, leading the way for substantial mantle enrichment.