Evidence for melting of subducting carbonate-rich sediments in the western Aegean Arc

Abstract

Subduction of sediments affects the composition of the Earth's mantle and the formation of melts in subduction zones. The sediment thickness of ~8 km along the Hellenic Trench is the largest observed globally and significant volumes of these sediments are subducted. Magmas erupting in the Aegean Arc reflect sediment subduction but they also assimilated sediments during ascent, making distinction of the two processes difficult. In this study, we present geochemical data from olivines in lavas and enclaves from the peninsula of Methana in the western Aegean Arc. The olivines have high forsterite (Fo) contents of 90–92% and contain Cr-spinel with Cr# >60 originating from a primitive melt in equilibrium with a highly depleted mantle wedge. The high δ18OVSMOW of ~6.5‰ of the olivines imply input of up to 6.5% sediment component into the mantle wedge. Incompatible element and radiogenic isotope ratios suggest that the sediment component is added as a carbonate-rich melt. Low Ni contents and small rounded sulphide inclusions in the Fo-rich olivines indicate the formation of an immiscible sulphide liquid during olivine crystallization. The input of carbonate-rich sediment reduced the mantle and the co-existing primitive melts, and possibly added S leading to sulphide saturation. This study shows that olivine crystals and their inclusions provide important insights into the mantle wedge, sediment subduction and its effects on the primary magma composition and can indeed be used to distinguish between a mantle and crustal origin of the sediment signature.