Oxygen isotope evidence for slab-derived fluids in the sub-arc mantle

Abstract

The subduction of oceanic lithosphere is thought to enrich the mantle in elements concentrated in altered oceanic crust and its sedimentary cover (for example, H_2O, CO_2 and alkalis). This enrichment is generally inferred from the geochemistry of island-arc lavas. More direct evidence—such as samples from the mantle with a clear crustal origin—is rare. Inclusions of silicate glass within mantle-derived minerals can have major- and trace-element compositions unlike basalt, and sometimes contain 'enriched' isotopic compositions of Sr, Nd and Pb, suggesting that the inclusions are partial melts of subducted oceanic crust or sediments. Alternatively, some of these alkali-rich inclusions may have been produced by melting peridotites to low degrees (possibly in the presence of volatiles). Here we present oxygen isotope data from silicate glass inclusions obtained from mantle olivine samples in an island-arc setting. These data provide direct evidence that the inclusions are derived from a source rich in material from the subducted oceanic crust and therefore that slab-derived fluids have infiltrated the sub-arc mantle wedge.