OXYGEN-ISOTOPE EVIDENCE FOR SLAB-DERIVED FLUIDS IN THE SUB-ARC MANTLE

The subduction of oceanic lithosphere is thought to enrich the mantle in elements concentrated in altered oceanic crust and its sedimentary cover (for example, H2O, CO2 and alkalis)(1,2). This enrichment is gen erally inferred from the geochemistry of island-arc lavas(3). More dir ect evidence-such as samples from the mantle with a dear crustal origi n(4)-is rare. Inclusions of silicate glass within mantle-derived miner als can have major- and trace-element compositions unlike basalt(5,6) and sometimes contain 'enriched' isotopic compositions of Sr, Nd and P b, suggesting that the inclusions are partial melts of subducted ocean ic crust or sediments(6). Alternatively, some of these alkali-rich inc lusions may have been produced by melting peridotites to low degrees ( possibly in the presence of volatiles)(7). Here we present oxygen isot ope data from silicate glass inclusions obtained from mantle olivine s amples in an island-are setting. These data provide direct evidence th at the inclusions are derived from a source rich in material from the subducted oceanic crust and therefore that slab-derived fluids have in filtrated the sub-are mantle wedge.