Insights into the nature of plume-ridge interaction and outflux of H2O from the Galápagos Spreading Centre

The flow of high-temperature and compositionally-enriched material between mantle plumes and nearby spreading centres influences up to 30% of the global mid-ocean ridge system and represents a significant, but currently unconstrained, flux of volatiles out of the mantle. Here we present new analyses of H2O, F, Cl and S in basaltic glass chips from an archetypal region of plume-ridge interaction, the Galapagos Spreading Centre (GSC). Our dataset includes samples from the eastern GSC, on ridge segments that are strongly influenced by the adjacent Galapagos mantle plume, and complements published analyses of volatiles largely from the western GSC. We use forward models of mantle melting to investigate the role of solid and melt-phase transport from a lithologically heterogeneous (peridotite-pyroxenite) mantle in plume-ridge interaction along approximately 1000 km of the GSC. Our results indicate that the observed geochemical and geophysical variations cannot be recreated by models which only involve solid-state transfer of material between the Galapagos mantle plume and the GSC. Instead, we show that the geochemical and geophysical data from the GSC are well-matched by models that incorporate channelised flow of volatile-rich melts formed at high-pressures (>3 GPa) in the Galapagos plume stem to the GSC. In addition, our new models demonstrate that channelised flow of enriched, plume-derived melt can account for up to ~60% of the H2O outgassed from regions of the GSC which are most strongly influenced by the Galapagos mantle plume.