Evidence for shallow dehydration of the subducting plate beneath the Mariana forearc: New insights into the water cycle at subduction zones
Abstract:Water is efficiently recycled at subduction zones. It is fluxed from the surface into the mantle by the subducted plate and back to the surface or crust through explosive arc volcanism and degassing. Fluids released from dehydrating the subducting plate are transfer agents of water. Geophysical modeling  and the geochemistry of arc glasses  suggest that at cold-slab subduction zones, such as the Mariana convergent margin, the downgoing plate mostly dehydrates beneath the volcanic arc front (≥ ~ 80 -100 km depth to slab) to trigger volcanism. However, there is a gap in our understanding of the water fluxes released beneath forearcs, as examples of forearc magmatism are extremely rare. Here, we investigate the Southernmost Mariana Forearc Rift (SEMFR), where MORB-like spreading occurred unusually close to the trench, sampling slab-derived aqueous fluids released at ~ 30 to 100 km depth from the subducted plate. Examining the trace element and water contents of olivine-hosted melt inclusions and glassy rinds from the young (2 - 4 Ma) and fresh SEMFR pillowed basalts provide new insights into the global water cycle. SEMFR lavas contain ~2 wt % H2O, and the olivine-hosted melt inclusions have the highest subduction-related H2O/Ce ratios (H2O/Ce = 6000 - 19000) ever recorded in arc magmas (H2O/Ce < 10600 and global averaged H2O/Ce < 3000). Our findings show that (i) slab-derived fluids released beneath forearcs are water-rich compared to the deeper fluids released beneath the arc system; and (ii) cold downgoing plates lose most of their water at shallow depths (~ 70 - 80 km slab depth), suggesting that water is efficiently recycled beneath the forearc (≥ 90%).
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