Composition of the slab-derived fluids released beneath the Mariana forearc: Evidence for shallow dehydration of the subducting plate

•We track the composition of the shallow aqueous fluids released beneath forearcs.•The shallow, slab-derived aqueous fluids are enriched in alkalis and water.•The shallow subduction outfluxes peak at ∼70–80 km depth to the subducting plate.•The subducting plate mostly dehydrates beneath the forearc.

In cold subduction systems, the downgoing plate is thought to mostly dehydrate (≥90% water) beneath the arc front. However, the composition of aqueous fluids released from the shallow subducting slab is poorly constrained because the occurrence of melt inclusions is rare in forearcs. The Southeast Mariana Forearc Rift (SEMFR) is a recently discovered site where extensionally induced magmatism occurred in Pliocene time unusually close to the trench. SEMFR basalts sampled fluids derived from the subducted plate at ∼30 to 100 km deep. Here, we use SEMFR basalts to obtain new insights into the composition of shallow slab-derived fluids by evaluating major element, trace element and volatile contents of olivine-hosted melt inclusions and glassy rinds. Olivine-hosted melt inclusions and their host glassy rinds both contain at least ∼2 wt% H2O, representing minimum estimates for magmatic water content. Melt inclusions have the highest volatile and alkali ratios (i.e., H2O/Ce=6000–19000H2O/Ce=6000–19000; Rb/Th=200–2600Rb/Th=200–2600; Cs/Th=0.4–20Cs/Th=0.4–20) yet recorded in glasses from subduction zones (arc magmas have mean H2O/Ce<2700H2O/Ce<2700; Rb/Th<40Rb/Th<40; Cs/Th<2Cs/Th<2). Our results indicate that shallow slab-derived fluids are water- and alkali-rich as compared to the deeper fluids released beneath the arc system. Shallow subduction outfluxes peak at ∼70–80 km slab depth, demonstrating that significant slab dehydration occurs beneath the forearc.