Limited water outflux from the subducted slab serpentinite via Mariana arc-basin magmatism

Abstract

The water cycle between Earth’s surface and interior controls the long-term stability of sea level. The discovery of ~24-km thick serpentinized mantle in the Pacific plate outside of the Mariana Trench suggests that water influx by plate subduction could be three times greater than previous estimates. However, the extent to which water hosted in the slab serpentinite is released by magmatism remains unclear. Here we present new boron isotopic data on Mariana frontal- and rear-arc volcanics, along with literature data on d11B, halogens and water, to examine the role of slab serpentinite in Mariana arc-basin magmatism. Our results indicate that the contribution of slab serpentinite occurs only in rear-arc volcanics, but is largely invisible in other Mariana arc-basin volcanics. Additionally, quantitative calculations suggest that the water influx of slab crust is sufficient to account for the water outflux through Mariana arc-basin magmatism, demonstrating a limited role of slab serpentinite. Given the extensive slab serpentinization inferred from well-developed bending faults throughout the entire Mariana arc system in the subducting plate, subduction of the 24-km slab serpentinite could potentially carry ~80% of bound water in the subducted slab to the mantle outside of the typical subduction zone region.