Hydrogen isotopes in Mariana arc melt inclusions: Implications for subduction dehydration and the deep-Earth water cycle

Water is carried into Earthʹs mantle at subduction zones via hydrous mineral phases in subducting lithospheric plates. A certain fraction of this water is released during subduction and leads to the generation of arc volcanism. The extent of lithosphere dehydration and associated hydrogen isotope fractionation are critical to understanding the global water cycle. To further understand the origin of subduction-related fluids and how water is exchanged between Earthʹs reservoirs, we have analyzed volatiles and δD of magmatic melt inclusions from the Mariana arc. We find high δD values, ranging from − 55‰ to − 12‰, indicating release of D-enriched fluids from the subducting plate into the mantle wedge. A consequence of this process is the formation of complementary hydrous mantle components that are D-enriched (mantle wedge) and D-depleted (slab). This implies that ocean island basalts (OIB) with recycled slab components should be characterized by low δD, while high δD signatures are expected from OIB containing recycled mantle wedge peridotite. These results have important implications for the hydrogen isotope evolution of terrestrial H2O reservoirs. Based on the magnitude of subduction-related fractionations, we show that terrestrial δD variations are inconsistent with steady-state exchange of water between the mantle and surface reservoirs. We suggest that the subduction process is responsible for the present-day δD difference observed between Earthʹs major water reservoirs.