Xenolith hydrogen isotope evidence for lower crustal hydration during cycles of flat slab subduction

Abstract Dehydrating slabs release aqueous fluids that can cause widespread metasomatism of the overlying lithosphere. Studies of mantle and crustal xenoliths provide critical mineralogical and geochemical constraints on these processes. The Colorado Plateau is a natural laboratory for documenting how chemical modification to the lithosphere impacted tectono-magmatic and geodynamic phenomena in the western United States since the latest Cretaceous. Diatremes of the Oligocene Navajo volcanic field host lower crustal xenoliths with evidence of pre- (M1) and post-hydration (M2) metamorphic textures and mineral assemblages. New hydrogen stable isotope results from hydrous minerals in felsic and mafic lower crustal xenoliths hosted in serpentinized ultramafic microbreccia diatremes show deuterium enrichment during M2. M1 minerals have δD values (Vienna standard mean ocean water, VSMOW) from −103‰ to −69‰ (−82‰ ± 12‰; mean ± 1σ), compared to M2 hydrous phases with values from −81‰ to −23‰ (−50‰ ± 14‰). Using available thermobarometry-based temperature constraints of ~500 °C for M2, we estimate the δD values of the hydrous fluids as ~−17‰ ± 15‰. We suggest that Laramide-age, slab-derived aqueous fluids permeated into the lower crust of the Colorado Plateau and that shallow subduction transports and releases water far inboard from subduction zones. Results suggest that this part of the global subduction cycle facilitates hydrous metasomatism of large swaths of continental lithosphere with implications for de-densification and uplift of continental plateaus, subsequent magmatism, and transport of volatiles and metals in modern and ancient flat slab systems.