XENOLITH HYDROGEN ISOTOPE EVIDENCE FOR LOWER CRUSTAL HYDRATION OF THE COLORADO PLATEAU BY SUBDUCTING SLAB-DERIVED FLUIDS

Widespread metasomatism of the western North American lithosphere is predicted to have occurred during the Laramide orogeny, mediated by fluids released from the shallowly subducting Farallon plate. The resultant chemical and physical changes to the continental lithosphere may have facilitated the tectono-magmatic and geodynamic phenomena that have impacted the western U.S. since the latest Cretaceous, such as pulses of Tertiary volcanism and mechanisms for the modern ~2 km elevation gain of the Colorado Plateau. Direct observations of the mantle and crust necessary to test these ideas are limited to xenolith studies. Diatremes located in the Oligocene Navajo Volcanic Field (NVF) of the central Colorado Plateau host mantle and crustal xenolith lithologies that preserve petrological evidence of pre- (M₁) and post-hydration (M₂) metamorphic textures and mineral assemblages. Here we present new hydrogen stable isotope results from hydrous minerals in felsic and mafic lower crustal xenoliths hosted in six serpentinized ultramafic microbreccia (SUM) diatremes from the NVF. Hornblende, muscovite, and biotite from M₁ assemblages have δ²H values (VSMOW) ranging from -103 to -70 ‰ (-82 ± 12 ‰). M₂ hydrous minerals including actinolite, biotite, white mica, epidote, and chlorite yield δ²H values from -82 to -22 ‰ (-50 ± 14 ‰). Using available thermobarometry-based temperature constraints of ~500 °C for M₂, we estimate δ²H values of water in equilibrium with these hydrous phases as ~-40 to +15 ‰ (-17 ± 15 ‰). We rule out local metamorphic fluids or interaction with meteoric fluids during/post diatreme emplacement because these should yield M₂ phases with similar or much lower values than M₁. These δ²H values are most consistent with M₂ mineral growth during reactions with slab-derived fluids. Our interpretations are consistent with published isotopic studies of mantle xenoliths from these locations, supporting the hypothesis that Laramide-age, slab-derived hydration permeated the mantle and lower crust of the Colorado Plateau.