HYDROGEN STABLE ISOTOPE CONSTRAINTS ON HYDRATION OF COLORADO PLATEAU LOWER CRUST FROM OLIGOCENE-MIOCENE NAVAJO VOLCANIC FIELD XENOLITHS

Prevailing ideas suggest that compositional changes due to hydration of the continental lithosphere during Laramide shallow slab subduction contributed to processes such as Tertiary magmatism and surface uplift in the Rocky Mountain and Colorado Plateau regions. Petrologic and isotopic studies of xenoliths are one of the few ways to directly constrain the composition of the mantle and crust to test these ideas. Here we use hydrogen stable isotope (δ²H values) of hydrous phases in lower crustal xenoliths from the Colorado Plateau to identify evidence of crustal hydration and its likely origin. We present new results from 24 lower crustal xenoliths of varying lithology from six diatreme localities in the Oligocene to early Miocene Navajo Volcanic Field (NVF) of the Colorado Plateau. Petrographic techniques were used to identify hydrous minerals and distinguish those associated with retrograde hydrous metasomatism versus the primary peak assemblages. Phases including hornblende, actinolite, biotite, white mica, zoisite, chlorite, and epidote were hand-picked using a stereoscope, and δ²H values (vs. VSMOW) were determined using continuous flow, isotope ratio mass spectrometry via high-temperature pyrolysis. Hornblende and biotite from the primary assemblages have δ²H values averaging -85 ± 13 ‰ (n= 4) and -74 ± 6 ‰ (n=3), respectively. In comparison, retrograde actinolite and biotite exhibit mean values of -54 ± 16 ‰ (n=14) and -45 ± 13 ‰ (n=7), respectively. Values from other retrograde phases range from -99.2 to -21.9 ‰. With few exceptions, the δ²H values from secondary hydrous minerals are much higher than unaltered lower crustal values, suggesting growth in equilibrium with deuterium-enriched fluids. Preliminary estimates of the water δ²H values in equilibrium with these minerals using retrograde temperature constraints from thermobarometry are in the range of -30 to +5 ‰. These estimates are similar to published interpretations based on δ²H values of hydrous phases from NVF upper mantle xenoliths and overlap with estimates of slab-water δ²H values. Although additional constraints on timing are needed, our results suggest that the Colorado Plateau lower crust was at least partially hydrated by Farallon slab-derived fluids during the Laramide.