Oxygen and Hydrogen Stable Isotope data from the Raft River Metamorphic Core Complex (Utah, USA)

Oxygen and hydrogen stable isotope analyses on quartz and muscovite veins from the footwall of the Raft River detachment shear zone (Utah) provide insight into the hydrology and fluid-rock interactions during ductile deformation. Samples were collected from veins containing 90-100% quartz, with orientations either at a high angle or sub-parallel to the surrounding quartzite mylonite foliation. Stable isotope analysis was performed on ten samples and compared to previous quartzite mylonite isotope datasets. The results indicate that the fluid present during deformation of the shear zone was meteoric in origin, with a δ2H value of approximately -100‰ and a δ18O value of approximately -13.7‰. Oxygen stable isotope O18O depletion correlates with muscovite content of the analyzed rocks. Many of the analyzed samples in this and other studies show an apparent lack of equilibrium between the oxygen and hydrogen isotope systems, which can be explained by hydrogen and oxygen isotope exchange at varying fluid-rock ratios. Our results suggest that the Raft River detachment shear zone had low static fluid-rock ratio (<0.1), yet experienced episodic influxes of fluids through semi-brittle structures. This fluid was then expelled out into the surrounding mylonite following progressive shearing, causing further 18O-depletion and fluid-related embrittlement.