Fluid inclusion evidence for metamorphic fluid evolution in the Black Hills, South Dakota

The mobilization of carbon as a fluid during metamorphism is an incompletely understood process. To gain direct evidence for the composition and setting of carbonic metamorphic fluids, I conducted a microthermometric and Raman spectroscopic investigation of fluid inclusions within metamorphic quartz veins in the Proterozoic metasedimentary rocks of the Black Hills, South Dakota, USA. The vein-hosting country rocks are dominantly metapelites and metagraywackes spanning the biotite to second-sillimanite metamorphic zones, deformed and metamorphosed during the Trans-Hudson collisional orogeny. Quartz within staurolite and sillimanite zone veins was recrystallized during or following the youngest deformation. Secondary fluid inclusions within the recrystallized quartz veins are CO2-H2O-N2-NaCl or CH4-N 2+/-H2O+/-NaCl mixtures. While the composition of CO2-bearing inclusions covers the entire range of the CO 2-H2O binary system, CH4-bearing inclusions are limited to aqueous-rich or non-aqueous-rich compositions, a product of fluid immiscibility in the CH4-H2O-NaCl system. A single carbonic fluid species is predicted by mineral-fluid equilibria during prograde metamorphism of graphite-bearing lithologies. An early CH4-bearing metamorphic fluid, a product of organic maturation, is oxidized to become CO2-bearing following an increase in XH2O and fO2 with prograde temperature increase. The transition between carbonic species requires only small shifts in fO2. The process accounts for observed variability in carbonic speciation at the thin-section scale. Quartz within biotite- and garnet-zone veins preserves an arrested deformation-recrystallization process. Relict quartz crystals contain densely populated zones of fluid inclusions with mixed CO2-CH4-N 2 composition and evidence for the preferential loss of H2O and autodecrepitation during the deformation process. The mixed composition is interpreted to be the product of fluid redistribution during the deformation process and not the composition of metamorphic fluid in equilibrium with graphite at the time of trapping.