LARGE-SCALE OPEN-SYSTEM BEHAVIOR OF CARBON DIOXIDE IN THE CONTINENTAL LITHOSPHERE DEDUCED FROM CLOSED-SYSTEM MODELING OF METAMORPHIC PHASE EQUILIBRIA IN THE WEPAWAUG SCHIST, CT
We present results of thermodynamic modeling of metamorphosed (Acadian orogeny) carbonate rocks from the Wepawaug Schist, CT. The ~10 km thick unit consists of metapelites ranging in grade from chlorite up to the staurolite-kyanite zone, and metacarbonates characterized by ankerite, biotite, amphibole, and diopside zones with increasing grade. We calculate two kinds of pseudosections from bulk compositions measured in low-grade precursor rocks: (1) closed-system (no infiltration) metamorphism and (2) metamorphism buffered by an H2O-CO2 fluid. By varying the XCO2 of this fluid, we explore the effect of infiltration on phase relations. Thus we utilize closed-system modeling to test whether open-system processes were necessary to produce the observed mineral assemblages.
Our results indicate that the purely closed-system approach consistently makes grossly inaccurate predictions of CO2 loss and mineral assemblages, and that an H2O-rich fluid – derived from dewatering metaclastic rocks or degassing intrusions – must have been present during metamorphism. For example, closed-system models predict that ankerite will be stable up to ~620 ˚C at 8 kbar, yet the ankerite-out isograd formed between ~500 and ~530 ˚C in nature. On the other hand, models buffered by H2O-rich fluids of XCO2 ~0.05–0.1 are in excellent agreement with observed field relationships, including the succession of prograde mineral zones. Closed-system models of devolatilization will likely severely underestimate orogenic CO2 fluxes in metaclastic-metacarbonate sequences, necessitating consideration of open-system volatile transport and reaction.