Release of CO2 from carbonate rocks during regional metamorphism of lithologically heterogeneous crust

Prograde regional metamorphism drives CO2 from carbonate rock to crustal fl uids that ascend and ultimately interact with the atmosphere and oceans. Th e observed loss of CO2 from metamorphic belts remains problematic, however, because the cooling that accompanies fluid ascent favors reactions that ad d CO2 to metacarbonate rock by removing CO2 from fluids. A new two-dimensio nal model of coupled mass transfer, chemical reaction, and heat transport w as developed to assess how rock devolatilization proceeds along the upward escape paths of crustal fluids during prograde metamorphism. The model is b ased on upper greenschist to lower amphibolite facies growth of amphibole i n metacarbonate layers and garnet and biotite in intercalated metapelite la yers of the Wepawaug Schist, Connecticut (Acadian orogeny). The modeling in dicates that during heating, CO2 concentrations were larger in metacarbonat e layers than in adjacent metapelite layers because amphibole growth in met acarbonates produced CO2, whereas garnet and biotite growth in metapelites produced H2O. The resulting cross-layer concentration gradients drove H2O i nto the metacarbonate layers and CO2 out by diffusion and the transverse co mponent of mechanical dispersion. Such cross-layer mass transfer can contin ually force rock decarbonation while fluids ascend, dominating the effects of cooling, unless fluid fluxes are large and prograde heating rates are sm all. Consequently, prograde metamorphism of carbonate-bearing sedimentary s equences containing significant amounts of pelitic rock will release CO2 to regionally migrating fluids in a wide range of orogenic settings, regardle ss of whether flow is in a direction of increasing or decreasing temperatur e. Regional CO2 release can be driven by outcrop-scale processes of volatil e exchange between contrasting lithologies.