Modeling open system metamorphic decarbonation of subducting slabs

Abstract

Fluids derived from the devolatilization of subducting slabs play a critical role in the melting of the mantle wedge and global geochemical cycles. However, in spite of evidence for the existence and mobility of an aqueous fluid phase during subduction metamorphism, the effect of this fluid on decarbonation reactions in subducting lithologies remains largely unquantified. In this study we present results from thermodynamic modeling of metamorphic devolatilization of subducted lithologies for pressures up to 6 GPa using an approach which considers fluid fractionation from source lithologies and infiltration from subjacent lithologies. This open system approach in which fluid flow is an intrinsic component of the chemical model offers an alternative to closed system models of subduction zone decarbonation. In general, our models simulating pervasive fluid flow in subducting lithologies predict CO2 fluxes measured from volcanic arcs more closely than models which assume purely channelized flow. Despite the enhanced effect of H2O‐rich fluid infiltration on subduction decarbonation, our results support the hypothesis that CO2 is returned to the deep mantle at convergent margins, particularly in cool and intermediate subduction zones. Our results demonstrate that for most subduction zones, a significant proportion of the CO2 derived from the slab is lost beneath the fore arc, and therefore CO2 flux estimates based on measurements within the volcanic arc alone may significantly underestimate the slab‐derived CO2 flux for individual margins. Nevertheless, our predicted global slab‐derived CO2 flux from convergent margins of 0.35–3.12 × 1012 mols CO2/yr is in good agreement with previous estimates of global arc volcanic flux. Because our predicted global slab‐derived CO2 flux is significantly less than atmospheric CO2 drawdown by chemical weathering, significant CO2 emission from other geologic regimes (e.g., hot spots) would be required to balance the global carbon cycle.