Volatiles, most notably CO2, are recycled back into the Earth's interior at subduction zones. The amount of CO2 emitted from arc volcanism appears to be less than that subducted, which implies that a significant amount of CO2 either is released before reaching the depth at which arc magmas are generated or is subducted to deeper depths. Few high-pressure experimental studies have addressed this problem and therefore metamorphic decarbonation in subduction zones remains largely unquantified, despite its importance to arc magmatism, palaeoatmospheric CO2 concentrations and the global carbon cycle. Here we present computed phase equilibria to quantify the evolution of CO2 and H2O through the subduction-zone metamorphism of carbonate-bearing marine sediments (which are considered to be a major source for CO2 released by arc volcanoes). Our analysis indicates that siliceous limestones undergo negligible devolatilization under subduction-zone conditions. Along high-temperature geotherms clay-rich marls completely devolatilize before reaching the depths at which arc magmatism is generated, but along low-temperature geotherms, they undergo virtually no devolatilization. And from 80 to 180 km depth, little devolatilization occurs for all carbonate-bearing marine sediments. Infiltration of H2O-rich fluids therefore seems essential to promote subarc decarbonation of most marine sediments. In the absence of such infiltration, volatiles retained within marine sediments may explain the apparent discrepancy between subducted and volcanic volatile fluxes and represent a mechanism for return of carbon to the Earth's mantle.
[1] 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 CO 2 fluxes measured from volcanic arcs more closely than models which assume purely channelized flow. Despite the enhanced effect of H 2 O-rich fluid infiltration on subduction decarbonation, our results support the hypothesis that CO 2 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 CO 2 derived from the slab is lost beneath the fore arc, and therefore CO 2 flux estimates based on measurements within the volcanic arc alone may significantly underestimate the slab-derived CO 2 flux for individual margins. Nevertheless, our predicted global slab-derived CO 2 flux from convergent margins of 0.35-3.12 Â 10 12 mols CO 2 /yr is in good agreement with previous estimates of global arc volcanic flux. Because our predicted global slab-derived CO 2 flux is significantly less than atmospheric CO 2 drawdown by chemical weathering, significant CO 2 emission from other geologic regimes (e.g., hot spots) would be required to balance the global carbon cycle.
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