Metamorphic devolatilization of subducted marine sediments and the transport of volatiles into the Earth's mantle

Volatiles, most notably CO2, are recycled back into the Earth's interior at subduction zones(1,2). The amount of CO2 emitted from arc volcanism appear s to be less than that subducted, which implies that a significant amount o f 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(3-5) have addressed this problem and therefore metamorphic decarbon ation in subduction zones remains largely unquantified, despite its importa nce to arc magmatism, palaeoatmospheric CO2 concentrations and the global c arbon cycle(6). Here we present computed phase equilibria to quantify the e volution of CO2 and H2O through the subduction-zone metamorphism of carbona te-bearing marine sediments (which are considered to be a major source for CO2 released by arc volcanoes(6)). Our analysis indicates that siliceous li mestones undergo negligible devolatilization under subduction-zone conditio ns. Along high-temperature geotherms clay-rich marls completely devolatiliz e 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 e ssential to promote subarc decarbonation of most marine sediments. In the a bsence of such infiltration, volatiles retained within marine sediments may explain the apparent discrepancy between subducted and volcanic volatile f luxes and represent a mechanism for return of carbon to the Earth's mantle.