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

Abstract

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.

Contrasting He–C relationships in Nicaragua and Costa Rica: insights into C cycling through subduction zones

“…The percentage of CO 2 emitted via the Central American arc is exceptionally low relative to global estimates of the output/input, V0.50 (calculated from global £ux estimates in [46] and [19]). Kerrick and Connelly [46] attributed low CO 2 arc emissions along arcs to two processes : (1) strong devolatilisation of clay-rich marls in the forearc region, provided the geotherm is su⁄ciently high, and (2) retention of CO 2 in carbonate-bearing marine sediments to depths greater than subarc depths. It is possible that both of these processes occur in the Central American subduction zone to limit CO 2 loss via arc volcanism.…”

Large‐scale rigid‐body rotation in the mantle wedge and its implications for seismic tomography

“…In this approach, we calculate the stable mineralogy and thermodynamic properties for each lithology as a function of pressure and temperature by Gibbs energy minimization [Connolly, 2005]. The phase relations obtained by such calculations for lithologies relevant here have been presented elsewhere [Kerrick and Connolly, 1998, 2001a, 2001b. Seismic velocities were computed as described by Connolly and Kerrick [2002] with shear moduli as summarized by Connolly [2005].…”

A piece of the deep carbon puzzle

“…The water in the fluid lowers the melting point of the rocks, eventually triggering melting of the mantle and eruptions at arc volcanoes. However, thermodynamic models of this devolatilization process assume that only water and carbon dioxide are removed from the slab 14,15 . If so, relatively little carbon can be stripped from the slab beneath the arc -not enough to account for the significant amounts of carbon found in arc magmas.…”