Cycling of CO2 and H2O constrained by experimental investigation of a model ophicarbonate at deep subduction zone conditions

Abstract

Ophicarbonates are carbonate-bearing serpentinites and therefore have high concentrations of CO2 and H2O. Their high concentrations of CO2 and H2O potentially make them important contributors to recycling of CO2 and H2O between Earth's surface and interior. However, the phase relations of ophicarbonates under deep subduction zone conditions remain poorly constrained, especially under conditions where melting may occur. Here, we present laboratory experiments on a model ophicarbonate at 3-7 GPa and 700-1100°C. We find that chlorite is stable at 3 GPa and ≤ 800°C, and phase A and balangeroite are stable at 7 GPa and ≤ 700°C. At all other conditions there is no stable residual H2O-bearing phase, providing support for the existence on an “anhydrous nose” in ultramafic rocks between 4-6 GPa in relatively warm subduction zones. We find that the solidus decreases with increasing pressure from between 900 and 1000°C at 3 GPa to between 700 and 800°C at 7 GPa. We find that the near-solidus melt is carbonate-rich and this melt coexists with a residual carbonate phase over a small temperature range. Using mass balance calculations, we estimate phase proportions as function of P and T to estimate mass fractions of CO2 remaining in the solid lithology. When compared to P-T paths of subducting slabs our results suggest that significant amounts of CO2 will be released from the slab in hot subduction zones, while significant amounts of CO2 will be retained in the slab in cold subduction zones of the modern Earth. In ancient subduction zones, ophicarbonate may have been more important to deep C fluxes due to the absence of thick carbonate sequences, and like modern Earth can either deliver significant amounts of CO2 to the mantle wedge or to the deep mantle depending on the P-T trajectories of subducting slabs.