Experimental determination of dissolved CO2 content in nominally anhydrous andesitic melts at graphite/diamond saturation – Remobilization of deeply subducted reduced carbon via partial melts of MORB-like eclogite

Abstract:

Experimental phase relations of carbonated lithologies [1] and geochemistry of deep diamonds [2] suggest that deep recycling of carbon has likely been efficient for a significant portion of Earth’s history. Both carbonates and organic carbon subduct into the mantle, but with gradual decrease of fO₂ with depth [3] most carbon in deep mantle rocks including eclogite could be diamond/graphite [4]. Previous studies investigated the transfer of CO₂ from subducted eclogite to the ambient mantle by partial melting in the presence of carbonates, i.e., by generation of carbonate-rich melts [5]. However, the transfer of carbon from subducted eclogite to the mantle can also happen, perhaps more commonly, by extraction of silicate partial melt in the presence of reduced carbon; yet, CO₂ solubility in eclogite-derived andesitic melt at graphite/diamond saturation remains unconstrained. CO₂content of eclogite melts is also critical as geochemistry of many ocean island basalts suggest the presence of C and eclogite in their source regions [6].

In the present study we determine CO₂ concentration in a model andesitic melt [7] at graphite/diamond saturation at conditions relevant for partial melting of eclogite in the convecting upper mantle. Piston cylinder and multi anvil experiments were conducted at 1-6 GPa and 1375-1550 °C using Pt/Gr double capsules. Oxygen fugacity was monitored with Pt-Fe sensors in the starting mix. Completed experiments at 1-3 GPa show that CO₂ concentration increases with increasing P, T, and fO₂ up to ~0.3 wt%. Results were used to develop empirical and thermodynamic models to predict CO₂ concentration in partial melts of graphite saturated eclogite. This allowed us to quantify the extent to which CO₂ can mobilize from eclogitic heterogeneities at graphite/diamond saturated conditions. With estimates of eclogite contribution to erupted basaltic lavas, the models developed here allow us to put constraints on the flux of CO₂ to mantle source regions coming from subducted crust and investigate the possible role this process may play in the deep carbon cycle.

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