Megan S. Duncan, Rajdeep Dasgupta, Kyusei Tsuno

Abstract

Knowledge of the carbon carrying capacity of peridotite melt at reducing conditions is critical to constrain the mantle budget and planet-scale distribution of carbon set at early stage of differentiation. Yet, neither measurements of CO₂ content in reduced peridotite melt nor a reliable model to extrapolate the known solubility of CO₂ in basaltic (mafic) melt to solubility in peridotitic (ultramafic) melt exist. There are several reasons for this gap; one reason is due to the unknown relative contributions of individual network modifying cations, such as Ca²⁺ versus Mg²⁺, on carbonate dissolution particularly at reducing conditions. Here we conducted high pressure, temperature experiments to estimate the CO₂ contents in silicate melts at graphite saturation over a compositional range from natural basalts toward peridotite at a fixed pressure (P) of 1.0 GPa, temperature (T) of 1600 °C, and oxygen fugacity (logfO₂ ~ IW+1.6). We also conducted experiments to determine the relative effects of variable Ca and Mg contents in mafic compositions on the dissolution of carbonate. Carbon in quenched glasses was measured and characterized using Fourier transform infrared spectroscopy (FTIR) and Raman Spectroscopy and was found to be dissolved as carbonate (CO₃^2-). The FTIR spectra showed CO₃^2- doublets that shifted systematically with the MgO and CaO content of silicate melts. Using our data and previous work we constructed a new composition-based model to determine the CO₂ content of ultramafic (peridotitic) melt representative of an early Earth, magma ocean composition at graphite saturation. Our data and model suggest that the dissolved CO₂ content of in reduced, peridotite melt is significantly higher than that of basaltic melt at shallow magma ocean conditions; however, the difference in C content between the basaltic and peridotitic melts may diminish with depth as the more depolymerized peridotite melt is more compressible. Using our model of CO₂ content at graphite saturation as a function of P–T-fO₂-melt composition, we predict that a superliquidus shallow magma ocean should degas CO₂. Whereas if the increase of fO₂ with depth is weak, a magma ocean may ingas a modest amount of carbon during crystallization. Further, using the carbon content of peridotite melt at logfO₂ of IW and the knowledge of C content of Fe-rich alloy melt, we also consider the core-mantle partitioning of carbon, showing that D_C^{metal/peridotite} of a shallow magma ocean is generally higher than previously estimated.

Duncan, M. S., Dasgupta, R., Tsuno, K. (2017). Experimental determination of CO₂ content at graphite saturation along a natural basalt-peridotite melt join: Implications for the fate of carbon in terrestrial magma oceans. Earth and Planetary Science Letters 466, 115-128. doi:10.1016/j.epsl.2017.03.008