Graphite-melt equilibria during mantle melting: constraints on CO2 in MORB magmas and the carbon content of the mantle

Abstract

Melts produced under fluid-absent, graphite-saturated conditions have tightly constrained oxidation states and CO₂ contents. A model is presented for the generation of mid-ocean ridge basalt (MORB) magma by fluid-absent partial melting of graphite-saturated upper mantle. The model results in melt ferric/ferrous ratios and oxygen fugacities which are consistent with observed values in glassy MORBs and oceanic peridotites. Increasing the degree of partial melting causes decreases in the ferric/ferrous ratio of the melt, the oxygen fugacity, and the concentration of CO₂ in the melt. Sensitivity analysis indicates that the initial ferric-iron concentration in the source region has the greatest effect on the results. For the range of probable initial values, the oxygen fugacity of the graphite-saturated primary MORB magma at 1 GPa is 2.5 log₁₀ units below the Ni–NiO buffer, and the CO₂ content of the melt ranges from 900 to 1800 ppm (mass). The amount of source region carbon consumed during partial melting is between 40 and 80 ppm. The dissolved CO₂ in MORBs represents a flux of 3.5 to 7×10¹⁰ kg/year which is significantly greater than estimates based on the measured CO₂ content of glassy MORB samples.