Decades of research have now firmly established that aqueous fluids in Earth's crust from groundwater to deep basinal brines to shallow mid-ocean ridge hydrothermal systems can contain metastable equilibria involving C-species in which methane does not participate. It now appears, however, that this situation does not extend to the upper mantle. Instead, field evidence from high pressure metamorphic rocks, experimental diamond-anvil cell and fluid inclusion studies, and theoretical models all indicate that, at pressures above about 2.0 - 3.0 GPa, methane and all other aqueous C-species with different oxidation states between -4.0 and + 4.0 can coexist in variable proportions. As a consequence, C-species with a mixture of oxidation states in aqueous fluids at high pressures in the upper mantle can control the oxidation state of their upper mantle environment, including, for example, redox reactions during diamond formation from supersolvus fluids.<br/>

Organic geochemistry of Earth's upper mantle

2019 Spring National Meeting

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