The Changing Character of Carbon in Fluids with Pressure: Organic Geochemistry of Earth’s Upper Mantle Fluids

Abstract

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 theoretical models, experimental diamond‐anvil cell and fluid inclusion studies, and high‐pressure metamorphic rocks all indicate a wide variety of aqueous C‐species at high pressures. 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. Furthermore, the aqueous fluids can coexist with immiscible hydrocarbon fluids. As a consequence, C‐species with a mixture of oxidation states in aqueous fluids and hydrocarbon fluids at high pressures in the upper mantle can influence the oxidation state of their upper mantle environment.