Immiscible Hydrocarbon and Aqueous Fluids Under Subduction Zone Conditions and Implications for the Deep Carbon Cycle

Abstract:

Subducting slabs recycle rocks into the deep Earth releasing fluids which may cause partial melting and possible oxidation of the mantle wedge. Recent theoretical studies¹ indicate that at pressures greater than about 3.0 GPa these fluids could contain high concentrations of organic and inorganic C-species with a wide range of C-oxidation states at equilibrium. If so, such fluids could play an important role in the deep carbon cycle, including the formation of diamond. However, direct experimental observations of the speciation in the fluids are needed.

We studied 1.0 M aqueous Na-formate and 1.0 M Na-acetate solutions in the diamond anvil cell using Raman spectroscopy at 300 ºC and 3.0 GPa for up to 60 hours. Our preliminary results indicate that formate rapidly decomposed to bicarbonate/carbonate species and methane, with no detectable H₂. Acetate decomposed much more slowly. Within the first two hours of heating, crystals of Na₂CO₃ precipitated in the fluid, and kept growing while immiscible droplets of hydrocarbon appeared and persisted throughout the experiments at elevated temperature and pressure. In the aqueous fluid, acetate and HCO₃^- were present during the first 6 hours, and then CO₃^2- and acetate after 20 hours of heating. The final HCO₃^- /CO₃^2- ratio was constant indicating a constant pH.

This is the first in situ observation of persistent immiscible fluid hydrocarbons formed from an aqueous precursor at upper mantle pressures. Our results suggest that Earth’s subduction zone fluids at high pressures might involve fluid hydrocarbon species as well as inorganic and organic aqueous C-species, which considerably broadens the picture of deep carbon sources, cycles and sinks.

[1] Sverjensky et at. (2014), Nat. Geosci. 7, 909–913.