Formation of hydrocarbons favored by high pressure at subduction zone conditions

Subduction zones enable carbon transport to the deep Earth. Inorganic and organic carbon in fluids released from the subducted slab are important in water-rock interactions. However, the potential role of reduced carbon species in fluids during subduction is still unclear. In particular, the pressure, temperature, and oxidation state conducive to the formation of fluid hydrocarbons are poorly understood. Here we focused on the role of pressure influencing the formation of hydrocarbons using diamond anvil cell experiments starting with 0.5 M Ca-acetate solutions over a broad range of pressures, from 1.6 to 4.6 GPa at temperatures of 300 and 350 °C. Droplets of immiscible hydrocarbon fluid coexisting with aqueous carbonate or bicarbonate and crystals of calcite or aragonite were formed at peak pressure and temperature conditions in the experiments. Greater quantities of hydrocarbons formed at higher pressure as revealed by measuring the in situ volume percent of the droplets. Measurement of the in situ Raman spectra of the co-existing aqueous solutions and analysis of the peak area ratios of acetate to HCO3− showed faster decrease at higher pressure, which indicated that the reactions forming hydrocarbons were accelerated by pressure. Compared to previous Na-acetate experiments at 300 °C and 3.0 GPa, this study found hydrocarbons that were more methane-rich and had more propane and less isobutane. In experiments at 350 °C, isobutane transformed to one or more aromatic hydrocarbons in an immiscible fluid. The air-dried aromatic hydrocarbons were measured using a UV laser, and show peaks of bitumen. Overall, our study supports the hypothesis that high pressures and high temperatures facilitate the possible occurrence of immiscible hydrocarbon fluids in the deep carbon cycle. [Display omitted] •Aragonite & hydrocarbon fluid formed in aqueous acetate decomposition experiments.•Acetate decomposition accelerates with high pressure at 300 °C and 350 °C.•Experiments in a diamond anvil cell measured by in situ Raman spectroscopy.•Theory: fluid methane formation favored by pressure, opposed by temperature.