Immiscible hydrocarbons in aqueous fluids at subduction zone conditions from in-situ experiments and thermodynamic modeling

Abstract:

Subduction zones are geological settings important for carbon transport to the deep Earth. Carbon in fluids released from the subducted slab is essential in water-rock interactions. Hydrocarbons in deep fluids have been studied by experiments and theoretical calculations in recent years at subduction zone conditions[1,2]. In diamond anvil cell experiments starting with Na-acetate solutions[1], immiscible hydrocarbon droplets containing mainly methane and isobutane formed at 300 °C and 3.0 GPa, measured by Raman spectroscopy. Here, we focused on the formation of hydrocarbons starting with 1.0 M Ca-acetate solutions over a wide range of pressures (1.5 – 4.6 GPa) and temperatures (300 – 350 °C). Droplets of immiscible hydrocarbon fluid coexisting with aragonite or calcite crystals were found at peak pressure and temperature conditions in the experiments. In fact, the higher the pressure, greater quantities of hydrocarbons were formed faster at 300 °C. Interestingly, less methane and more propane formed compared to the previous Na-acetate experiments. In experiments at around 350 °C, aromatic hydrocarbons formed which caused strong fluorescence using a 532 nm laser. Increase of temperature is the main reason for forming aromatic hydrocarbons in our experiments. We also set up a thermodynamic model to modify the reactions in the experiments using a chemical mass transfer model. In the model, different types of reactions happen from extremely dilute to 1.0 M Ca-acetate fluids, which cause changes of pH and logfO2. Graphite first precipitated from dilute aqueous fluid, then immiscible hydrocarbon fluids formed when the reactant of Ca-acetate in the system reached 0.1 M but shortly disappear. However, no graphite formed in our experiments. If Graphite was suppressed, aragonite coexisted with hydrocarbons until the end of reaction at a higher fO2. Our study supports the possible cooccurrence of hydrocarbons and carbonate minerals without graphite in subduction zones.

[1] Huang et al., 2017

[2] Li 2017