ABIOTIC METHANE PRODUCTION AT MANTLE PRESSURES

We have designed and conducted experiments to make in situ observations of hydrocarbon formation at the pressure conditions of the Earth's upper mantle. Specifically, we have used both laser and resistively heated diamond anvil cells to examine an assemblage initially consisting of FeO, CaCO₃ and H₂O. We have spanned pressures from 2 to 11 GPa and temperatures from 200 to 1500 C.

Synchrotron x-ray diffraction was used to identify solid phases and Raman and infrared spectroscopy were used to detect hydrocarbon species. At high pressures and temperatures, FeO oxidizes to form Fe₃O₄-magnetite. Methane formation is particularly strong near 5 GPa and 500 C, but forms at all conditions of this study. Furthermore, the results are shown to be consistent with multi-phase thermodynamic calculations based on the statistical mechanics of soft particle mixtures. The assemblage FeO-CaCO₃-H₂O was studied by previous workers at 5 GPa and 1,500 C using an apparatus by which run products were analyzed after pressure and temperature quench. Contrary to the previous work, our in situ observations thus far have not provided conclusive evidence to indicate the presence of heavier hydrocarbons, yet we cannot rule out the possibility. The observation of methane formation at mantle pressures is significant because it demonstrates the existence of abiogenic pathways for the formation of hydrocarbons in the Earth's interior and suggests that the hydrocarbon budget of the bulk Earth may be larger than conventionally assumed. It is conceivable that heavier hydrocarbons may yet be produced by high-pressure carbonate reduction.