SIMPLE HYDROCARBON SYNTHESIS IN PLANETARY INTERIORS

It has previously been shown that carbonate minerals, such as calcite, can be reduced at mantle pressures and temperatures in the presence of FeO and H₂O to produce methane (Scott et al., 2004). We have performed new experiments in the Fe-calcite-H₂O system at pressures from 2 to 10 GPa and temperatures from 500 to 2000 ºC. We used both laser and resistively heated diamond anvil cells for all experiments and a combination of Raman spectroscopy and synchrotron X-ray diffraction for analysis. Our motivation was to explore a more reducing system than previous experiments in order to favor the methane-forming reaction. We were particularly interested to see if, by improving the methane yield, we can detect the formation of heavier hydrocarbons.

As expected, by using Fe rather than FeO as the reducing agent, we observe increased methane production as indicated by Raman spectroscopy. Powder X-ray diffraction data confirm previous results that the oxidation of iron at these conditions produces magnetite. We also detect Ca(OH)₂ and a few diffraction lines from an as yet unidentified phase. Furthermore, we have conducted experiments with Au-lined gaskets to conclusively demonstrate that steel gaskets are not necessary for these reactions to proceed. Finally, spectroscopic evidence suggests the formation of a methane-hydrate structure upon temperature quench from 5 GPa and 650 ºC (by room temperature the pressure had decreased to 2 GPa; we cannot further constrain the P-T conditions of this clathrate formation from our current dataset). Notably, we have not detected the presence of hydrocarbons heavier than methane.