Generation of methane in the Earth's mantle: in situ high pressure-temperature measurements of carbonate reduction

Abstract

We present in situ observations of hydrocarbon formation via carbonate reduction at upper mantle pressures and temperatures. Methane was formed from FeO, CaCO(3)-calcite, and water at pressures between 5 and 11 GPa and temperatures ranging from 500 degrees C to 1,500 degrees C. The results are shown to be consistent with multiphase thermodynamic calculations based on the statistical mechanics of soft particle mixtures. The study 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.

Fig. 1.

Typical Raman spectra from the heating of FeO, calcite, and water near 5 GPa. The broad O-H stretching vibration of ice VII or liquid water near 3,200 cm^–1 is ubiquitous in these H₂O-rich samples. (A) Raman spectrum produced by heating FeO, calcite, and water to 1,500°C at 5.7 GPa. In isolated regions methane is the dominant Raman-active component. (B) A spectrum produced after resistively heating to 600°C and a decrease in pressure to ≈2 GPa; the production of methane is clearly indicated by the C-H stretching vibration at 2,932 cm^–1.

Fig. 2.

X-ray diffraction pattern for temperature and pressure quenched sample after external heating to 600°C at 5 GPa. Fe₃O₄-magnetite dominates the spectrum; weak lines suggest a Ca-Fe oxide.

Fig. 3.

Raman spectra (Left) of low-pressure bubbles (Right) formed upon decompression to ≈0.5 GPa at room temperature after laser heating to ≈1,500°C at 5.7 GPa. Note the absence of O-H stretching vibrations because the bubble has displaced the surrounding H₂O. Bubbles are visible near the bottom, left side, and slightly right of center.

Fig. 4.

Results of thermochemical calculations. Concentrations at 500°C (a) and 1,500°C (b). CH₄ is predicted to be prevalent at 500°C above 0.9 GPa, as shown in a, but H₂ becomes dominant at 1,500°C, as shown in b. Concentrations are shown per kg of H₂O/CaCO₃/FeO mixture. Liquid phases are denoted by (l), and solid phases are denoted by (s). Lines representing more than one species are labeled with a /.