HYDROCARBON STABILITY AND THE REDOX STATE OF EARTH’S MANTLE

The source and origin of hydrocarbons on Earth has a long and contentious history. One school of thought maintains that most hydrocarbons present in Earth’s crust originate from inorganic processes in Earth’s mantle. The conventional view maintains that hydrocarbons are derived from original organic matter of biological origin.

Oxygen fugacity (fO₂) is the most important factor controlling speciation of Carbon at upper mantle conditions. Oxygen thermobarometry of abyssal spinel peridotites shows that mantle beneath “hot-spots” may be sufficiently reducing for C-O-H phases to be in equilibrium with graphite (Δlog fO₂ ~FMQ-2; Bryndzia and Wood, 1990). Under such conditions, CH₄ is stable.

Thermodynamic simulations of CH₄ and species in the system [(1/n)C_nH_2n+2 + (n-1)/nH2] predict that higher chain liquid alkanes would form as a result of CH₄ reduction and be stable in Earth’s mantle at P ≥ 4 GPa and at T = 1000 K. Such models require unrealistically high fugacity of hydrogen (fH₂), inconsistent with petrological based estimates of mantle fO₂. It is also unlikely that high fH₂ could be retained in the mantle over geological time, H₂ likely having degassed during Earth’s early history.

EOS based models of carbonate reduction predict that at 7 GPa and 500^oC, reduction of calcite will produce CH₄ as the dominant C-O-H phase at upper mantle conditions (100 to 200 km and fO₂ ~FeO-Fe₃O₄). At 7 GPa and 1500^oC no methane is predicted, only CO₂, CO and H₂.

Results of DAC experiments by Scott et al., (2004) with FeO-CaCO₃-H₂O at 5 GPa and T = 500 to 1500^oC generated CH₄. Calculations indicate that at 500^oC and P < 7 GPa, CH₄ is favored. At 1500^oC, CO₂ and CO are favored due a reforming reaction with CH₄. fO₂ was not buffered in these experiments but was more oxidizing than FMQ, as suggested by inferred reactions of starting materials:

8FeO + CaCO₃ + 2H₂O = 4Fe₂O₃ + CH₄ + CaO and

12FeO + CaCO₃ + 2H₂O = 4Fe₃O₄ + CH₄ + CaO

In these experiments fO₂ is probably at Δlog fO₂ ~FMQ+2 where conditions are too oxidizing for methane to be stable. The obvious conclusion based on petrological, experimental and thermodynamic models is that fO₂ in Earth’s upper mantle is too oxidizing for liquid hydrocarbons to ever have been stable. Methane is only stable in Earth’s mantle under sufficiently reducing conditions where C-O-H phases are in equilibrium with graphite.