Redox history of the Earth's interior since approximately 3900 Ma: implications for prebiotic molecules.

The history of the oxidation state in the Earth's mantle has been constrained using (a) the whole-rock abundances of Cr and V in ancient volcanics, and (b) the composition of Cr-rich spinels in ancient volcanics. Results indicate that the Earth's mantle has been at-or-near its current oxidation state (+/- 0.5 log-unit fO2) since at least 3600 Ma, and probably since at least 3960 Ma. Volatiles released into the Earth's atmosphere by high-temperature (T > or = 1300 K) volcanism during this time have been dominated by H2O, CO2, and SO2. This blend of volatiles is known to provide smaller yields of prebiotic, organic molecules by atmospheric and surface processes than gas mixtures containing higher concentrations of reduced species such as H2, CO, and H2S (e.g., Miller, 1998; Zolotov and Shock, 2000). The results discussed in this article independently support the conclusion of Canil (1997, 1999). If the atmosphere was reducing (e.g., CH4, H2, H2S, NH3, CO) at any time during the last approximately 3900 Ma, high-temperature volcanic outgassing was not the cause of it.