Redox history of the Earth's interior since similar to 3900 Ma: Implications for prebiotic molecules

The history of the oxidation state in the Earth's mantle has been constrain ed using (a) the whole-rock abundances of Cr and V in ancient volcanics, an d (b) the composition of Cr-rich spinels in ancient volcanics. Results indi cate that the Earth's mantle has been at-or-near its current oxidation stat e (+/-0.5 log-unit fO(2)) since at least 3600 Ma, and probably since at lea st 3960 Ma. Volatiles released into the Earth's atmosphere by high-temperat ure (T greater than or equal to 1300 K) volcanism during this time have bee n dominated by H2O, CO2, and SO2. This blend of volatiles is known to provi de smaller yields of prebiotic, organic molecules by atmospheric and surfac e processes than gas mixtures containing higher concentrations of reduced s pecies such as H-2, CO, and H2S (e.g., Miller, 1998; Zolotov and Shock, 200 0). The results discussed in this article independently support the conclus ion of Canil (1997, 1999). f the atmosphere was reducing (e.g., CH4, H-2, H 2S, NH3, CO) at any time during the last similar to 3900 Ma, high-temperatu re volcanic outgassing was not the cause of it.