Mantle redox state

The following reactions rapidly changed the mantle redox state to a more oxidized level and would explain the outgassing of H2, H2S and SO2. 

The role of sulfur in these vents is complex and often obscured by its multiple redox states and by uncertainties in the degree of equilibration. Studies by Styrt et al. (1981), Arnold and Sheppard (1981), Skirrow and Coleman (1982), Kerridge et al.(1983), Zierenberg et al. (1984), and others have shown that the sulfur in these deposits is enriched in S relative to a mantle source (typical 5 " S ranges are between 1 and 5%c), implying small additions of sulfide derived from sea water. 

This requirement is fulfilled for electric discharges in a reduced atmosphere containing methane, ammonia, and water, as in the original Miller experiment. It has also been observed for atmospheres based on N2 and CO or CO2 on the condition that H2 or methane is also present in snfflcient amonnts (19). A neutral atmosphere (based on N2, CO2, and water) wonld produce much lower yields of organics (by several orders of magnitude). In the absence of other species to be oxidized, the rednction of CO2 reqnires the concomitant thermodynamically nnfavorable conversion of water into O2 (as in photosynthesis). However, even if the atmosphere was nentral when life arose, as nsnaUy believed, the Earth was not nniform with respect to redox state simply becanse the rednced state of the mantle and the high volcanic activity favored the occnrrence of locally rednced environments (for instance, in hydrothermal vents in the oceans). Then, a preservation of the hydrogen content of the early atmosphere or the diversity of environments on the early Earth is likely to have made amino acid formation possible, at least at specific places. 

Wadhwa M. (2001) Redox state of Mars upper mantle and crust from Eu anomalies in shergottite pyroxenes. Science 291, 1527-1530. 

Canil D. (1999) Vanadium partitioning between orthopyroxene, spinel and sihcate melt and the redox states of mantle source regions for primary magmas. Geochim. Cosmochim. Acta 63, 557-572. 

Lecuyer, C. Ricard, Y. 1999. Long-term fluxes and budget of ferric iron implication for the redox state of the Earth s mantle and atmosphere. Earth and Planetary Science Letters, 165,197-211. 

Fundamental to this process is the oxidation state of the Earth s mantle, for equilibria between elemental carbon and C-O-H fluids control the proportions of oxidized and reducing gases degassed from the mantle. Particularly important here is knowing the extent to which the oxidation state of the mantle has changed with time. There have been a number of models which claim that the redox state of the mantle was different in the past due either to the separation of the Earth s core (Kuramoto St Matsui, 1996), the subduction of ferric iron (Lecuyer Si Ricard, 1999), or intense mantle plume activity (Kump et al., 2001). 

A number of studies have used xenoliths from Archaean subcontinental lithosphere to make inferences about the oxidation state of the early mantle. For example Woodland and Koch (2003) showed that the subcontinental lithosphere beneath the Kaapvaal Craton displays a systematic decrease in oxygen fugacity with depth (Fig. 5.11). However, it should be remembered that the highly depleted nature of the Archaean subcontinental lithosphere means that it is atypical of the mantle as a whole and may not therefore be useful as an indicator of mantle redox conditions (Chapter 3, Section 3.1.3.2). 

Deciding between these competing options requires a quantitative knowledge of oxygen sources and sinks in the late Archaean/early Proterozoic (Catling Claire, 2005). Initial attempts at such a quantification suggest that the redox states of the continental crust and/or the mantle may hold the key to this puzzle (Claire et al., 2005), although this more sophisticated approach is still in its infancy. 

Kuramoto, K. and Matsui, T., 1996. Partitioning of H and C between the mantle and core during the core formation in the Earth its implications for the atmosphere evolution and the redox state of early mantle. /. Geophys. Res., 101, 14909-32. 

---