Abstract

The current state of knowledge regarding redox conditions of rocks from asteroidal bodies (as represented by various classes of meteorites), the Moon and Mars is discussed here. In the case of the differentiated meteorite parent bodies, the redox conditions range over at least six orders of magnitude (from ~5 log units below the Iron-Wüstite buffer to slightly above it) and are determined in large part by the compositions of the undifferentiated precursor materials that accreted to form these parent bodies. Lunar basalts record oxygen fugacities ranging from close to the Iron-Wüstite (IW) buffer to ~2 log units below it. The current best estimate of the oxygen fugacity of the lunar mantle is ~1 log unit below IW. Martian crustal rocks represented by the Shergottite-Nakhlite-Chassignite group of meteorites record a wide range of oxygen fugacities. The basaltic shergottites range from slightly below the IW buffer to ~2 log units above it, whereas the cumulate nakhlites (and chassignites) are relatively oxidized (~3–4 log units above the IW buffer). Following early metal-silicate and crust-mantle differentiation on Mars, the depleted martian mantle is likely to have been reduced (close to the IW buffer or slightly lower). Metasomatism and secondary (hydrous) alteration are likely to have produced silicate reservoirs on Mars that are relatively more oxidized (most likely ≥3 log units above the IW buffer). The redox conditions on the other terrestrial planets (Mercury and Venus) are not well constrained. Based on the limited information from remote spacecraft and telescopic observations of surface rocks on these planets, it is inferred that silicate reservoirs on Mercury are highly reduced; those on Venus are likely to be somewhat more oxidized than on Mercury, possibly similar to the lower mantle of Earth.

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