Oxygen fugacity in the Martian mantle controlled by carbon: New constraints from the nakhlite MIL 03346

Abstract

This trend can be explained by polybaric graphite-CO-CO 2 equilibria in the Martian mantle. Shergottites would have formed at pressures between 1.2 and 3.0 GPa, and nakhlite parent liquids formed at pressures >3.0 GPa, consistent with geochemical and petrologic data for the shergottites and nahklites. Carbon buffering in the Martian mantle could be responsible for variation in fO 2 in Martian meteorites (rather than assimilation or crustal interaction), as well as C-H-O fluids that could be the source of ~30 ppb CH 4 detected by recent spacecraft missions. The conundrum of an oxidized current mantle and basalts, but reduced early mantle during core-mantle equilibrium exists for both the Earth and Mars. A polybaric buffering role for graphite can explain this discrepancy for Mars, and thus it may not be necessary to have an oxidation mechanism like the dissociation of MgFe-perovskite to account for the oxidized terrestrial mantle.

The effects of sulfur, silicon, water, and oxygen fugacity on carbon solubility and partitioning in Fe-rich alloy and silicate melt systems at 3 GPa and 1600 °C: Implications for core–mantle differentiation and degassing of magma oceans and reduced planetary mantles

“…It is widely argued that the log f O 2 of primary Martian basalts is down to IW-1 and therefore carbon is present in the Martian mantle as graphite/diamond (e.g., Herd et al, 2002;Karner et al, 2007;Wadhwa, 2008;Righter et al, 2008). However, the water content of Martian basalts is subject to considerable controversy, with arguments for both quite dry and hydrous primary magmas with hydrogen content up to 3000 ppm but as low as a few ppm (e.g., Filiberto and Treiman, 2009;McCubbin et al, 2010aMcCubbin et al, , 2010bMcCubbin et al, , 2012Gaillard et al, 2012).…”

Silicate mineralogy of martian meteorites

“…Several chronometers including K-Ar, Ar-Ar, Rb-Sr, and Sm-Nd show that Nakhla is 1.2-1.3 billion years old (Stauffer, 1962;Podosek, 1973;Nakamura et al, 1977;Papanastassiou and Wasserburg, 1974), which is quite consistent with all the other martian Clinopyroxenites. Smith et al (1983) first noted high Ni contents in both Nakhla olivine and pyroxene grains and postulated this required relatively oxidizing conditions, near the QFM buffer (e.g., see Righter et al, 2008).…”

Hydrothermal jarosite and hematite in a pyroxene-hosted melt inclusion in martian meteorite Miller Range (MIL) 03346: Implications for magmatic-hydrothermal fluids on Mars

“…First, it is a highly oxidized rock; in fact, it has been referred to as the most oxidized martian meteorite found to date (Dyar et al, 2005) with the mesostasis indicating an oxygen fugacity close to the QFM buffer (Dyar et al, 2005;Righter et al, 2008). This is thought to be the result of an oxidizing magmatic environment, not a product of martian or terrestrial weathering (Dyar et al, 2005;Righter et al, 2008). Yet, the oxidation event appears to have occurred in the late magmatic stage since the cumulus pyroxene cores from MIL 03346 exhibit low ferric iron (Domeneghetti et al, 2006(Domeneghetti et al, , 2007, which indicates that the high ferric iron contents are concentrated in cumulus pyroxene rims and mesostasis.…”