Fe3+ partitioning during basalt differentiation on Mars: insights into the oxygen fugacity of the shergottite mantle source(s).

Abstract:

The partitioning of Fe³⁺ between silicate melts and minerals is a key parameter to understand magmatic processes, as it is directly linked to oxygen fugacity (fO₂). fO₂ is, a priori, not a constant during magmatic processes, and its evolution depends on the compatibility of Fe³⁺. We have experimentally determined the partition coefficients of Fe³⁺ between augite, pigeonite, and silicate melt, and use them to constrain the fO₂of the martian mantle and of differentiated martian basalts.

A series of experiments on various martian basaltic compositions were performed under controlled fO₂ in one-atmosphere gas-mixing furnaces. Fe³⁺/Fe_total ratios in silicate melts and pyroxenes were determined using synchrotron Fe K-edge XANES on the 13 IDE beamline at APS (Argonne). Fe³⁺ mineral/melt partition coefficients (DFe³⁺) for augite and pigeonite were obtained with a relative uncertainty of 10-15 %. Both are constant over a wide range of oxygen fugacity (FMQ-2.5 to FMQ+2.0). DFe³⁺ for augite and pigeonite are broadly consistent with previous data by [1], but DFe³⁺ for augite is significantly higher (by a factor of 2) than the indirect determinations of [2]. Since augites in [2] are extremely poor in iron compared to ours (0.18 wt% vs 13 wt% FeO), this strongly suggests that DFe³⁺ varies with Mg#, indicating that Fe³⁺is more compatible than previously thought in terrestrial mantle pyroxenes (3 wt% FeO) as well.

Crystallization paths for shergottite parental melts have been calculated using the MELTS software, combined with our partition coefficients. fO₂ in the residual melts is calculated from the models of [3] and [4]. It stays relatively constant at high temperatures, but increases very strongly during the latest stages of crystallization. These results explain the large range of fO₂ determined in enriched shergottites. In order to estimate the fO₂ of the martian mantle, only the highest temperature phases in the most primitive martian samples should be used. The most primitive shergottites record a mantle fO₂ around FMQ-2.5, consistent with the lowest fO₂estimated for surface basalts [5].

[1] McCanta et al. (2004) Am Min 89:1685-1693; [2] Mallmann and O’Neill (2009) J Petrol 50:1765-1794; [3] Righter et al. (2013) Am Min 98:616-628; [4] Kress and Carmichael (1991) CMP 108:82-92; [5] Schmidt ME et al. (2014) EPSL 384:198-208.