An experimental study into the behaviour of sulfur during Martian differentiation

A series of experiments related to igneous processes within Mars are presented, with a particular emphasis on the behaviour of sulfur during Martian magmatism. Of most significance are a series of one-atmosphere experiments conducted on a broad range of silicate melt compositions, under oxygen fugacity (fO2) conditions ranging from FMQ-1.67 to FMQ+1.6: a range which brackets much of the variation observed in Terrestrial and Martian igneous rocks. Quenched glasses from these experiments have been analysed by X-ray absorption Near Edge Spectroscopy (XANES) and Secondary Ion Mass Spectrometry (SIMS) to determine the speciation of their dissolved sulfur, as well as its overall concentration. The XANES spectra collected indicate the presence of the sulfide and sulfate species, and the progressive replacement of the former by the latter as fO2 increases. Clear differences are observed between the S6+/ΣS ratios of different silicate melts equilibrated under the same fO2 conditions, demonstrating that the major-element composition of a silicate melt has some measureable influence over sulfur speciation. The component most closely correlated with this change is FeO, whose presence in the silicate melt shifts the exchange between sulfide and sulfate towards higher fO2 conditions. This is interpreted as an outcome of coupling between sulfur and iron in the melts, and on this basis a quantitative relationship between log S6+/ΣS and log (Fe2O3/FeO2) is proposed. Such a relationship potentially provides an alternative to direct measurement of S6+/ΣS, but warrants further study to establish its generality. SIMS analyses of the most oxidized melts indicate a strong inverse correlation between their sulfate concentration and the abundance of the tetrahedrally coordinated cations Si and Ti, suggesting that sulfate solubility is governed by competition for tetrahedral sites in silicate melts. Additional experiments performed at 1.5GPa establish the sulfur concentration at sulfide saturation (SCSS) of a putative Martian primary melt at 1400°C. Experiments performed at the same pressure but under more oxidizing conditions constrain the sulfide-sulfate transition to between -1.5 and +0.8 log units relative to the Fayalite-Magnetite-Quartz (FMQ) buffer. This inference is supported by XANES spectra for these experiments, which show no evidence for mixed speciation. In addition to sulfur, the elements Sc, Y and a selection of REEs were also investigated at 1.5GPa. Partitioning coefficients were measured for these elements between a likely primary Martian melt, and three phases believed to be present in the Martian mantle: high- and low- calcium pyroxene and Fo76 olivine. The values obtained closely approximate those predicted by the lattice-strain model, suggesting that these elements partitioning behaviour is unaffected by the unusually Ferich chemistry of this melt. REE profiles generated by a simple partial melting model demonstrate the complexity of Martian meteoritesâ petrogenesis, including the need for chemically separate reservoirs within Marsâ mantle. To assist in further investigations of Martian magmatism, in particular those involving sulfur, a method is presented for the manufacture of polycrystalline olivine capsules. These are suitable as containers for olivine-saturated melts in one-atmosphere experiments, and permit studies involving a wider range of melt compositions than those allowed by natural olivines (the presently used alternative). The equipment required for manufacture is readily available in most experimental petrology laboratories. Use of these capsules avoids common problems associated with using wire loops, such as their propensity to absorb iron, or to react with the sulfur in the experimental charge.