¹Molly C.McCanta,²M. Darby Dyar
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.113978]
¹Department of Earth and Planetary Sciences, University of Tennessee, 1621 Cumberland Ave, Knoxville, TN 37996, United States of America
²Department of Astronomy, Mount Holyoke College, 50 College St, South Hadley, MA 01075, United States of America
Copyright Elsevier

Pyroxene spectral features in the visible near-infrared (VNIR) and mid-infrared (MIR) wavelengths are affected by oxidation resulting from traditional metamorphic processes as well as impact metamorphism. The observed effects are due to modifications in the crystal arising from changes in crystallization temperature or pressure or from substituting Fe³⁺ for Fe²⁺. Highly oxidized pyroxenes from terrestrial mantle xenoliths and shock experiments indicate that the spectral effects of oxidation are greater in clinopyroxene than orthopyroxene because clinopyroxene can accommodate more Fe³⁺ structurally. Changes in clinopyroxene VNIR related to increasing oxidation include a shift in the 0.8 μm absorption band to shorter wavelengths and a strengthening of the Fe²⁺↔Fe³⁺ intervalence charge transfer (IVCT) band, which reduces the band depth of the 1.0 μm feature by ~20%. Although shocked clinopyroxenes are oxidized to similar levels to that seen in the mantle xenoliths, the effects of shock overprint those of oxidation in the VNIR. These include a decrease of ~76% intensity of the 2.35 μm feature and a decrease of ~70% intensity of the 1.0 μm feature. In the MIR, the effects of oxidation and shock are minimal, resulting in a 5% overall decrease in band depth. These shifts and changes can be interpreted as a result of changes in the polyhedra surrounding the Fe cations which reduce crystal field splitting and the order of the crystal structure. Determination of planetary surface composition through VNIR remote sensing methods requires careful consideration of potential changes induced via shock and/or oxidation processes.

¹Brendan A.Anzures,¹Stephen W.Parman,¹Ralph E.Milliken,²Olivier Namur,³Camille Cartier,⁴Sicheng Wang
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.07.024]
¹Brown University, Department of Earth and Planetary Sciences, USA
²KU Leuven, Department of Earth and Environmental Sciences, Belgium
³CRPG/CNRS, University of Lorraine, France
⁴Centre for Mined Land Rehabilitation, Sustainable Minerals Institute, The University of Queensland, Brisbane 4072, Australia
Copyright Elsevier

The NASA MESSENGER mission revealed that lavas on Mercury are enriched in sulfur (1.5-4 wt.%) compared with other terrestrial planets (<0.1 wt.%), a result of high S solubility under its very low oxygen fugacity (estimated ƒO₂ between IW-3 and IW-7). Due to decreasing O availability at these low ƒO₂conditions, and an abundance of S^2-, the latter acts as an important anion. This changes the partitioning behaviour of many elements (e.g. Fe, Mg, and Ca) and modifies the physical properties of silicate melts. To further understand S solubility and speciation in reduced magmas, we have analysed 11 high pressure experiments run at 1 GPa in a piston cylinder at temperatures of 1250 to 1475 °C and ƒO₂ between IW-2.5 to IW-7.5. S K-Edge XANES is used to determine coordination chemistry and oxidation state of S species in highly reduced quenched silicate melts. As ƒO₂ decreases from IW-2 to IW-7, S speciation goes through two major changes. At ∼IW-2, FeS, FeCr₂S₄, Na₂S, and MnS species are destabilized, CaS (with minor Na₂S) becomes the dominant S species. At ∼ IW-4, Na₂S is destabilized, MgS becomes the dominant S species, with lesser amounts of CaS. The changes in S speciation at low ƒO₂affect the activities of SiO₂, MgO and CaO in the melt, stabilizing enstatite at the expense of forsterite, and destabilizing plagioclase and clinopyroxene. These shifts cause the initial layering of Mercury’s solidified magma ocean to be enstatite-rich and plagioclase poor. Our results on S speciation at low ƒO₂ are also applicable to the petrologic evolution of enstatite chondrite parent bodies and perhaps early Earth.