Effect of sulfur speciation on chemical and physical properties of very reduced mercurian melts. (1st October 2020)

• Record Type: Journal Article
• Authors: Anzures, Brendan A.; Parman, Stephen W.; Milliken, Ralph E.; Namur, Olivier; Cartier, Camille; Wang, Sicheng
• Is Part Of: Geochimica et cosmochimica acta. Volume 286(2020)
• Page Start: 1
• Page End: 18
• Publication Date: 2020-10-01
• Subjects: XANES spectroscopy -- Oxygen fugacity -- Sulfide saturation -- Sulfide speciation; Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2020.07.024
• Languages: English
• ISSNs: 0016-7037

Abstract

Highlights: At low ƒO2 conditions sulfur acts as an important anion. In very reduced basaltic melts (<IW-2), FeS is destabilized in the melt structure. CaS and Na2 S are the primary sulfur species from IW-2 to IW-4. MgS becomes the dominant species at oxygen fugacities below IW-4. Mercury's solidified magma ocean should be enstatite-rich and plagioclase poor. Abstract: 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 ƒO2 conditions, 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 1250–1475 °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, FeCr2 S4, Na2 S, and MnS species are destabilized, CaS (with minor Na2 S) becomes the dominant S species. At ∼IW-4, Na2 S is destabilized, MgS becomes the dominant S species, with lesser amounts of CaS. The changes in S speciation at low ƒO2 affect 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.