Sulfur Concentration of Martian Magmas at Sulfide Saturation at High Pressures and Temperatures - Implications for Martian Magma Ocean and Magmatic Differentiation

Sulfur is critical for a wide range of processes of terrestrial planets including thermal evolution of core and atmosphere and geochemistry of mantle and crust. For Mars, sulfur is particularly important because it may be abundant in the core [1] while SO ₂ and H₂ S might have exerted a strong greenhouse climate in the past [2]. A critical parameter that affects sulfur distribution during differentiation is the sulfur carrying capacity of mantle melts. However, most experiments constraining sulfur content at sulfide saturation (SCSS) are conducted on FeO poor (~5-12 wt.%) basalts [3] and recent experiments on high-FeO (~16-22 wt.%, [4]) Martian basalts are restricted to ≤0.8 GPa [5]. To constrain SCSS of Martian magmas at mantle conditions, we simulated basalt-sulfide melt equilibria (S added as 15-30 wt.% FeS) in Gr capsules using a piston cylinder at 1-3 GPa and 1500-1700 °C. Two starting compositions, equivalent to olivine-phyric shergottites Yamato980459 (Y98; ~17.53 wt.% FeO) and NWA 2990 (NWA; ~16.42 wt.% FeO) and thought to be primary magma [6] were used. A composition Y98+1.4 wt.% H₂O was also explored to constrain the effect of water on SCSS. All experiments produced quenched sulfide and silicate melts ± opx . FeS species in the NWA glasses was confirmed from peaks at 300-400 cm^-1 in Raman spectra [7]. At 1600 °C, SCSS, measured using EPMA, decreases with pressure, 4800 to 3500 ppm from 1 to 2.5 GPa for Y98, ~5440 to 4380 ppm from 1 to 2 GPa for Y98+1.4 wt.% H₂O, and 5000 to 3000 ppm from 1 to 3 GPa for NWA. At 2 GPa, SCSS of NWA increases with temperature, 3300 to 4600 ppm from 1500 to 1700 °C. Combining new and previous experiments on Martian basalts [5] (a total of 28 SCSS data with FeO* of 9.3-32.78 wt.%), a preliminary equation of the form LnS (ppm) = a + b.P + c/T +d.XSiO₂ + e.XAl₂O₃ + f.LnXFeO was fitted, where P is in GPa, T in K, and X represents mole fraction of a given oxide. Our study suggests that at conditions of final melt-mantle equilibration [6], primary Mars basalts likely have ~2970-3480 ppm S. This implies that shergottites with 1300-2700 ppm S [4] might have either experienced sulfur degassing during the ascent thereby causing the Noachian greenhouse conditions by sulfur-rich gases or sulfide residue separation during segregation from deeper mantle sources. Furthermore, the inverse relationship of SCSS and pressure suggests that Martian magma ocean may have gained sulfur through interaction a sulfur-rich atmosphere and depositing sulfide phases at deeper part of the magma ocean. References: [1] Stewart et al. (2007) Sci., 316, 1323-1325. [2] Johnson et al. (2008) JGR, 113, 1-15. [3] Holzheid and Grove, AM, 87, 227-237. [4] Meyer (2008). Mars Meteor. Comp. [5] Righter et al. (2009) EPSL, 288, 235-243. [6] Filiberto and Dasgupta (2011) EPSL, 304, 527-537. [7] Klimm and Botcharnikov (2010) AM, 95, 1574-1579.