The fate of sulfide during igneous processes of terrestrial planetary bodies
Doctor of Philosophy
In order to constrain the S flux from Martian mantle to crust and atmosphere by magmatism, we simulated basalt-sulfide melt equilibria using two synthesized Martian meteorite compositions in both anhydrous and hydrous conditions at 1–5 GPa and 1500–1700 ̊ C (Chapter 2). We developed an empirical parameterization to predict Martian basalt SCSS (sulfur content at sulfide saturation) as a function of depth, temperature, and melt composition. Our SCSS model suggests that sulfur-rich gases released by magmatism might have caused the greenhouse conditions during the late Noachian. SCSS of Martian magma ocean was at least six times greater than in the Earth magma ocean, which might have been key to make Mars a planet richer in sulfur compared to Earth. We also measured bulk S concentration of 7 Martian meteorites and modeled the behavior of S during both isobaric crystallization of primary Martian magmas and isentropic partial melting of Martian mantle (Chapter 3). Our modeling indicates that in addition to degassing induced S loss, mixing between basaltic melts and cumulus minerals could also be responsible for the bulk sulfur contents in some Martian meteorites. In this case, a significant quantity of S could remain in Martian crust. Our modeling also suggests that the mantle source of Martian meteorites contain <700–1000 ppm S if melting degree estimation of 2–17 wt.% based on compositions of shergottites is relevant. In order to constrain the sulfur budget of the Earth mantle, and sulfur and chalcophile element systematics of primitive mid-ocean ridge basalts (MORB) and oceanic island basalts (OIB), we developed a model to describe the behavior of sulfide and Cu during decompression melting in mid-ocean ridge (Chapter 4) and ocean islands (Chapter 5). We coupled thermodynamic models and experimental constraints on isentropic decompression melting at different mantle potential temperatures with existing empirical SCSS model developed for basalts on Earth. The goal was to track the S contents in the partial melt and the mode of residual sulfide as a function of initial sulfur content of the mantle source, melting degree, depth, temperature, and change in partial melt composition. The fractionation of Cu, a chalcophile element, is also modeled to derive an internally consistent set of inferences about the geochemistry of both S and Cu in partial melt parental to MORB and OIB. Our modeling suggests that primitive MORBs (Mg#>60) with S and Cu mostly concentrated in 800-1000 ppm and 80-120 ppm are likely mixture of sulfide undersaturated high degree melts and sulfide saturated low degree melts derived from depleted peridotite containing 100-200 ppm S. Our modeling also suggests that mixing of eclogite partial melt with peridotite partial melt is critical to reconcile S and Cu contents in low-F (<10%) OIBs. In order to constrain SCSS of intermediate to high-Ti mare basalts, we conducted basalt-sulfide melt equilibria experiments using a piston cylinder at 1.0-2.5 GPa and 1400-1600 °C, on two compositions similar to high- and intermediate-Ti mare basalts (Chapter 6). We derived a new SCSS parameterization for high-FeO* silicate melts of variable TiO2 content, which can be used to predict SCSS of any mafic-ultramafic system within the calibration range, including VLT (very low Ti) to high-Ti lunar basalts as well as Martian basalts. We constrained that the mass fraction of sulfide in the mantle cannot exceed 0.03 wt.% (120 ppm S), and probably ≤0.025 wt.% (90 ppm S), which falls near the lower end of the S abundance in depleted terrestrial mantle similar to MORB source and is in agreement with previous calculations of highly siderophile elements depletion in the bulk silicate moon.
sulfide; igneous processes