Sulfur extraction via carbonated melts from sulfide-bearing mantle lithologies - Implications for deep sulfur cycle

Transport of sulfur via mantle-derived partial melts from deep Earth to the surface reservoirs is a critical step in the deep global sulfur cycle. Given sulfur is stored mostly in sulfide phases in mantle lithologies, the critical parameter is sulfur concentration at sulfide saturation (SCSS) of mantle-derived magmas. At Earth's mantle beneath oceans and continents the largest volume of rocks is affected by CO₂±H₂O-induced melting [1]. However, although SCSS of wide ranging silicate melts is extensively studied [e.g., 2], the SCSS of carbonatitic and carbonated silicate melts are yet to be obtained.

We performed piston-cylinder and multi-anvil experiments in graphite capsules at 2.5-6.0 GPa and 1350-1650 °C to investigate the SCSS of carbonatitic and carbonated silicate melts. All experiments produced quenched Fe±Ni-sulfide melt blobs + carbonated melt matrix ± ol ± cpx ± opx ± gt, with melt composition on a CO₂-free basis varying from 9 to 37 wt.% SiO₂, 1 to 6 wt.% Al₂O₃, and 4 to 11 wt.% FeO*. SCSS measured using EPMA increases with SiO₂, FeO*, and T but decreases with P; the effect of composition is more pronounced than P-T. With 33 wt.% Ni in the sulfide phase, the SCSS is 800-3000 ppm whereas for Ni-free sulfide-melt equilibria it is 2000-4000 ppm as melt composition evolves from carbonatitic to a silicate melt with 15-18 wt.% CO₂. Comparison of our measured SCSS with the existing SCSS models for silicate melts show that the latter fail to capture the solubility of S in CO₂-rich melts. Using a new SCSS model, we constrained the efficiency of S extraction from the mantle beneath mid-oceanic ridges and continents via low-degree carbonated melts. Deep carbonated melts beneath ridges are expected to mobilize 5-15% of the initial sulfur before nominally-volatile-fee peridotite melting begins. In continental mantle, deep kimberlitic melt can act as an agent to enrich the shallow mantle in sulfide as it evolves to a carbonatitic melt upon reactive cooling. Finally, application of our data to subduction zones suggests that low-degree carbonatitic melt is not an efficient agent to strip-off residual sulfide from the subducting oceanic crust.

[1] Dasgupta, 2018, AJS [2] Ding et al., 2018, GCA