The thermodynamic controls on sulfide saturation in silicate melts with application to Ocean Floor Basalts.

The sulfur solubility minimum and maximum in silicate melt

“…The second reaction describes a heterogeneous equilibrium between silicate melt (m) and vapour that governs the dissolution of sulfur from the vapour as S 2in the silicate melt (e.g., Fincham and Richardson 1954;Moretti andOttonello 2005, 2003;Moretti and Papale 2004;Scaillet 2009, 2014;Baker and Moretti 2011;Gaillard et al 2011Gaillard et al , 2013Gaillard et al , 2015Baumgartner et al 2017;Moretti 2021) where a m i, x m i, and w m i are the activity, mole fraction, and weight fraction, respectively, in the silicate melt of the i th ion (either S 2or oxide [O 2-], in this case); and CS2-is referred to as the sulfide capacity (e.g., Fincham and Richardson 1954;O'Neill 2020). The final term of equation (4b) makes the approximation that a m i can be replaced by x m i, and this approximation is adopted throughout.…”

“…When the silicate melt is saturated with sulfide melt, the chemical potential of FeS in the silicate melt (μ m FeS) and the sulfide melt are equal. The "sulfide content at sulfide-saturation" (S 2-CSS) is the dissolved S 2concentration in the silicate melt in equilibrium with sulfide melt (e.g., Shima and Naldrett 1975;O'Neill and Mavrogenes 2002;Smythe et al 2017;O'Neill 2020):…”

“…Although we are confident that the trends and insights derived from this choice are robust, the exact behaviour depends strongly on the chosen values of CS2-and CS6+. Therefore, the specific values of various variablesincluding the precise values of fO2 where shifts in behaviour are predicted to occurwill likely vary strongly with T, composition of the silicate component, and the CS2-andCS6+ parameterisation used (e.g., O'Neill andMavrogenes 2002, 2019;Moretti and Ottonello 2005;Nash et al 2019;O'Neill 2020;Boulliung and Wood 2021;Moretti 2021).…”

“…Melt inclusions from arcs and ocean islands can be S-rich (up to ~5000 ppm ST), hydrous (up to ~6 wt% H2O for arcs and ~3 wt% H2O for ocean islands), and relatively oxidised (up to +3 FMQ) (e.g., Wallace 2005;Moussallam et al 2019;Cottrell et al 2021) and may therefore provide useful fO2 estimates based on this technique. However, this will require accurate knowledge of CS2-, CS6+, S 2-CSS, and S 6+ CSS at the relevant conditions (especially the effects of T and silicate melt compositionincluding the influence of H2O), as these parameters strongly influence the fO2 of the transition from S 2--to S 6+ -dominated silicate melt (e.g., O'Neill andMavrogenes 2002, 2019;Li and Ripley 2005;Moretti and Ottonello 2005;Baker and Moretti 2011;Chowdhury and Dasgupta 2019;Nash et al 2019;Zajacz and Tsay 2019;O'Neill 2020;Boulliung and Wood 2021).…”

“…Despite its potential importance, the existence of an SS min with varying fO2 has been somewhat mischaracterised in the literature. As emphasized by O'Neill (2020), this is at least in part due to a lack of clarity regarding the independent variables and the path followed by sulfur fugacity (fS2) with increasing fO2 for specific natural or experimental processes (e.g., and the number and identity of additional S-bearing phase(s) with which the silicate melt is saturated (e.g., Jugo et al, 2005). In this paper, we explore the conditions and paths for which an SS min occurs and some of the implications for magmatic and volcanic processes.…”