Sulfur and chlorine solubility in Mt. Unzen rhyodacitic melt at 850 °C and 200 MPa

Abstract

The solubilities of S and Cl have been determined at 850 °C, 200 MPa, and fO 2 ∼NNO in a rhyodacitic melt of the Unzen volcano in equilibrium with a fluid phase of mixed composition (H–O–Cl–S). The partitioning of Cl between silicate melt and fluid shows a strong deviation from ideal behavior. The Cl content of melts coexisting with fluids containing up to 50 mol% Cl is approximately 0.85 wt.%. With the addition of sulfur, the Cl concentration decreases by 0.2 wt.% Cl (2 wt.% S added in the system; ∼50 mol% Cl in fluid). This effect is most probably explained by a more ideal mixing of the Cl-bearing species in S-bearing fluids. However, changes of melt composition with the addition of S (depletion of iron) can also influence Cl solubility. The solubility of sulfur in the melt is a complex function of the activity of fluid components, the FeO tot content of the melt, and the speciation of S. With increasing S content in the system from 0 to 2 wt.% S, the amount of pyrrhotite (Pyr) increases (up to 2.2 vol.%), the FeO tot content of the melt decreases (from 2.8 to 0.9 wt.% FeO tot), and S solubility increases slightly (up to 100 ppm). At the investigated conditions in the Cl-bearing systems, S solubilities up to 200 ppm have been determined, indicating that the addition of Cl to the system enhances S solubility. The influence of Cl on the solubility of S can be related to nonideal mixing in the fluid, in particular, the increase in activity of S-bearing fluid species and a decrease of H 2S/SO 2 ratios in the fluid. The first effect leads to higher solubility of sulfide in the melt and the second one favors the dissolution of additional sulfate in the silicate melt at studied conditions.