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 fO2 ~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 FeOtot 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 FeOtot content of the melt decreases (from 2.8 to 0.9 wt.% FeOtot), 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.