Solubility of H 2O- and chlorine-bearing fluids in basaltic melt of Mount Etna at T = 1050–1250 °C and P = 200 MPa

Abstract

An experimental study on the solubility of H–O–Cl-bearing fluids in basaltic melts has been conducted at total pressure of 200 MPa, in the temperature range from 1050 to 1250 °C and at redox conditions corresponding to lof fO 2 ∼ QFM + 1 (QFM = quartz–fayalite–magnetite oxygen buffer). A natural trachybasaltic sample of Mount Etna's July–August 2001 eruption has been used as starting material. Water concentrations in the melt (H 2O melt) increase from 4.6 up to 5.4 wt.% with an increase in Cl concentration in the melt (Cl melt) from 0 to 2.5 wt.%. The H 2O melt dramatically decreases to ∼ 2–3 wt.% at Cl melt > 2.5 wt.%. The saturation of the melt with Cl is reached at 4 wt.% Cl melt which is significantly higher than predicted by the existing models. Although increasing total Cl concentration in the system (bulk Cl in fluid and melt) leads to significant extraction of cations like Na, K and Fe from the melt into the fluid(s), the observed systematic increase in H 2O melt cannot be directly attributed to the change in melt composition, indicating that the presence of Cl may decrease the activity coefficient of dissolved H 2O in the melt. The complex non-linear relationships between H 2O and Cl concentrations in basaltic melt and fluid phase point to a strong non-ideality of mixing between fluid components, presumably resulting in an increase in the activity coefficient of Cl in the fluid and in the formation of two fluid phases (vapor and liquid) at the studied conditions. The effect of temperature on the solubility of H–O–Cl-bearing fluids in trachybasaltic melt and, hence, on the mixing properties of the fluid phase is not resolved from the obtained experimental data. The experimentally determined partition coefficient of Cl between fluid(s) and melt ( K d′Cl, based on a molar ratio) exponentially increases from 0.04 up to 14 with increasing Cl melt and shows almost linear dependence on bulk Cl content of the fluid(s). The K d′Cl values < 1 are observed at Cl melt lower than 1.0 wt.%, explaining the almost constant Cl concentrations (∼ 0.2 wt.%) in melt inclusions and variations of H 2O/Cl and S/Cl ratios in melt inclusions and volcanic gasses with ongoing magma degassing and changes in eruptive activity of Mt. Etna. Thus, the obtained data represent an experimental database necessary for the accurate modeling of natural processes of magma degassing at basaltic volcanoes.