Chlorine partitioning between a basaltic melt and H2O-CO2 fluids.

Partitioning experiments between a basaltic melt from Mt. Etna and a low-density hydrous fluid or vapor containing H2O or H2O–CO2 were performed at 1200–1260 °C, at pressures between 1 and 200 MPa, either near the nickel–nickel oxide (NNO) buffer or at two log units above it (NNO+2), and with different chloride concentrations. Most of the experiments were done at chloride-brine-undersaturated conditions, although at the highest Cl concentrations explored brine saturation might have been reached. The average partition coefficients (DCl fluid/melt) over the range of Cl concentrations were derived on a weight basis by plotting the calculated concentrations of Cl in the fluid phase versus the measured ones in the melt. For H2O–Cl experiments in which the Cl concentration in the melt was ≤0.4 wt.%, a negative dependence between DClf/m and pressure is observed. DClfluid/melt in H2O+Cl-bearing experiments ranges between 11–14 at 1 and 25 MPa to 6 at 200 MPa at NNO; and between 4 at 50 MPa and 13 at 100 MPa at ΔNNO≥2. Addition of CO2 at NNO yields lower partition coefficients than in CO2-free conditions over the pressure range investigated. The negative pressure dependence observed for H2O–Cl experiments disappears when CO2 is present in the system. Overall, once CO2 is introduced in the system, Cl fugacity in the silicate melt tends to increase, thus resulting in a decrease of DCl f/m. Application to Mt. Etna shows that the composition of the volcanic plume in terms of Cl records very shallow pressures of magma equilibration with its exsolved fluid.