CHLORINE IN SILICATE MELTS: CONTROLS ON DISSOLUTION, EVAPORATION AND BRINE EXSOLUTION (Invited Presentation)

The halogens, particularly chlorine, are important constituents of volcanic systems and volcanic gases stabilising, in solution, metal cations and playing a key role in ore-formation. In melts, Cl is, on a molar basis, twice as effective as H₂O at suppressing liquidus temperatures. In this study, we report experimentally-determined thermodynamic activities of Cl in silicate melts, measured using an Ag-AgCl buffer that controls the Cl fugacity in the experiment. Because Ag is virtually insoluble in silicate melt the presence of the Cl^- buffer does not perturb its major-element composition. We performed experiments at pressures and temperatures from 5-20 kbar and 1200-1500 °C, respectively, in a piston-cylinder apparatus. The effect of oxygen fugacity on Cl solubility was determined by employing either Re-ReO₂ or C-CO₂ oxygen buffers, in addition to the Cl buffer. Our experiments show that: (1) Cl solubility in haplobasalt at 15 kbar/1400°C can reach 5 wt%, even at Cl₂ fugacities, 𝑓(𝐶𝑙₂), as low as 0.0035 bar. (2) Cl concentration increases linearly with the square root of Cl fugacity at fixed oxygen fugacity, f(O₂), obeying Henry’s Law up to ~3% Cl in haplobasaltic melt. (3) Cl solubility decreases with the fourth root of oxygen fugacity at fixed √𝑓(𝐶𝑙₂). This implies that Cl^- replaces O^2- in the silicate framework as follows:

Cl₂^(gas)+O^2-(melt)=2Cl^-(melt)+½O₂^(gas) [1]

The equilibrium constant of equation [1] equates to the chloride capacity:

C_Cl = (Cl/√f(Cl₂)) x ⁴√f(O₂) [2]

Equation [2] was used to parameterise Cl content as a function of pressure, temperature, the fugacities of Cl₂ and O₂ and variations in major element composition. We found that Cl content increases with temperature and decreases with pressure while Ca, Fe, K and Si are the most important compositional controls on chloride solubility, and <4.3 % H₂O has negligible effect. Parametrisation of the Cl capacity of silicate melts with regards to their chemical composition, temperature and pressure dependence results with a simple model, ChlorCalc. We find that Cl degasses late, and at shallow depths (0-5MPa) during magma ascent; and basaltic, silica poor, melts exhibit higher Cl solubilities, and thus lower tendences to degas HCl, than silica-rich melts which are poorer in Ca and Fe.

Finally, experiments were conducted at NaCl saturation to determine the compositions of fluids that interact with silicate melt. Our results show a linear relationship between NaCl activity and Cl content and that basalt dissolves approximately 8 times more Cl than rhyolite at a given NaCl activity.