176 D6. Noble Gases and Halogens to Unravel Crustal Processes and Their Control on Energy Resources

Tuesday, 11 October 2022
1:30 PM - 5:30 PM
601

CHLORINE IN SILICATE MELTS: CONTROLS ON DISSOLUTION, EVAPORATION AND BRINE EXSOLUTION

Paper number: 176-3
Program: Technical Sessions
Time: 1:50 PM – 2:05 PM

Authors

Abstract

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(𝐢𝑙2), 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 βˆšπ‘“(𝐢𝑙2). This implies that Cl- replaces O2- in the silicate framework as follows:

Cl2(gas)+O2-(melt)=2Cl-(melt)+Β½O2(gas) [1]

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

CCl = (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.

Geological Society of America Abstracts with Programs. Vol. 54, No. 6

doi: 10.1130/abs/2022AM-376429

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