Chlorine diffusion in rhyolite under low-H2O conditions

Abstract

The chlorine diffusivity in rhyolitic melts containing <1.2 wt% H2O was determined at 650 °C–950 °C and 1.5–11.8 MPa. Diffusion experiments were carried out by reacting rhyolitic obsidian with four types of chlorine sources including a pure Cl2 fluid, Cl2 + H2O fluid mixture, molten NaCl, and NaCl aqueous solution. In the pure Cl2 experiments, the chlorine diffusivity ranged from 7.7 × 10−18 m2/s (650 °C) to 8.2 × 10−16 m2/s (950 °C); it is much lower than previously reported values for rhyolitic melt. The Cl2 + H2O fluid mixture experiments revealed that the chlorine diffusivity strongly depends on the water content: it increased by 1–2 orders as the water content of the melt increased to ~1 wt%. In the molten NaCl experiments, sodium infiltrated into the melt while potassium diffused out. The chlorine diffusivity was higher than that in the pure Cl2 experiments by a factor of 4–10. A single experiment using NaCl solution as a chlorine source resulted in the highest Cl diffusivity, probably due to the high H2O and sodium contents. The activation energy of chlorine diffusion was 135–202 kJ/mol, and it is comparable to the activation energy in more mafic melts. Because the diffusivity of chlorine is much smaller than that of H2O and CO2, a strong diffusive fractionation is expected to occur during magma vesiculation and degassing. Calcium diffusion was observed as a by-product of chlorine diffusion under anhydrous conditions. In the pure Cl2 experiments, calcium diffusivity ranged from 6.2 × 10−17 m2/s (650 °C) to 3.9 × 10−14 m2/s (950 °C), and the activation energy was 177 ± 15 kJ/mol. In molten NaCl experiments, the calcium diffusivity was higher by one order of magnitude, and the activation energy was similar to that for the pure Cl2 experiments.