Noble gas solubilities in silicate melts: New experimental results and a comprehensive model of the effects of liquid composition, temperature and pressure

Abstract

New experimental data of Ar and Ne solubility at pressures up to 360 MPa in alkali-basaltic (Mt. Etna, Italy) and rhyolitic (Vulcano Island, Italy) melts are presented. Solubility experiments have been conducted in internally heated pressure vessels at 1200 °C under nominally anhydrous conditions. Ar and Ne contents dissolved in the experimental glasses were then measured by quadrupole mass spectrometry. Over the pressure range investigated, Ar and Ne solubilities vary linearly with Ar and Ne pressures and can be described by Henry's constant (k Ar,Ne = P Ar, Ne / x Ar, Ne, where P Ar, Ne is the partial pressure of Ar or Ne and x Ar, Ne is the molar fraction of Ar or Ne in the melt) of 7.6 ± 0.8 × 10 5 and 1.9 ± 0.4 × 10 5 MPa, respectively for Ar and Ne in the basaltic melt and 1.5 ± 0.2 × 10 5 and 3.8 ± 0.2 × 10 4 MPa, respectively for Ar and Ne in the rhyolitic melt. In accordance with existing models, rhyolitic melts show higher noble gas solubilities than basaltic melts, Ne solubility being higher than that of Ar in a given composition. We propose a semi-empirical model of noble gas (Ar, Ne and He) solubility calibrated on a very large set of measurements in natural and synthetic silicate melts. The model expands the concept of ionic porosity in terms of porosity accessible for noble gas dissolution in melt, taking into account the large-scale structural effects of cations, as well as temperature and pressure. The model is valid over a wide range of temperatures (800–1600 °C), pressures (up to 3 GPa) and compositions, being useful for both geological and physico-chemical studies.