NOBLE-GAS SOLUBILITIES IN SILICATE MELTS AND GLASSES - NEW EXPERIMENTAL RESULTS FOR ARGON AND THE RELATIONSHIP BETWEEN SOLUBILITY AND IONICPOROSITY

New measurements of the solubility of Ar in basaltic, rhyolitic, ortho clasic, and albitic melts and glasses at Ar pressures of 250-10,000 ba r and temperatures of 400-1300 degrees C are presented and combined wi th other solubility measurements for a wider range of melt composition s to parameterize the effects of pressure, temperature, and melt compo sition on Ar solubility. Argon solubility in melts and glasses is roug hly linear with Ar pressure under these conditions. At near-liquidus t emperatures, solubility in melts is approximately independent (within similar to 10%) of temperature, while some results below 600-700 degre es C show an increase in solubility with decreasing temperature, perha ps reflecting differences in the nature of Ar solubility in glasses an d melts. There is also a positive, linear correlation between the ''io nic porosity'' of melt and the logarithm of Ar solubility. This correl ation is better than previously noted correlations between inert gas s olubility and melt density and volume, and provides a useful means of predicting how Ar solubility varies with melt composition. The solubil ities of He, Ne, Kr, and Xe are also positively correlated with ionic porosity, but are increasingly sensitive to ionic porosity as the size of the gas atom increases, suggesting that with more efficient packin g of the melt structure the availability of sites that can incorporate inert gas atoms decreases more rapidly for larger atoms than for smal ler atoms. Comparison of inert gas solubilities with those of molecula r CO2 and molecular H2O in rhyolitic melts shows that solubilities dec rease in the order H2Omol much greater than He > Ne > Ar > CO2,(mol) a pproximate to Kr > Xe. The much higher solubility of molecular H2O com pared to the other neutral gas species (and molecular CO2) suggests th at it is not merely passively occupying interstitial ''holes'' in the melt structure as is thought to be the case for the rare gases (and li kely for molecular CO2), but rather it is stabilized in the melt struc ture by chemical bonds (e.g., by hydrating cations or through hydrogen bonds).