Argon solubility drop in silicate melts at high pressures: A review of recent experiments

Controversy surrounding Ar solubility in silicate melts at high pressure (HP) has led us to perform a suite of Ar solubility experiments on silicate melts with a variety of compositions up to ~ 25 GPa. These results of these experiments strongly constrain the structure of the melts at HP, and confirm the features first observed in olivine composition melts, resolving the controversy over the solubility of Ar in silicate melts at the upper mantle conditions.The Ar solubility experiments show that argon contents in aluminosilicate melts reach a maximum that persists to pressures as high as 17 GPa well above that observed for Al-free melts. A subsequent drop in argon solubility correlates well with the expected void loss in the melt structure predicted by molecular dynamics simulations [Nevins, D. and Spera, F.J. (1998). Molecular dynamics simulations of molten CaAl2Si2O8: dependence of structure and properties on pressure, Am. Mineral. 83: 1220–1230]. This discontinuous drop for argon solubility at high pressure should be a universal behaviour of silicate melts, and our experiments show that the onset of pressure for reduced argon solubility has a positive correlation with the Al/(Al + Si) ratio of the melts. Thus the experimental data for Ar solubility are entirely consistent with known natural samples from upper mantle conditions.