Volatile Solubility in Silicate Melts: Implications for Degassing and Rejuvenation of Silicic Magma Chambers

Tuttle and Bowen 1958 were among the first scientists emphasizing the role of water on the generation of partial melting. In their memoir they do not only report on phase equilibria in water-bearing granitic systems, but also on water solubility in rhyolitic melts. In the last two decades, special attention was given to water solubility in silicate melts and the pioneering water solubility data of Tuttle and Bowen (based on the effect of water on the refraction index of glasses) were perfectly reproduced with sophisticated methods. The investigation of volcanic gases and of glass inclusions in minerals clearly demonstrated the presence of additional volatile species in silicate melts. Accurate models for volatile solubilities in silicate can be used to predict magma degassing conditions (from concentrations of CO2 and H2O in glass inclusions), to estimate the depth of magma storage and to model processes occurring in magma chambers as a result of replenishment, mixing or gas sparging events.

The solubility of C-O-H-bearing fluids was investigated in a variety of felsic to mafic melts (rhyolite, dacite, andesite, ferrobasalt, tholeiitic and alkali basalts, tephriphonolite) in the pressure range 200 of 500 MPa. The C-O-H solubility varies strongly as a function of the melt composition (relative variations up to 100%), implying that the solubility of C-O-H fluids predicted by the existing empirical and thermodynamic models for rhyolites and basalts can not be extrapolated to other compositions. Furthermore, the data illustrate that the solubility of CO2 is strongly decreasing with decreasing fO2 at reducing conditions (QFM – 1 log unit). The role of the anhydrous composition and of fO2 on volatile incorporation mechanisms in silicate melts coexisting with mixed fluids is still not explored sufficiently to be applied systematically to natural systems and is a challenge for future studies.