Raman spectroscopy has been employed to determine the solubility mechanisms of H2O in silicate melts. In melts that have a three-dimensional network structure (e.g., melts on the join Na2O−Al2O3−SiO2), water reacts with bridging oxygens to form two OH groups per broken oxygen bond. At the same time some of the three-dimensional network is broken down to chain units, accompanied by the expulsion of Al3+ from tetrahedral coordination. In melts that have nonbridging oxygen (NBO), water reacts with both nonbridging oxygen and network modifiers (e.g., Na+) to form Si−OH bonds and M(OH) or M(OH)2 complexes. The anhydrous portion of the network becomes more polymerized.
The formation of chain units at the expense of three-dimensional network units in melts implies that the liquidus boundaries involving pyroxenes and silica minerals or feldspar minerals shift to higher silica contents. Liquidus fields of silica minerals or feldspar minerals are depressed relative to those of pyroxene minerals. This prediction is supported by published observations of phase relations in hydrous basalt and andesite systems. Similar logic can be used to explain the formation of partial melts of andesitic bulk composition from hydrous peridotite in the upper mantle.
We propose that trace-element crystal–liquid partition coefficients involving highly polymerized melts will decrease with increasing water content because of the formation of the less polymerized chain units in the melt. Partition coefficients involving less polymerized melts (e.g., picrite and komatiite) may increase because the degree of polymerization of the melt is increased as a result of dissolved water.