The density of andesitic melts and the compressibility of dissolved water in silicate melts at crustal and upper mantle conditions

•First in situ measurements of the density of dry and hydrous andesitic melts.•A thermal equation of state was derived from the experimental data.•Partial molar volume of dissolved water is independent of silicate melt composition.

Many magmatic processes, including the extraction of partial melts, fractional crystallization and buoyancy overpressure, depend on the density of the silicate melt phase. Because of its low molecular weight, water has a disproportionately large effect on melt density and accurate data on the effect of water on melt density is required to model density-dependent processes. However, the volumetric properties of the hydrous component at crustal and mantle conditions remain debated. In this study, we determined the density of dry and hydrous (0-9 wt% H2O) haplo-andesitic melts from 0.7 to 4.0 GPa and 1600 to 1930 K using the X-ray absorption method in a Paris-Edinburgh press to determine the partial molar volume of water at high pressure. Our in situ data were combined with atmospheric pressure data to derive the first experimental equation of state for andesitic melts valid for crustal and subduction zone conditions, with KT=17.2GPa and K′¯=6.3−2.5+3.3 at 1273 K. In addition, the partial molar volume and compressibility of dissolved water in andesitic melts were derived from the in situ data, with V0¯=19.2−1.9+2.4cm3/mol, K¯T=2.7−2.2+7.6GPa and K′¯=3.6−2.7+13.5 at 1273 K. Combined with available literature data for other melt compositions, our results support a compositionally independent partial molar volume of water in silicate melts, at least within the currently attainable precision of the in situ measurements. This observation greatly simplifies the construction of future models for magma density.