The Density of Hydrous Magmatic Liquids

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Science  26 Feb 1999:
Vol. 283, Issue 5406, pp. 1314-1317
DOI: 10.1126/science.283.5406.1314

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Density measurements on several hydrous (≤19 mole percent of H2O) silicate melts demonstrate that dissolved water has a partial molar volume ( H2 O) that is independent of the silicate melt composition, the total water concentration, and the speciation of water. The derived value for H2 O is 22.9 ± 0.6 cubic centimeters per mole at 1000°C and 1 bar of pressure, whereas the partial molar thermal expansivity (∂ H2 O/∂T) and compressibility (∂ H2 O/∂P) are 9.5 ± 0.8 × 10−3 cubic centimeters per mole per kelvin and −3.2 ± 0.6 × 10−4 cubic centimeters per mole per bar, respectively. The effect of 1 weight percent dissolved H2O on the density of a basaltic melt is equivalent to increasing the temperature of the melt by ∼400°C or decreasing the pressure of the melt by ∼500 megapascals. These measurements are used to illustrate the viability of plagioclase sinking in iron-rich basaltic liquids and the dominance of compositional convection in hydrous magma chambers.

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