The density of hydrous magmatic liquids

Density measurements on several hydrous (less than or equal to 19 mole perc ent of H2O) silicate melts demonstrate that dissolved water has a partial m olar volume ((V) over bar(H2O),) that is independent of the silicate melt c omposition, the total water concentration, and the speciation of water. The derived value for (V) over bar(H2O) is 22.9 +/- 0.6 cubic centimeters per mole at 1000 degrees C and 1 bar of pressure, whereas the partial molar the rmal expansivity (partial derivative (V) over bar(H2O)/partial derivative T ) and compressibility (partial derivative (V) over bar(H2O)/partial derivat ive P) are 9.5 +/- 0.8 x 10(-3) cubic centimeters per mole per kelvin and - 3.2 +/- 0.6 x 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 similar to 400 d egrees C or decreasing the pressure of the melt by similar to 500 megapasca ls. These measurements are used to illustrate the viability of plagioclase sinking in iron-rich basaltic Liquids and the dominance of compositional co nvection in hydrous magma chambers.