DETERMINATION OF THE PARTIAL MOLAR VOLUME OF SIO2 IN SILICATE LIQUIDS AT ELEVATED PRESSURES AND TEMPERATURES: A NEW EXPERIMENTAL APPROACH - THE IMPORTANCE OF MELT DENSITY AND SOURCE REGION SIZE

Abstract

This study presents a new approach for determining the partial molar volume and its pressure derivative for a silicate liquid component through an experimental determination of the isothermal pressure dependence of the solubility in the liquid of a crystalline phase having the composition of the component. Because this approach allows the determination of partial molar volumes of liquid components at elevated pressure, it has the potential to detect pressure-induced structural changes associated with particular components in silicate liquid through their influence on partial molar volumes. To illustrate the approach, an experimental determination of the solubility of quartz in a rhyolitic liquid was used to determine the partial molar volume of SiO2 at pressures up to 35 kbar and a temperature of 1350°C. The 1 bar partial molar volume for SiO2 determined in this way, 2.635 +/- 0.009 J/bar, is slightly smaller than the 2.690 +/- 0.006 J/bar determined by Lange and Carmichael (1987) (all uncertainties are 1σ). The isothermal pressure dependence of the partial molar volume of SiO2 at 10 to 35 kbar (-8.69 x 10-6 +/- 6.1 x 10-7 J/bar2) is approximately one-half of the value determined by Kress and Carmichael (1991) at 1 bar (-1.96 x 10-5 +/- 0.2 x 10-6 J/bar2). Our high pressure determination can be reconciled with the existing 1 bar volume and compressibility data if the isothermal pressure dependence of the partial molar volume of SiO2 in silicate liquids decreases rapidly between 1 bar and 10 kbar, then remains approximately constant to at least 35 kbar.