NONLINEAR TEMPERATURE-DEPENDENCE OF LIQUID VOLUMES IN THE SYSTEM ALBITE-ANORTHITE-DIOPSIDE

The temperature-dependent thermal expansivities of glasses and liquids in the ternary albite-anorthite-diopside have been determined using a combination of calorimetry, dilatometry and Pt and Ir double bob Archimedean densitometry. Supercooled liquid volumes and molar thermal expansivities were determined across the glass transition using a combination of scanning calorimetry and dilatometry, based upon the equivalence of relaxation of volume and enthalpy in the vicinity of the glass transition. Superliquidus volumes were determined using double Pt bob Archimedean densitometry at temperatures up to 1,650-degrees-C and double Ir bob densitometry at 1,800-degrees-C. Experimental access to liquid volumes near the glass transition temperatures (680-920-degrees-C) and at superliquidus temperatures (1,400-1,800-degrees-C) for these compositions results in the observation of a non-linear temperature dependence of molar volume, i.e., temperature-dependent thermal expansivities. The diopside composition exhibits the largest temperature dependence of thermal expansivity, decreasing by approximately 50% between 800 and 1,500-degrees-C. Linear extrapolation of the high-temperature volume data of diopside to 810-degrees-C would result in a 3% overestimation of the molar volume. The temperature dependence of the molar volume of anorthite is approximately linear. The thermal expansivities of the liquids in the albite-anorthite-diopside system appear to converge at high temperature. This study uses a combination of methods that allows interpolation rather than extrapolation of the extant melt-volume data into the petrologically meaningful (subliquidus) temperature range.