OLIVINE-MELT AND ORTHO-PYROXENE-MELT EQUILIBRIA

The use of non-regular solution models for silicate melts allows saturation temperatures to be calculated with an accuracy of +/-10 K for Mg and Fe, olivine or orthopyroxene components; and +/-20 K for Mn, Co and Ni components. This accuracy is comparable to that of the temperature measurement in the experiments with which the models are calibrated. The errors in the temperature calculation are less than a third of those associated with a regular solution model of mineral-melt equilibria. The values of thermodynamic properties predicted by these empirical solution models are larger than those found calorimetrically, but provide a better fit to the existing experimental data. The calculation of thermochemical properties of olivine and orthopyroxene species in both the crystalline and melt phases allows the calculation of mineral-melt K(DM/Mg)alpha/Ls; the values calculated are within one standard error of those reported in the literature. Eruption temperatures calculated from the composition of Hawaiian tholeiite glasses range from 1135 to 1185-degrees-C, and are comparable to measured lava temperatures. These temperatures are lower than those calculated for Atlantic MORB confirming that extensive fractional crystallisation has occurred.