Silica activity and P total in igneous rocks

Abstract

The variation of silica activity with temperature and pressure for a variety of silica buffers (mineral pairs) allows Ptotal to be calculated for a wide range of igneous rocks. The method also depends on evaluating (∂ log aSiO2/∂P)T and (Δ log aSiO2/ΔT)p; the former is equivalent to the partial molar volume of silica in silicate liquids, while the latter is estimated from published experiments on natural melts. Results for calc-alkaline rhyolites with phenocrysts of quartz, olivine or orthopyroxene, and iron-titanium oxides, range from 3.45 to 9.58 kilobars; a pantellerite is intermediate at 7.53 kilobars. At 1327° C, the silicate inclusions in diamond equilibrated at 63.5 kilobars, and the kimberlite crystallisation path intersected the baddeleyite-zircon reaction at 55.7 kilobars. Two trachybasalts would equilibrate with their lherzolite xenoliths at 17.0 and 21.0 kilobars at surface quenching temperatures. Potassic lavas such as orendites and ugandites at 1300° C would be in equilibrium with mantle olivine-orthropyroxene at 35.1 and 69.0 kilobars respectively. Basalts and basaltic-andesites could equilibrate (at 1100° C) with quartz at between 24.9 and 26.8 kilobars; quartz can therefore be considered a possible high pressure “xenocryst” in lavas with low Sr87/Sr86 ratios. Andesites will equilibrate at 1300° C with the mantle at a depth of 75 kilometres; at greater depths andesite will have a basaltic precursor. In general, lavas with low silica activity will equilibrate at greater depths in the mantle than those with higher silica activities.