Silica activity and the classification of alkalic and tholeiitic basalts Available to Purchase

The thermodynamic classification of basalts introduced by Carmichael et al. (1970) is based on ^aSiO₂/melt vs. T°C data for the olivine/low-Ca pyroxene reaction. This classification is not consistent with geochemical classification schemes, and when ^aSiO₂/melt data for a representative composition in the critical plane of silica undersaturation in the basalt tetrahedron are compared with the olivine/low-Ca pyroxene buffer curve, some rocks classified as alkali olivine basalts are shown to be tholeiitic basalts.