Physico-chemistry of the Silicate Melts

Abstract

Some aspects of the physico-chemistry of the silicate melts are discussed, with special reference to the melts, basaltic or rhyolitic in composition. Characteristic features of some ternary or quaternary systems pertinent to the basalt problems are first explained. Nearly simultaneous crystallization of olivine, monoclinic pyroxene, and plagioclase in various basaltic rocks in nature as determined by thermal experiments shows close agreement with the crystallization in these systems. An important role of oxygen partial pressure in the basaltic magma is clarified by the different crystallization trends in the system MgO-FeO-Fe₂O₃-SiO₂. Enrichment in either iron or silica can be explained by the difference in the PO₂ value. The so called “granite system” of Or-Ab-Q-H₂O affords a basis for the petrochemistry of the rhyolitic rocks. The remarkable difference in the tie lines of alkali feldspar and liquid in the rhyolitic or pantelleritic rocks can be ascribed to the presence in the pantelleritic liquid of normative Ns and Ac molecules. Origin of the felsic volcanic rocks of the Taupo district, New Zealand is discussed, in relation to the shift of the minimum melting points with the PH₂O in the granite system. A new hypothesis of explosive volcanism is proposed by Yoder based on the study of the An-Di-H₂O system. He regards the sudden relief of pressure on the hydrous magma as the prime cause for such explosive activity.