RECENT ADVANCES IN EXPERIMENTAL PETROLOGY

Abstract

Recent advances in various aspects of experimental petrology is reviewed. It is noticed that the research projects have been shifting from the studies on the rocks and minerals on or near the Earth's surface, to those in the crust and finally to those in the upper mantle, corresponding to the progress in the high pressure experimental technique. EPMA and Mossbauer spectrograph have been prooved indispensable as the identification method in these fields. At first origin of granites is discussed by means of various studies on the system albite-orthoclase-quartz-H20, with or without anorthite or various mafic minerals. It is now generally agreed that majority of granites are formed from liquid phases, either the primary magmas or secondary liquids resulted from melting of pre-existing rocks. Formation of basaltic magmas by partial melting of lherzolite in the upper mantle is discussed by means of the studies on the forsterite-diopside-silica system at atmospheric pressure or higher pressures. It is evident from these studies that pressure has close relation to the composition of the magmas thus formed, indicating increase in alkalis with increasing pressures. Olivine (α-form) and spinel (γ-form) transition in the system Mg2Si04 Fe2SiO4 has been studied at several laboratories at pressures up to 250kb. During these studies an intermediate phase of “modified spinel” (β-form) has been found in the compositions close to pure Mg2SiO4, making the phase diagram of Mg2SiO4-Fe2SiO4 very complicated. Decomposition of clinopyroxenes of the system MgSiO3-FeSiO3 into spinel and stishovite at very high pressures was also found. Behaviors of several minerals of the upper mantle, such as amphiboles and garnets, have been studied at moderate pressures. It is noted that ugrandite-and pyaralspite-series of garnets form continuous solid solutions at higher pressures. Genesis of andesite, one of the most fascinating subjects in modern petrology, is discussed with the results of experimental works, especially by the study of forsterite-diopside-silica-H20 system at various pressures. Kushiro presented a hypothesis of direct derivation of andesite from hydrous lherzolite by partial melting at depths of about 80 km. Relation between andesite magma generation and the plate tectonics is also discussed. Shock-wave experiments can produce ultrahigh pressures and temperatures up to 10Mb and 10, 000°C, though the time being in the order of 10-6 second, and provide a basis for experimental studies of shock metamorphism, which has been observed only in some terrestrial impact craters. Importance of this new category of metamorphism is discussed. In spite of remarkable advances in the experimental studies under high pressures, the author still believes that atmospheric pressure studies have to be continued as the basis on which high pressure studies are constructed. They also provide many informations relevant to the interpretation of ystallization of magmas and metamorphic processes of crustal rocks on the Earth's surface. Finally interdisciplinary and international nature of the experimental petrology is discussed.