Forsterite-Diopside-Silica: Pyroxenes and Their Reactions

The system forsterite-diopside-SiO2 (Fo-Di-SiO2 or “FDS”) is one of the most illuminating systems in basalt petrology; it is also one of the most complex, and indeed, it is only because of the very diligent reinvestigation of the system by Kushiro (especially 1972) and his colleagues that we can now begin to appreciate the subtleties of crystallization, melting, and reaction in this system. Like Fo-An-SiO2, this system illustrates the reaction relationships of olivine and pyroxene, but in this case there are three varieties of pyroxene, all involved in reactions with olivine and to some extent with each other. These synthetic pyroxenes represent all the varieties of pyroxene occurring in silica-saturated basaltic rocks (augite, pigeonite, and hypersthene), and the system now under discussion forms a foundation for understanding many of the complex relations among these pyroxenes in nature. The system at atmospheric pressure can be applied to a large variety of natural basaltic rocks, including layered intrusions as well as volcanic rocks, and at high pressures, it furnishes several interesting and relatively simple models for generating the classical spectrum of basalt magma types in the mantle.