Thermochemistry of Garnets and Aluminous Pyroxenes in the CMAS System

Abstract

The bulk chemistry of the earth’s upper mantle, as deduced from analysis of exotic fragments from explosive igneous pipes and from geochemical arguments, can be “modeled” to quite a good approximation by the simple system CaO-MgO-A1₂0₃-SiO₂. The only significant departures from this quaternary system in Green and Ringwood’s (1967) “pyrolite III”, which is their favored mantle composition, are 8.0 percent FeO and 0.4 percent Cr ₂0₃. FeO plays a role in the ferromagnesian minerals very similar to that of MgO, which dominates it by a factor of ten in molar amount, hence its influence should be minor. Cr ₂0₃ is closely similar to A1₂0₃ in its behavior and again greatly inferior in amount. The quaternary system yields all of the major phases which coexist in peridotites, namely olivine, orthopyroxene, clinopyroxene, spinel, garnet and plagioclase. For these reasons the simple system has been very useful in geophysical phase-equilibrium investigations.