System MgO–Al2O3–SiO2

Abstract

The three-component chemical system MgO–Al2O3–SiO2 (MAS) accounts for 89 % of the Earth’s mantle. Phase relations in the MAS system are fundamental for understanding the mineral composition of the mantle; other components only modify but do not qualitatively alter these phase relations. The importance of the MAS system has been recognized early, as is evident from the large number of experimental studies carried out in this system over a time period of several decades. However, the quality of these studies varies substantially. While the phase relations for spinel and garnet peridotites have been studied repeatedly and are known in detail, the equilibrium relations for other MAS compositions are known only partially and often from older studies which have not been re-investigated by more modern experimental and analytical techniques. The approach used here builds on the detailed experimental work carried out for the spinel and garnet peridotites, and extends this information to the whole MAS system by maintaining internal consistency. Earlier experimental studies were reviewed to obtain the full understanding of the complex phase relations at low pressures, and to derive an internally consistent set of parameters. These parameters were then used to calculate phase relations for the whole MAS system in the T-P range corresponding to the crust and upper mantle.