System Na₂O-MgO-Al₂O₃-SiO₂

Abstract

Phase relations in the NMAS system are directly applicable to the classic problem of the origin of the tholeiitic and alkali basalts. The two magma types are separated at 1 atm by a thermal divide between forsterite and albite (Schairer and Yoder 1961). Yoder and Tiley (1962) showed that this thermal divide breaks down at high pressures and is replaced by a new thermal divide between enstatite and jadeite, responsible for the change in the composition of the basaltic melts from tholeiitic at lower pressures to alkaline at higher pressures. This reflects the depth of origin, which is shallow for the tholeiitic magmas, typically generated under the mid-ocean ridges, but greater for the alkali basaltic magmas originating under the older and thicker oceanic lithosphere.