Incongruent Melting: The System Forsterite-Silica

Up to now, we have concentrated on developing the phase relations in and around the critical plane in the basalt tetrahedron, described in Chap. 2 . This critical plane is the plane which separates nepheline-normative bulk compositions from hypersthene-normative ones. Hypersthene is modelled, in the fundamental basalt tetrahedron, by its magnesian end member, enstatite, MgSiO3 (or MgO·SiO2). Enstatite appears at the apex of another triangular plane in the tetrahedron, the plane Di-Ab-En, called the plane of silica saturation by Yoder and Tilley (1962). This plane marks the transition from (enstatite + forsterite)-normative bulk compositions to (enstatite + quartz)-normative bulk compositions. It also truly marks the division between solid, equilibrium assemblages of enstatite + forsterite, to the left, and enstatite + quartz, to the right. However, it does not precisely divide the crystal + liquid equilibria En + Fo + L and En + Q + L. Instead, an important phenomenon exists near the plane, wherein the assemblage forsterite + L extends to more silica-rich compositions than the plane itself, with or without enstatite, thus providing a possibility for liquids to become silica-oversaturated by fractional crystallization of forsterite. To see why this is so, we must now examine the so-called incongruent melting behaviour of enstatite in the system Fo-SiO2.