Forsterite-Anorthite-Silica: Incongruent Melting in a Ternary System

In the ternary system Di-An-Ab, we saw that the complete solid solution of plagioclase had a dominant influence on the course of liquids in the ternary system. We shall now see that the incongruent melting of orthopyroxene in the system Fo-SiO2 is carried over into a ternary system when a component such as anorthite is added. This ternary system is one of the most useful in illustrating the importance of the reaction relationship of olivine in igneous petrology. The system is the An-analogue of the right half floor of the basalt tetrahedron (Fig. 2.1 ), and as such is fundamental in establishing the role of the plane of silica saturation, modelled in this system by the line An-En. This role is like that of a check valve: liquids which pierce the plane towards SiO2 cannot return, at atmospheric pressure, because of the incongruent melting of En to Fo + L.