Further assuming that cotectic ratios of plagioclase to pyroxene equal ~7:3 (Morse, pers.
1993) differs from the cotectic basaltic magmas inferred to be parental to Archaean megacrystic, layered intrusive, ocean basin, and lunar anorthosite.
A cotectic cumulate origin, for example, is inconsistent with the lack of optical continuity of orthopyroxene in individual patches and the lack of crystal-settling structures.
Although there is a tendency for the bulk composition of magma chamber M to migrate incrementally toward the composition of invading magma Y, this is immediately counterbalanced as magma M enters the pyroxene stability field, thereby resulting in rapid crystallization of pyroxene that drives the liquid away from the enstatite composition back toward the pyroxene-plagioclase cotectic
(Boudreau and McBirney 1997).
In this diagram the volume is expressed in Cartesian coordinates and the cotectic
faces that are visible in the orientation shown are labeled.
Projection of these liquid compositions onto experimentally determined phase diagrams reveals that the slightly higher silica content of Mauna Loa magmas causes evolving liquids to intersect the olivine-pyroxene cotectic