Figure 5
Schematic illustration of the effect of pressure on forsterite-enstatite equilibria in the hydrous environment.
Primary magmas in equilibrium with magnesian olivine and magnesian orthopyroxene become progressively more silica-rich with decreasing depth53. (a) At high pressure and in hydrous conditions, congruent melting of magnesian orthopyroxene results in primary basalt melt. (b) At lower pressure and in hydrous conditions the liquidus field of forsterite expands relative to that of enstatite, with the result that, at some point, enstatite melts incongruently to produce primary andesite melt. (c) Schematic illustration showing primary basalt and andesite magmas and their possible fractionation trends.
