Graphical analysis of the orthopyroxene-pigeonite-augite-plagioclase equilibrium at liquidus temperatures and low pressure

There are both natural and experimental observations of the coexistence of three pyroxenes-orthopyroxene (Opx) + pigeonite (Pig)+ augite (Aug)-with pl agioclase (Pl). Commonly, the assemblage occurs as an intermediate product in the following fractionation trend of a mafic magma: Opx + Aug + Pl --> O px + Aug + Pig + Pl --> Aug + Pig + Pl. To clarify the phase-equilibria con straints on the existence of this mineral assemblage, we have graphically a nalyzed the change in topology of an isobaric-isoplethic section, Ol-Aug-Pl -Qtz [with fe = 25-50%, where fe = Fe/(Fe + Mg), and An = 50%], arising fro m an increase in the fe-value of silicate liquid. The analysis shows that t he stability field of the mineral assemblage Opx + Aug + Pig + Pl is restri cted, and can only crystallize in the interval between two invariant points -T-1(4) (Ol + Opx + Pig + Aug + Pl + L) and T-2(4) (Qtz + Opx + Pig + Aug Pl + L)-that emerge successively during expansion of a liquidus volume of pigeonite within the isobaric-isoplethic section Ol-Aug-Pl-Qtz. At fe-value s lower than T-1(4) the 3-pyroxene assemblage cannot exist due to the absen ce of a contact surface between the primary volumes of plagioclase and pige onite. At fe-values greater than T-2(4), the assemblage is unstable due to separation of the primary volumes of orthopyroxene: and augite.