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There are both natural and experimental observations of the coexistence of three pyroxenes- orthopyroxene (Opx) + pigeonite (Pig) + augite (Aug)-with plagioclase (Pl). Commonly, the assemblage occurs as an intermediate product in the following fractionation trend of a mafic magma: Opx + Aug + Pl → Opx + Aug + Pig + Pl → Aug + Pig + Pl. To clarify the phase-equilibria constraints on the existence of this mineral assemblage, we have graphically analyzed 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 from an increase in the fe-value of silicate liquid. The analysis shows that the stability field of the mineral assemblage Opx + Aug + Pig + Pl is restricted, 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-values lower than T 1  4 , the 3- pyroxene assemblage cannot exist due to the absence of a contact surface between the primary volumes of plagioclase and pigeonite. At fe-values greater than T 2  4 , the assemblage is unstable due to separation of the primary volumes of orthopyroxene and augite.

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