Analysis of the termination topology of the plagioclase + alkali feldspar + liquid (PAL) curve in the system Ab-Or-An(-H2O) was conducted using thermodynamic solution models for ternary feldspar melt and solid solutions for a low activity of H2O and a pressure of 2 kbar. Under these conditions the PAL curve terminates within the ternary system and does not lie within a thermal valley near its termination. Rather, the adjacent liquidus surfaces show a peritectic-like topology with the lowest temperatures lying on the Ab-Or(-H2O) sideline. The calculated PAL curve has an almost constant slope to its termination; this is in disagreement with the geometric conclusion of Henderson (1984). Using Henderson’s geometric method supplemented by an analytical approach, we show that no sharp decrease in An is required as the termination is approached.

Behavior of bulk compositions close to and on the line that connects the consolute point of the solvus (K) with the melt composition at the termination (C) of the PAL curve has been investigated computationally. Bulk compositions slightly displaced from the K-C line will undergo complete resorption of one of the feldspars before the solidus is reached. However, compositions on the K-C line will show coprecipitation of two feldspars during equilibrium crystallization with the subtraction of a crystalline component from each of the solids, a process that we term “partial resorption.” Partial resorption removes the mass-balance requirement that the PAL boundary must curve away from K.

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