Experimental constraints on the high-temperature termination of the anhydrous 2 feldspar + L curve in the feldspar system at 11.3 kbar

The location of the eutectic and the limits of mutual crystalline solubility were determined by reversed experiments at 11.3 kbar in the anhydrous system anorthite-sanidine. These results differ greatly from those of earlier work by Ai and Green (1989) and indicate that the anhydrous eutectic lies at 19 ± 2 wt% An and 1280 ± 10°C. This eutectic composition lies much closer to the H₂O-saturated eutectic than indicated by the previous data. Therefore, the amount of An depletion during crystallization of two feldspars from a hydrous melt along the 2 feldspar + L surface in the system An-Or-Ab-H₂O is significantly less than previously thought. Maximum mutual crystalline solubilities with various solution models for ternary feldspars permit assessment of these models and indicate that only the ternary feldspar models of Nekvasil and Burnham (1987) and Elkins and Grove (1990) yield calculated liquidus relations in good agreement with the data. -Authors