Abstract

The reaction 3 anorthite = grossular + 2 kyanite + quartz has been determined experimentally over the ranges 900-1250 °C and 19-28 kbar in a piston-cylinder apparatus using a NaCl pressure medium. Two to four weight percent Li2MoO4 flux enabled reversal of the equilibrium 200 °C lower than previously achieved. A point at 650 °C, 13.5 ± 0.6 kbar was calculated from published studies of 4 zoisite + quartz = 5 anorthite + grossular + 2 H2O and 2 zoisite + kyanite + quartz = 4 anorthite + H2O, and the addition of this point constrains the equilibrium curve to P (in kilobars) = 22.80T (in °C) — 1093. Use of this equation with the plagioclase-garnet-Al2SiO5-quartz geobarometer results in reduction of the uncertainty of calculated pressures by a factor of two over previous calibrations. Calculated paleopressures are 400 bars higher for the assemblage with silli-manite than those given by Newton and Haselton (1981). Analysis of the data provides an estimate of the 1 bar, 298 K entropy of anorthite that is 3.3 J/K greater than the third-law entropy, implying 4% Al-Si disorder in anorthite.

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