The assemblage diaspore plus quartz in metamorphic rocks: A petrological, experimental and thermodynamic study

The upper pressure limit of pyrophyllite is given by the equilibria (i) pyrophyllite = diaspore + quartz and (ii) pyrophyllite = diaspore + coesite. High-P experimental investigations carried out to locate equilibrium (i) yield brackets between 497 degrees C/24.8 kbar and 535 degrees C/25.1 kbar, and between 500 degrees C/23 kbar and 540 degrees C/23 kbar. Equilibrium (ii) was bracketed at 550 degrees C between 26.0 and 28.3 kbar. In the experimental P-T range, equilibria (i) and (ii) are metastable with respect to kyanite. A stable P-T grid is calculated using thermodynamic data derived under consideration of the present experimental results. According to these data, the lower pressure limit of the assemblage diaspore + quartz according to equilibrium (i) range from about 12 kbar/300 degrees C to 20 kbar/430 degrees C (in the presence of pure water). The upper stability of diaspore + quartz is limited by the reaction diaspore + quartz = kyanite + H2O at about 450 degrees C (nearly independent of pressure) and, to higher pressure, by the quartz = coesite transition. Equilibrium (ii) is metastable over the whole P-T range. Natural occurrences of the diaspore-quartz assemblage in metamorphic rocks in Sulawesi, New Caledonia, Amorgos and the Vanoise are characterized by minerals indicative of high-P such as ferromagnesiocarpholite, glaucophane, sodic pyroxene and lawsonite. The metamorphic P-T conditions of these rocks are estimated to be in the range 300-400 degrees C, >8 kbar. These data are compatible with the derived P-T stability field of the diaspore + quartz assemblage. We conclude that, in metamorphic rocks, diaspore + quartz is, as ferrocarpholite, an indicator for unusual low-T/very high-P settings.