The assemblage diaspore+quartz in metamorphic rocks: a petrological, experimental and thermodynamic study |
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| Authors: | T THEYE C CHOPIN K-D GREVEL & E OCKENGA |
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| Institution: | Research Group on High-Pressure Metamorphism, Institut für Mineralogie, Ruhr-Universität Bochum, D-44780 Bochum, Germany,;Institut für Geowissenschaften der Technischen Universität Braunschweig, Abt. für Mineralogie, Postfach 3329, D-38023 Braunschweig, Germany |
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| Abstract: | 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 °C/24.8 kbar and 535 °C/25.1 kbar, and between 500 °C/23 kbar and 540 °C/23 kbar. Equilibrium (ii) was bracketed at 550 °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 °C to 20 kbar/430 °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 °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 occurrences600.S 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 ferro-magnesiocarpholite, glaucophane, sodic pyroxene and lawsonite. The metamorphic P–T conditions of these rocks are estimated to be in the range 300–400 °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. |
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| Keywords: | diaspore high-pressure metamorphic rocks pyrophyllite reversal experiments thermodynamic calculations |
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