The Influence of Retrograde Cation Exchange on Granulite P-T Estimates and a Convergence Technique for the Recovery of Peak Metamorphic Conditions
Chemical relationships in garnet-orthopyroxene-plagioclase-quartz rocks are governed principally by three equilibria: the Fe-Mg exchange reaction between garnet and orthopyroxene, the solubility of alumina in orthopyroxene coexisting with garnet, and the reaction of garnet and quartz to form orthopy...
| Published in: | Journal of Petrology |
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| Main Authors: | , |
| Format: | Text |
| Language: | English |
| Published: |
Oxford University Press
1994
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| Subjects: | |
| Online Access: | http://petrology.oxfordjournals.org/cgi/content/short/35/2/543 https://doi.org/10.1093/petrology/35.2.543 |
| Summary: | Chemical relationships in garnet-orthopyroxene-plagioclase-quartz rocks are governed principally by three equilibria: the Fe-Mg exchange reaction between garnet and orthopyroxene, the solubility of alumina in orthopyroxene coexisting with garnet, and the reaction of garnet and quartz to form orthopyroxene and plagioclase. Various thermobarometric calibrations of these equilibria have been applied to granulite-facies gneisses from two areas of the Proterozoic Complex of East Antarctica, and a wide range of P - T estimates is obtained for each area. Some of this P - T variation reflects the different thermodynamic data and mineral mixing models used by each calibration, but other differences are attributed to the effects of retrograde Fe-Mg exchange. An inter-specimen spread of temperatures in each area, obtained for mineral core compositions with a single calibration of the garnet-orthopyroxene exchange reaction, is attributed to a variable extent of Fe-Mg exchange on cooling from peak metamorphic conditions. A similar spread of pressures from the garnet-orthopyroxene alumina solubility barometer indicates that this calibration is also reset by retrograde Fe-Mg exchange. In contrast, pressures from the garnet-orthopyroxene-plagioclase-quartz barometer for mineral cores show little variation between specimens from the same area, indicating that this equilibrium is relatively insensitive to changes in the Fe-Mg distribution coefficient and that derived pressures are more likely to reflect peak metamorphic conditions than those from the alumina solubility barometer. Temperatures can be corrected for Fe-Mg exchange using the Fe-Mg distribution coefficient required to bring pressures from the exchange-sensitive alumina solubility barometer into agreement with reference pressures calculated from the exchange-insensitive garnet-orthopyroxene-plagioclase-quartz barometer. These corrected temperatures are closure temperatures for Al diffusion, which in many cases are likely to be good estimates for the peak metamorphic ... |
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