Granulites,CO2 and graphite |
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| Institution: | 1. Graduate School of Life and Environmental Sciences, University of Tsukuba, Ibaraki 305-8572, Japan;2. Faculty of Life and Environmental Sciences, University of Tsukuba, Ibaraki 305-8572, Japan;3. Department of Geology, University of Johannesburg, Auckland Park 2006, South Africa;4. School of Earth Sciences and Resources, China University of Geosciences Beijing, 29 Xueyuan Road, Beijing 100083, China;5. Centre for Tectonics Resources and Exploration, Department of Earth Sciences, University of Adelaide, SA 5005, Australia;6. Department of Geology, University of Kerala, Kariyavattom, Trivandrum, India;7. Département des Sciences de la Terre, Université d''Antananarivo, B.P. 906, 101 Antananarivo, Madagascar;1. Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada;2. Rogue Geoscience, 616 Nelson Street, Kimberley, British Columbia V1A 2M6, Canada;3. Natural Resources Canada, Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada;1. Departamento de Geodinámica, Universidad del País Vasco, UPV/EHU, PO Box 644, E-48080 Bilbao, Spain;2. Servicios Generales de Investigación—SGIKER, Universidad del País Vasco, UPV/EHU, PO Box 644, E-48080 Bilbao, Spain |
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| Abstract: | Externally derived, pure CO2 that mixes with a carbon-(under)saturated C-O-H  fluid in lower crustal granulites may result in graphite precipitation if the host-rock oxygen fugacity (fO2rock) is below the upper fO2 limit of graphite. The maximum relative amount of graphite that can precipitate varies between a few mol% up to more than 25 mol%, depending on pressure, temperature, and host-rock redox state. The maximum relative amount of graphite that can precipitate from an infiltrating CO2 fluid into a dry granulite (CO fluid system) varies between zero and a few mol%. Thermodynamic evaluation of the graphite precipitation process shows that CO2 infiltration into lower crustal rocks does not always result in a carbon (super)saturated fluid. In that case, graphite precipitation is only possible if carbon saturation can be reached as a result of the reaction CO2 → CO + ½ O2. Graphite that has been precipitated during granulite facies metamorphic conditions can subsequently be absorbed by a COH fluid during retrograde metamorphism. It is also possible, however, that significant amounts of graphite precipitate from a COH fluid during retrograde metamorphism. This study shows that interpreting the presence or absence of graphite in granulites with respect to CO2 infiltration requires detailed information on the P–T–fO2rock conditions, the relative amount of CO2 that infiltrates into the rock, and whether H2O is present or not. |
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