Oxygen isotope exchange processes and disequilibrium between calcite and forsterite in an experimental C-O-H fluid |
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| Institution: | 1. Max Planck Research Group for Marine Isotope Geochemistry, Institute for Chemistry and Biology of the Marine Environment (ICBM), University of Oldenburg, Carl-von-Ossietzky-Str. 9-11, 26129 Oldenburg, Germany;2. Microbiogeochemistry, Institute for Chemistry and Biology of the Marine Environment (ICBM), University of Oldenburg, Carl-von-Ossietzky-Str. 9-11, 26129 Oldenburg, Germany;3. Hydrogeology and Landscape Hydrology, Institute for Biology and Environmental Sciences (IBU), University of Oldenburg, Oldenburg, Germany;1. U.S. Geological Survey, 431 National Center, Reston, VA 20192, USA;2. Department for Isotope Biogeochemistry, Helmholtz-Centre for Environmental Research (UFZ), Permoserstrasse 15, 04318 Leipzig, Germany;3. Institute of Earth Surface Dynamics, University of Lausanne, 1015, Switzerland;4. Max-Planck-Institute for Biogeochemistry, Beutenberg Campus, P.O. Box 100164, 07701 Jena, Germany;5. Department of the Geophysical Sciences, The University of Chicago, Chicago 60637, USA;6. Department of Earth Sciences, University of Oregon, Eugene, OR 97403, USA;7. Department of Earth Sciences, The University of Western Ontario, London, Ontario N6A 3K7, Canada |
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| Abstract: | Oxygen isotope exchange between calcite and forsterite were investigated in the presence of a large amount of supercritical fluid. The experiments were conducted in standard cold-seal pressure vessels at 680°C and 500 MPa in the T-P-XCO2 stability field of the calcite-forsterite assemblage for 2, 5, 10, 20, 40 and 80 days, respectively. The weight ratio of mineral to fluid in the starting mixture is 1.46; the fluid was a mixture of H2O and CO2 with the mole fraction of CO2 being 0.1. The results show that the oxygen isotope exchange between the minerals was accomplished via mineral-fluid exchange by a dual-mechanism, i.e. initial rapid exchange due to Ostwald ripening of both calcite and forsterite, followed by a slower diffusion-controlled process. Furthermore, for the given fluid composition, calcite shows a greater rate of dissolution-recrystallization and oxygen isotope exchange with fluid than forsterite. As a result, oxygen isotope fractionations between calcite and forsterite and between the minerals and the fluid can simply pass the equilibrium fractionations with time and even lead to crossover behavior. Once diffusion becomes a primary mechanism for further isotope exchange in the three-phase system, the rate of oxygen diffusion in calcite is equal to, or slightly less than that in forsterite. |
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