Mineralogy and stable isotope geochemistry of hydrothermally altered oceanic rocks |
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| Authors: | DS Stakes JR O&#x;Neil |
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| Institution: | Department of Oceanography, Oregon State University, Corvallis, OR 97331 (U.S.A.);U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 (U.S.A.) |
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| Abstract: | Data for the diffusion of cations in pyroxenes are relevant to a variety of sub-solidus processes including order-disorder and exsolution. Similar data must also be available if the reliability of geobarometers and geothermometers involving pyroxenes is to be assessed. Two types of diffusion experiment have been performed to determine cation diffusion rates in pyroxenes: (1) interdiffusion between single crystals of diopside and polycrystalline sinters enriched in Al and Fe, and (2) interdiffusion between single crystals of diopside and a glass of the same composition which was isotopically enriched in26Mg and43Ca. Following high-temperature annealing for periods up to several hundred hours, analysis of the diffusion couples, using an electron microprobe and an ion microprobe respectively, failed to show any measurable diffusion profiles. From these “null result” experiments the diffusion coefficients (D) for Al and Fe in diopside are estimated to be less than4×10?14cm2s?1 at 1200°C, and values ofD for Ca and Mg in diopside are estimated to be less than7 × 10?14cm2s?1 at 1250°C. These rates are significantly slower than published tracer-type diffusion data for Ca and Al.A review of studies of order-disorder, microstructural coarsening, and diffusion in pyroxenes suggest that activation energies for cation exchange are typically in excess of 60 kcal mol?1. Transport rates will be assisted, and activation energies lowered by sample non-stoichiometry, inhomogeneities, high dislocation densities and the presence of water.The collective data for Al, Mg and Ca diffusion in diopside indicate diffusion coefficients? 10?15cm2s?1 at 1200°C. A comparison with data for diffusion in garnet, olivine and spinel suggests that pyroxenes may have the highest blocking temperatures. |
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