High U/Th partitioning by clinopyroxene from alkali silicate and carbonatite metasomatism: an origin for Th/U disequilibrium in mantle melts? |
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| Authors: | S F Foley C M Petibon G A Jenner & B A Kjarsgaard |
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| Institution: | Institut für Geologische Wissenschaften, Universität Greifswald, Jahnstrasse 17a, 17487 Greifswald, Germany,;Earth Sciences Department, Memorial University of Newfoundland, St. John's, Newfoundland, A1B 3X5, Canada,;Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario, K1A 0E8, Canada |
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| Abstract: | Clinopyroxene/melt pairs in strongly potassic silicate and carbonatite melts exhibit unusually high U/Th partitioning ratios of ? 3 and ? 2, respectively. These values are much higher than those found for aluminous clinopyroxenes in peridotite, and have the potential to cause significant (230Th)/(238U) isotope enrichment in volcanics. The potassic silicate (lamproite) and carbonatite melts correspond closely to the main agents of mantle metasomatism, indicating that clinopyroxene in metasomatized regions of the mantle may greatly affect U/Th disequilibria. Recycling of alkali pyroxenite veins in the oceanic lithosphere formed by solidification of melt in the extremities of the MORB melting region presents an alternative to eclogite recycling in MORB and OIB genesis. |
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