Composition of primary kimberlite melt in a garnet lherzolite mantle source: constraints from melting phase relations in anhydrous Udachnaya-East kimberlite with variable CO2 content at 6.5 GPa |
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| Institution: | 1. V.S. Sobolev Institute of Geology and Mineralogy, Russian Academy of Science, Siberian Branch, Koptyga Ave. 3, Novosibirsk 630090, Russia;2. Novosibirsk State University, Pirogova St. 2, Novosibirsk 630090, Russia;3. Department of Earth and Planetary Material Science, Tohoku University, 6-3 Aza-Aoba, Aramaki, Aoba-ku, Sendai 980-8578, Japan;1. Gemological Institute, China University of Geosciences, Wuhan 430074, P.R. China;2. School of Earth Sciences, China University of Geosciences, Wuhan 430074, P.R. China;1. V.S. Sobolev Institute of Geology and Mineralogy of Siberian Branch of Russian Academy of Sciences, Novosibirsk 630090, Russian Federation;2. Novosibirsk State University, Pirogova st. 2, Novosibirsk 630090, Russian Federation;1. State Key Laboratory of Petroleum Resources and Prospecting,China University of Petroleum, Beijing 102249, China;2. Setate Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;3. Deep & Early Earth Processes (DEEP) Research Group, Department of Geology, University of Johannesburg, 2006 Auckland Park, South Africa;4. Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences, Irkutsk 664033, Russia;5. College of Geosciences, China University of Petroleum, Beijing 102249, China;1. Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, Russian Academy of Sciences (IGEM RAS), Moscow, Russia;2. Lomonosov Moscow State University, Moscow, Russia;3. AC ALROSA Research Geological Prospecting Enterprise, Arkhangelsk, Russia |
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| Abstract: | The critical issue in the study of kimberlites, known as principal host rocks of diamonds, is the reconstruction of their primary melt composition, which is poorly constrained due to contamination by xenogenic materials, significant loss of volatiles during eruption, and post-magmatic alteration. It is generally accepted that the last equilibration of primary kimberlite melt with surrounding mantle (garnet lherzolite) occurred beneath cratons at 5–7 GPa (150–230 km depths). However, the subliquidus mineral assemblages obtained in kimberlite melting experiments at mantle pressures differ from lherzolite, probably owing to unaccounted loss of CO2. Here we present experiments at 6.5 GPa and 1200–1600 °C on unaltered kimberlite with an addition of 2–22 mol% CO2 over its natural abundance in the rock (13 mol%), but keeping proportions of other components identical to those in an exceptionally fresh anhydrous kimberlite from Udachnaya-East pipe in Siberia. We found that the partial melt achieves equilibrium with garnet lherzolite at 1500 °C and 19–23 mol% CO2 in the system. Under these conditions this melt contains (mol%): SiO2 = 9, FeO = 6–7, MgO = 23–26, CaO = 16, Na2O = 4, K2O = 1, and CO2 = 30–35. We propose, therefore, the alkali-rich carbonatitic composition of primary kimberlite melt and loss of 34–45 mol% (34–46 wt%) CO2 during ascent of the kimberlite magma to the surface. |
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