Origin of K-rich diamond-forming immiscible melts and CO2 fluid via partial melting of carbonated pelites at a depth of 180–200?km |
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| Institution: | 1. V.S. Sobolev Institute of Geology and Mineralogy, 3 Koptyuga ave., Novosibirsk 630090, Russia;2. Novosibirsk State University, 2 Pirogova st., Novosibirsk 630090, Russia;3. Research School of Earth Sciences, The Australian National University, Canberra 2601, Australia;4. Institute of Geological Sciences, University of Bern, Bern 3012, Switzerland;5. Vinogradov Institute of Geochemistry, 1a Favorsky st., Irkutsk 664033, Russia;1. Gemological Institute of America (GIA), 50 west 47th Street, New York City, NY 10036, USA;2. Canadian Centre for Isotopic Microanalysis, Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada;1. Department of Geosciences, University of Texas at Dallas, 800 West Campbell Road, Richardson, TX 75080-3021, USA;2. 652 Muckleshoot Circle, La Conner, Washington 98257, USA;1. Departamento de Geología Regional, Instituto de Geología, Universidad Nacional Autónoma de México, 04510 México, DF, Mexico;2. National Taiwan Normal University, Department of Earth Sciences, 88 Tingzhou Road Section 4, Taipei 11677, Taiwan;3. Department of Geology, St. Mary''s University, Halifax, Nova Scotia, Canada B3H 3C3;4. Department of Energy, Halifax, Nova Scotia B3J 3J9, Canada;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;4. School of Physical Sciences, University of Tasmania, Hobart, Tasmania 7001, Australia |
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| Abstract: | Melt inclusions in kimberlitic and metamorphic diamonds worldwide range in composition from potassic aluminosilicate to alkali-rich carbonatitic and their low-temperature derivative, a saline high-density fluid (HDF). The discovery of CO2 inclusions in diamonds containing eclogitic minerals are also essential. These melts and HDFs may be responsible for diamond formation and metasomatic alteration of mantle rocks since the late Archean to Phanerozoic. Although a genetic link between these melts and fluids was suggested, their origin is still highly uncertain. Here we present experimental results on melting phase relations in a carbonated pelite at 6 GPa and 900–1500 °C. We found that just below solidus K2O enters potassium feldspar or K2TiSi3O9 wadeite coexisting with clinopyroxene, garnet, kyanite, coesite, and dolomite. The potassium phases react with dolomite to produce garnet, kyanite, coesite, and potassic dolomitic melt, 40(K0.90Na0.10)2CO3·60Ca0.55Mg0.24Fe0.21CO3 + 1.9 mol% SiO2 + 0.7 mol% TiO2 + 1.4 mol% Al2O3 at the solidus established near 1000 °C. Molecular CO2 liberates at 1100 °C. Potassic aluminosilicate melt appears in addition to carbonatite melt at 1200 °C. This melt contains (mol/wt%): SiO2 = 57.0/52.4, TiO2 = 1.8/2.3, Al2O3 = 8.5/13.0, FeO = 1.4/1.6, MgO = 1.9/1.2, CaO = 3.8/3.2, Na2O = 3.2/3.0, K2O = 10.5/15.2, CO2 = 12.0/8.0, while carbonatite melt can be approximated as 24(K0.81Na0.19)2CO3·76Ca0.59Mg0.21Fe0.20CO3 + 3.0 mol% SiO2 + 1.6 mol% TiO2 + 1.4 mol% Al2O3. Both melts remain stable to at least 1500 °C coexisting with CO2 fluid and residual eclogite assemblage consisting of K-rich omphacite (0.4–1.5 wt% K2O), almandine-pyrope-grossular garnet, kyanite, and coesite. The obtained immiscible alkali?carbonatitic and potassic aluminosilicate melts resemble compositions of melt inclusions in diamonds worldwide. Thus, these melts entrapped by diamonds could be derived by partial melting of the carbonated material of the continental crust subducted down to 180–200 km depths. Given the high solubility of chlorides and water in both carbonate and aluminosilicate melts inferred in previous experiments, the saline end-member, brine, could evolve from potassic carbonatitic and/or silicic melts by fractionation of Ca-Mg carbonates/eclogitic minerals and accumulation of alkalis, chlorine and water in the residual low-temperature supercritical fluid. Direct extraction from the hydrated marine sediments under conditions of cold subduction would be another possibility for the brine formation. |
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