Thermal and redox equilibrium conditions of the upper-mantle xenoliths from the Quaternary volcanoes of NW Spitsbergen,Svalbard Archipelago |
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| Institution: | 1. Saint Petersburg State University, Institute of Geosciences, Universitetskaya nab. 7/9, St. Petersburg, 199034, Russia;2. Institute of Precambrian Geology and Geochronology, Russian Academy of Sciences, nab. Makarova 2, St. Petersburg, 199034, Russia;3. Department of Earth and Environmental Science, Ludwig Maximilians Universität, Theresienstr. 41/III, 80333, Munich, Germany;4. Polar Sea Geological-Prospecting Expedition, ul. Pobedy 24, St. Petersburg-Lomonosov, 198412, Russia;1. Department of Petrology, Geological Faculty, Moscow State University, Leninskie gory, Moscow 119234, Russia;2. Institute of Experimental Mineralogy, Russian Academy of Sciences, Institutskaya Str. 4, Chernogolovka, 142432, Russia;3. Sobolev Institute of Geology and Mineralogy, Siberian Branch, Russian Academy of Sciences, Ak. Koptyuga pr. 3, Novosibirsk 630090, Russia;4. Natural History Museum, Cromwell Road, London SW7 5BD, UK;5. Dipartimento di Scienze della Terra, Università di Firenze, I-50121 Firenze, Italy;1. Tomsk State University, pr. Lenina 36, Tomsk, 634050, Russia;2. V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia;3. Gorno-Altaiskaya Ekspeditsiya Joint-Stock Company, Maloeniseiskoe, Altai Territory, 659370, Russia;1. School of Mining, Petroleum & Geophysics Engineering, University of Shahrood, Iran;2. CCFS-GEMOC, Department of Earth and Planetary Sciences, Macquarie University, Australia;1. Department of Physics and Earth Sciences, University of Ferrara, Italy;2. Goethe Universität, Facheinheit Mineralogie, Frankfurt, Germany;3. Department of Geosciences, Colorado State University;1. Department of Geology, Lund University, Sölvegatan 12, 223 62 Lund, Sweden;2. Geological Survey of Denmark and Greenland (GEUS), Øster Voldgade 10, 1350 Copenhagen K., Denmark;3. Centre for Exploration Targeting (CET), University of Western Australia, 35 Stirling Highway, Crawley, Perth, WA 6009, Australia;4. Avannaa Resources Ltd., Dronningens Tværgade 48 st. tv., 1302 Copenhagen K, Denmark;5. Nordic Center for Earth Evolution, Natural History Museum of Denmark, Østre Voldgade 5–7, 1350 Copenhagen K., Denmark;6. Department of Geoscience, Swedish Museum of Natural History, SE-104 05 Stockholm, Sweden |
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| Abstract: | Upper-mantle xenoliths in Cenozoic basalts of northwestern Spitsbergen are rocks of peridotite (spinel lherzolites) and pyroxenite (amphibole-containing garnet and garnet-free clinopyroxenites, garnet clinopyroxenites, and garnet and garnet-free websterites) series. The upper-mantle section in the depth range 50–100 km is composed of spinel peridotites; at depths of 80–100 km pyroxenites (probably, dikes or sills) appear. The equilibrium conditions of parageneses are as follows: in the peridotites—730–1180 °C, 13–27 kbar, and oxygen fugacity of − 1.5 to + 0.3 log. un.; in the pyroxenites—1100–1310 °C, 22–33 kbar. The pyroxenite minerals have been found to contain exsolved structures, such as orthopyroxene lamellae in clinopyroxene and, vice versa, clinopyroxene lamella in orthopyroxene. The formation temperatures of unexsolved phases in orthopyroxene and clinopyroxene are nearly 100–150 °C higher than the temperatures of the lamellae–matrix equilibrium and the equilibrium of minerals in the rock. The normal distribution of cations in the spinel structure and the equilibrium distribution of Fe2 + between the M1 and M2 sublattices in the orthopyroxenes point to the high rate of xenolith ascent from the rock crystallization zone to the surface. All studied Spitsbergen rock-forming minerals from mantle xenoliths contain volatiles in their structure: OH−, crystal hydrate water H2Ocryst, and molecules with characteristic CH and CO groups. The first two components are predominant, and the total content of water (OH– + H2Ocryst) increases in the series olivine → garnet → orthopyroxene → clinopyroxene. The presence of these volatiles in the nominally anhydrous minerals (NAM) crystallized at high temperatures and pressures in the peridotites and pyroxenites testifies to the high strength of the volatile–mineral bond. The possibility of preservation of volatiles is confirmed by the results of comprehensive thermal and mass-spectral analyses of olivines and clinopyroxene, whose structures retain these components up to 1300 °C. The composition of hypothetic C–O–H fluid in equilibrium (in the presence of free carbon) with the underlying mantle rocks varies from aqueous (> 80% H2O) to aqueous–carbonic (~ 60% H2O). The fluid becomes essentially aqueous when the oxygen activity in the system decreases. However, there is no strict dependence of the redox conditions on the depth of formation of xenoliths. |
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