PT conditions and trace element variations of picroilmenites and pyropes from placers and kimberlites in the Arkhangelsk region,NW Russia |
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| Affiliation: | 1. Department of Geology, University of Johannesburg, P.O. Box 524, Auckland Park, 2006, South Africa;2. School of Geographical and Earth Sciences, University of Glasgow, Glasgow, G12 8QQ, United Kingdom;3. School of Geosciences, University of Edinburgh, Edinburgh EH9 3JW, United Kingdom;4. Geological Survey of Western Australia, Mineral House, 100 Plain Street, East Perth, Western Australia 6004, Australia;5. Department of Geological Sciences, University of Cape Town, Rondebosch 7701, South Africa;1. Department of Geological Sciences, University of Cape Town, Rondebosch 7701, South Africa;2. Planetary Geosciences Institute, Earth and Planetary Sciences Dept., The University of Tennessee, Knoxville, TN 37996, USA;1. Vinogradov Institute of Geochemistry SB RAS, Favorskogo str. 1A, Irkutsk 664033, Russia;2. Institute of Geology and Geophysics, Chinese Academy of Sciences, 19 Beitucheng West Road, Chaoyang, Beijing 100029, China;3. Institut für Mineralogie und Kristallographie, Universität Wien, Althanstr. 14, 1090 Wien, Austria;1. V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of Russian Academy of Science, Koptyuga ave. 3, Novosibirsk 630090, Russia;2. Novosibirsk State University, Pirogova st. 2, Novosibirsk 630090, Russia;3. Department of Earth and Planetary Materials Science, Graduate School of Science, Tohoku University, 6-3 Aramaki, Aza-Aoba, Aoba-ku, Sendai 980-8578, Japan;1. A.P. Vinogradov Institute of Geochemistry, 1a Favorsky st., Irkutsk, 664033, Russia;2. V.S.Sobolev Institute of Geology and Mineralogy, 3 Koptyuga ave., Novosibirsk, 630090, Russia;3. Novosibirsk State University, 2 Pirogova st., Novosibirsk, 630090, Russia |
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| Abstract: | 
Compositions of picroilmenite and pyrope concentrates from Carboniferous sandstones in the Arkhangelsk kimberlite province were analyzed by EPMA and LAM ICP MS in Analytic Center of V.S. Sobolev’s Institute of Geology and Mineralogy, SD RAS, Novosibirsk. The results from single grain thermobarometry (Ashchepkov et al., 2010, Ashchepkov et al., 2011, Ashchepkov et al., 2012) for garnet, spinel, ilmenite and clinopyroxene suggest heating of the base of the lithospheric mantle to 1400 °C (45 mw/m2) at 7.0–7.5 GPa and to 900 °C (35 mw/m2) at 3.5–5.5 GPa in an interval corresponding to a lens enriched in chromite and clinopyroxene. The pipes from the eastern fields reveal smoother mantle geotherms and lower temperature PT paths. Mantle columns beneath the kimberlites from northern (Verkhotinskoe field) and western pipes (Kepinskoe field) show heating from the lithosphere base to 5.0 GPa and stepped PT paths shown by chromites probably due to interaction with magmas which caused local Ti-enrichment near 3.0 and 5.5 GPa. The PT paths in the mantle columns beneath the alnöite pipes reveal higher temperature and relatively shallow PT conditions with two major clusters around 3.0 and 5.0 GPa. Trace element patterns for garnets vary from S-type typical of harzburgites to those with a hump in MREE (middle REE) typical for pyroxenites. Lherzolitic garnets with sinusoidal decrease of LREE show distinctive HFSE enrichment. Trace element ratios (Sm/Er)n and (La/Yb)n of garnets correlate positively with pressures estimates by single grain thermobarometry (Ashchepkov et al., 2010, Ashchepkov et al., 2011, Ashchepkov et al., 2012) but only poorly with Cr2O3 content. Enrichment in HFSE of all garnets is related to metasomatism that accompanied the picroilmenite-forming event.Ilmenites reveal two compositional trends. One corresponds to fractionation within conduits at the lower mantle (6.0–7.0 GPa) without contamination. A second trend at <6.0 GPa, formed due to assimilation fractional crystallization (AFC), is characterized by Fe and Cr increase with decreasing pressure. Similar trace element patterns of the various in HREE in ilmenites, possibly partly due to garnet assimilation from wall rock peridotites. The PT conditions and geochemistry for the minerals from the Carboniferous sediments are similar to those from the Lomonosovskoe deposit and Arkhangelskaya pipe (Lehtonen et al., 2009). |
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| Keywords: | Mantle Kimberlite Pyrope Ilmenite Placer Thermobarometry Transect Arkhangelsk |
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