Alteration of Al-rich inclusions inside amoeboid olivine aggregates in the Allende meteorite |
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| Institution: | 1. Institute of Geochemistry and Petrology, Clausiusstrasse 25, ETH Zürich, CH-8092 Zürich, Switzerland;2. Origins Laboratory, Department of Geophysical Sciences, The University of Chicago, IL 60637, USA;1. Department of Earth, Planetary, and Space Sciences, University of California Los Angeles, 595 Charles E Young Dr E, Los Angeles, CA 90095, USA;2. Institut für Planetologie, University of Münster, Wilhelm-Klemm-Straße 10, 48149 Münster, Germany;1. CRPG, CNRS, Université de Lorraine, UMR 7358, Vandoeuvre-lès-Nancy 54501, France;2. LSCE, CNRS, UMR 8212, Gif-sur-Yvette 91198, France;3. Université Grenoble Alpes, ISTerre, CNRS, UMR 5275, Grenoble 38000, France;4. Department of Geology and Geophysics, University of Hawaii-Manoa, 1680 East-West Road, Honolulu, HI 96822, USA;5. Natural History Museum, Burgring 7, A-1010 Vienna, Austria;6. IMPMC, CNRS & Muséum national d’Histoire naturelle, UMR 7590, CP52, 57 rue Cuvier, 75005 Paris, France;1. Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, Kosygin St. 19, Moscow, 119991, Russia;2. University of Hawai‘i at Mānoa, 1680 East-West Road, Honolulu, HI, 96822, USA;3. Institute for Dynamics of Geosphere, Russian Academy of Sciences, Moscow, Russia |
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| Abstract: | Lightly altered Al-rich inclusions in amoeboid olivine aggregates have cores containing primary melilite + fassaite + spinel + perovskite and no secondary alteration products. In moderately altered inclusions, whose cores now contain only fassaite + spinel + perovskite, melilite was replaced by a fine-grained mixture of grossular + anorthite + feldspathoids and perovskite was partially replaced by ilmenite. In heavily altered inclusions, fassaite has been replaced by a mixture of phyllosilicates + ilmenite and the remaining primary phases are spinel ± perovskite. In very heavily altered inclusions, no primary phases remain, the spinel having reacted to form either phyllosilicates or a mixture of olivine + feldspathoids. This sequence of alteration reactions may reflect successively lower solar nebular equilibration temperatures. During alteration, SiO2, Na2O, K2O, FeO, Cr2O3, H2O and Cl were introduced into the inclusions and CaO was lost. MgO may have been lost during the melilite reaction and added during formation of phyllosilicates. Electron microprobe analyses indicate that the phyllosilicates are a mixture of Na-rich phlogopite and chlorite or Alrich serpentine. Thermodynamic calculations suggest that, at a solar nebular water fugacity of 10−6, Na-rich phlogopite could have formed from fassaite at ~470 K and chlorite from Na-rich phlogopite at ~328 K. Olivine mantling Al-rich inclusions is not serpentinized, suggesting that these objects stopped equilibrating with the nebular gas above 274 K. |
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