Formation of spinel-, hibonite-rich inclusions found in CM2 carbonaceous chrondrites

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We report petrography, mineral chemistry, bulk chemistry, and bulk isotopic compositions of a suite of 40 spinel-rich inclusions from the Murchison (CM2) carbonaceous chondrite. Seven types of inclusions are identified based on mineralogy: spinel-hibonite-perovskite; spinel-perovskite-pyroxene; spinel-perovskite-melilite; spinel-hibonite-perovskite-melilite; spinel-hibonite; spinel-pyroxene; and spinel-melilite-anorthite. Hibonite-bearing inclusions have Ti-poor spinel compared to the hibonite-free ones, and spinel-hibonite-perovskite inclusions have the highest average bulk TiO{sub 2} contents (7.8 wt%). The bulk CaO/Al{sub 2}O{sub 3} ratios of the inclusions range from 0.005-0.21, well below the solar value of 0.79. Hibonite-, spinel-rich inclusions consist of phases that are not predicted by condensation calculations to coexist; in the ... continued below

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Simon, S B; Grossman, L; Hutcheon, I D; Phinney, D L; Weber, P K & Fallon, S J November 3, 2005.

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We report petrography, mineral chemistry, bulk chemistry, and bulk isotopic compositions of a suite of 40 spinel-rich inclusions from the Murchison (CM2) carbonaceous chondrite. Seven types of inclusions are identified based on mineralogy: spinel-hibonite-perovskite; spinel-perovskite-pyroxene; spinel-perovskite-melilite; spinel-hibonite-perovskite-melilite; spinel-hibonite; spinel-pyroxene; and spinel-melilite-anorthite. Hibonite-bearing inclusions have Ti-poor spinel compared to the hibonite-free ones, and spinel-hibonite-perovskite inclusions have the highest average bulk TiO{sub 2} contents (7.8 wt%). The bulk CaO/Al{sub 2}O{sub 3} ratios of the inclusions range from 0.005-0.21, well below the solar value of 0.79. Hibonite-, spinel-rich inclusions consist of phases that are not predicted by condensation calculations to coexist; in the equilibrium sequence, hibonite is followed by melilite, which is followed by spinel. Therefore, hibonite-melilite or melilite-spinel inclusions should be dominant instead. One explanation for the 'missing melilite' is that it condensed as expected but was lost due to evaporation of Mg and Ca during heating and melting of spherule precursors. If this theory were correct, melilite-poor spherules would have isotopically heavy Mg and Ca. Except for one inclusion with F{sub Mg} = 4.3 {+-} 2.6{per_thousand}/amu and another with isotopically light Ca (F{sub Ca} = 3.4 {+-} 2.0{per_thousand}/amu), however, all the inclusions we analyzed have normal isotopic compositions within their 2{sigma} uncertainties. Thus, we found no evidence for significant mass-dependent fractionation. Our preferred explanation for the general lack of melilite among hibonite-, spinel-bearing inclusions is kinetic inhibition of melilite condensation relative to spinel. Because of similarities between the crystal structures of hibonite and spinel, it should be easier for spinel to form from hibonite than for melilite to do so.

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PDF-file: 45 pages; size: 0 Kbytes

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  • Journal Name: American Mineralogist, Papike volume, vol. 91, no. 10, October 1, 2006, pp. 1675 - 1687

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  • Report No.: UCRL-JRNL-216859
  • Grant Number: W-7405-ENG-48
  • Office of Scientific & Technical Information Report Number: 895091
  • Archival Resource Key: ark:/67531/metadc878722

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  • November 3, 2005

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  • Sept. 22, 2016, 2:13 a.m.

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  • Dec. 9, 2016, 4:25 p.m.

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Simon, S B; Grossman, L; Hutcheon, I D; Phinney, D L; Weber, P K & Fallon, S J. Formation of spinel-, hibonite-rich inclusions found in CM2 carbonaceous chrondrites, article, November 3, 2005; Livermore, California. (digital.library.unt.edu/ark:/67531/metadc878722/: accessed April 26, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.