Berry, FJ

Bland, PA

Hoffman, E

Krot, AN

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CV3 CHONDRITES:  HOW OXIDIZED OR REDUCED?  P. A. Bland1, E. Hoffman2, A. N. Krot3 and F. J. Berry4, 1Department of Mineralogy, Natural History Museum, Cromwell Road, London SW7 5BD, UK (phib@nhm.ac.uk), 1Physics Department, Morgan State University, Baltimo re, MD 21251, USA, 3Hawai’i Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawai’i at Manoa, Honolulu, HI 96822, USA, 4Department of Chemistry, The Open University, Milton Keynes MK7 6AA, UK.  Introduction:  CV chondrites were initially divided into the reduced and oxidized subgroups based on ap-parent oxidation state defined petrographically (relative proportions of metal vs. magnetite and Ni content in sulfides) [1]. Recently, the oxidized subgroup was sub-divided further into the Bali-like and Allende-like sub-groups also largely based on the secondary mineraliza-tion: the former contains fayalite and phyllosilicates, whereas the latter contains fayalitic olivine, nepheline and sodalite [2,3]. It was shown that fayalite and faya-litic olivine postdate oxidation of metal to magnetite and form by preferential replacement of magnetite; this results in a decrease of modal abundance of magnetite in the oxidized subgroup [3,4]. In order to quantify mo-dal abundances of metal, magnetite and sulfides in CV chondrites, including their fine-grained matrices, we used Mössbauer spectroscopy. A knowledge of the abundance of Fe in a specimen allows absorption intensities in a Mössbauer spectrum to be converted to absolute wt% values of Fe-bearing phases. Previous work used Mössbauer spectroscopy to constrain the abundance of magnetite in bulk sam-ples of Vigarano and Allende [5]. This has now been extended to other CV chondrites. Experimental:  Samples (~0.3 g) were crushed and a 0.1 g aliquot taken for analysis and placed in a lead holder (ca. 1.25 cm2). Iron-57 Mössbauer spectra were recorded at 298K with a microprocessor controlled Mössbauer spectrometer using a 57Co/Rh source. Results:  Bulk chemical data of CV's [6,7] indicates wt% Fe of 23.2 ± 0.9. Therefore, from our Mössbauer spectra we can calculate that ALH 85006 contains 8.0 wt% magnetite, Grosnaja 2.6 wt%, Kaba 10.2 wt%, Mokoia 3.2 wt%, Efremovka 3.3 wt%, and Leoville 3.8 wt%. Vigarano is found to contain 4.1 wt% magnet-ite in the bulk meteorite (3 analyses taken from different areas of the meteorite), and 2.3 wt% in a matrix separate. We do not detect magnetite in Allende or ALH 84028, thus, we estimate that magnetite is present in these meteorites at levels of <0.5 wt%. Mössbauer spectra also allow us to constrain the abundance of FeNi-metal, sulfides, and Fe-containing silicates. Discussion:  Our data indicate that the oxidized Bali-like and Allende-like, and the reduced CV sub-groups, can be separated based on their FeNi-metal, sulfide, and magnetite abundances. The Allende-like meteorites have low abundances of FeNi-metal, sul-fides, and magnetite; metal and sulfides are Ni-rich. The Bali-like meteorites have variable to high abundances of magnetite, but low (or absent) metal and and low sulfides; metal and sulfides are largely Ni-rich. The re-duced CV chondrites have intermediate magnetite abundance, and high contens of metal and sulfides; metal and sulfides are largely Ni-poor. Our recent work [4] may explain this range in abun-dance for the opaque phases within CV3’s. Mineralogi-cal observations and thermodynamic analysis suggest that magnetite, Ni-rich metal and sulfides formed by oxidation of low-Ni-metal-sulfide nodules either in the solar nebula or more likely in an asteroidal environ-ment; a degree of this oxidation was variable through CV chondrites. Subsequently, magnetite of the oxidized opaque nodules was preferentially replaced by fayalite (in Bali-like meteorites), fayalitic olivine (in the Allende-like meteorites), Ca-Fe-rich pyroxenes and andradite in the presence of aqueous solutions in an asteroidal set-ting. References: [1] McSween H. Y., Jr. (1977) GCA, 41, 1777–1790. [2] Weisberg et al. (1997) MAPS, 32, A138–A139. [3] Krot et al. (1998) MAPS, 33, 1065–1085. [4] Krot et al. (2000) MAPS, submitted. [5] Bland P. A. et al. (1999) MAPS, 34, A12–A13. [6] Kallemeyn G. W. and Wasson J. T. (1981) GCA, 45, 1217–1230. [7] Jarosewich E. (1990) Meteoritics 25, 323–337.  63rd Annual Meteoritical Society Meeting 5240.pdf

CV3 chondrites: How oxidised or reduced?

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