Mathematics – Logic
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30s.530k&link_type=abstract
Meteoritics, vol. 30, no. 5, page 530-531
Mathematics
Logic
3
Asteroids, Calcium-Aluminum-Rich Inclusions, Chondrules, Meteorites, Allende, Solar Nebula
Scientific paper
CV3 chondrites experienced various degrees of late-stage modification, including: fayalitic rims around forsteritic grains, secondary mineralization (e.g., nepheline, sodalite, magnetite) in CAIs and chondrules, and formation of phyllosilicates [1]. Our literature survey show that these secondary features probably have a related origin [2]. Although an asteroidal origin is generally accepted for most phyllosilicates [3] and proposed for magnetite [4], the other secondary features have been attributed to reactions of CV3 components with a hot (> 1500 K) and oxidized (H2O/H2 about 1) nebular gas [5]. The Allende meteorite is considered to be a primitive CV3, because phyllosilicates are absent in the matrix and metamorphic effects are not apparent [6]. However, all other secondary features are well-developed. Studies of dark inclusions (DIs) in CV3s [7] provide a key to understanding the secondary features in Allende. Mineralogical, chemical and isotopic data indicate that DIs and CV3s have related origins. Matrices of the DIs resemble the Allende matrix, but the chondrules show a wider range of alteration textures. In some DIs, the chondrules have only fayalitic rims like those in Allende, but in others there are chondrule-shaped regions of porous fayalitic olivine. In DIs of intermediate type, the chondrules consist of forsteritic cores, which have fayalitic rims, surrounded by porous fayalitic olivine. We conclude that the sequence observed among DIs reflects various degrees of replacement of chondrules by fayalitic olivine. The presence of veins of fayalitic olivine, nepheline and Ca-pyroxene throughout DIs [7] which experienced the most pervasive alteration argues for asteroidal processing of DIs. Like Kojima and Tomeoka [7], we infer that porous fayalitic olivine formed by metamorphic dehydration of phyllosilicates on an asteroid. We suggest that the Allende host and Allende-like DIs represent the initial stages of the alteration sequence from material similar to reduced CV3s. Nearly all of the matrix, the outer edges of the chondrules and the mesostases were aqueously altered to phyllosilicates. Magnetite and Ni-rich sulfides formed at this time. Subsequent heating of Allende resulted in equilibration of opaque assemblages at about 770 K and high fO2 and fS2 [4], dehydration of the phyllosilicates and formation of fayalitic rims and plate-like matrix olivine. Nepheline and sodalite, which occur in veins and/or intergrowths with fayalitic olivine in Allende DIs and matrix, and as alteration products in chondrules and CAIs along with grossular, wollastonite, hedenbergite, and andradite, probably formed simultaneously with fayalitic olivine. Inferred initial ^26Al/^27Al ratios in grossular and sodalite, show that CAI alteration occurred at least 2-4 Myr after CAI crystallization [8]. Variations in the initial ^129I/^127I for chondrules and CAIs in Allende suggest that the alteration lasted for about 10 Myr [9]. Oxidation during aqueous alteration and dehydration of the Allende components also provides a simple explanation for the absence of phyllosilicates in the Allende matrix, depletions in planetary noble gases [10], interstellar SiC, diamonds, and graphite [11]. We thank K. Tomeoka, T. Kojima and P. Buchanan for critical discussions. This work was supported by NASA grant NAGW-3281. References: [1] Krot A. N. and Scott E. R. D. (1995) LPS XXVI, 803-804. [2] Scott E. R. D. et al. (1995) LPS XXVII. [3] Keller L. P. et al. (1994) GCA, 58, 5589-5598. [4] Blum J. D. et al. (1989) GCA, 53, 543-556. [5] Palme H. et al. (1991) Meteoritics, 25, 383. [6] Weinbruch S. et al. (1994) GCA, 58, 1019-1030. [7] Kojima T. and Tomeoka K. (1994) Meteoritics, 29, 484. [8] Davis A. M. et al. (1994) LPS XXV, 315-316. [9] Swindle T. D. et al. (1988) GCA, 52, 2215-2227. [10] McSween H. Y. Jr. (1977) GCA, 41, 1777-1790. [11] Huss G. R. and Lewis R. S. (1995) GCA, 59, 115-160.
Krot Alexander N.
Scott Edward R. D.
Zolensky Michael E.
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