Mathematics – Logic
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30q.576s&link_type=abstract
Meteoritics, vol. 30, no. 5, page 576
Mathematics
Logic
Alteration, Aqueous, Calcium-Aluminum-Rich Inclusions, Chondrites, Carbonaceous, Chondrules, Rims, Condensation, Matrix, Meteorites, Allende, Efremovka, Mekoia
Scientific paper
Our review of mineralogical variations among CV3 chondrites suggests that all components, chondrules, matrices, and CAIs, were affected by various degrees of secondary mineralization. Chondrules and CAIs are rimmed with fayalitic olivine [1, 2]; metal in all components is oxidized and sulfidized to magnetite, Ni-rich metal and sulfides [3]; silicates in all components are aqueously altered to phyllosilicates [4]; and nepheline, sodalite, wollastonite, and hedenbergite replace primary minerals in CAIs [5]. In those CV3s with altered CAIs, nepheline etc. are also present in chondrule mesostases [6] and in matrices [7]. Correlated occurrences of secondary minerals indicate that they have related origins. CV3 chondrites can be divided into three kinds according to their secondary features. Reduced CV3s (e.g., Efremovka) lack magnetite [8] and show minimal secondary features. Oxidized CV3s [8] generally show all features: those like Mokoia contain minor fayalitic rims, nepheline, etc, whereas those like Allende lack phyllosilicates but contain well developed fayalite rims and abundant nepheline, etc. Allende-like CV3 chondrites also contain abundant plate-like matrix olivine (Fa(sub)45-55). Similarities in chemistry and O isotopic composition and petrographic observations suggest that fayalitic rims and plate-like matrix olivine have related origins [1, 9]. The presence of secondary minerals in all components implies that alteration postdated component formation. The absence of secondary minerals in reduced CV3s indicates that CV3 oxidized formed from CV3 reduced-like material. Oxidized and reduced materials coexist in some breccias indicating a common parent asteroid. Nebular origins are widely accepted for most secondary features. To form fayalitic rims and matrix , Palme and colleagues [10, 11] suggest that chondritic components were briefly exposed to a hot (>1500 K), highly oxidizing nebula with H2O/H2 to about 1. Such an environment could have resulted from vaporization after >1000-fold dust/gas enrichment [11]. Fe-rich olivine will not condense until most Mg has condensed into forsterite [11]. The steep compositional gradients between adjacent fayalite and forsterite limit the duration of fayalite condensation to a period of several hours [2]. There are several inconsistencies in this late-stage evaporation-condensation model. Fayalitic rims occur inside chondrules and formed by alteration, not by condensation. Forsterite and enstatite grains that supposedly condensed from the nebula are absent on chondrule rims and in chondrites. Magnetite, Ni-rich metal and sulfides are present inside matrix olivine, inconsistent with equilibrium calculations. I-Xe data suggest that sodalite formation in Allende lasted for about 10 Myr, which is inconsistent with a nebular origin [12]. Asteroidal alteration is favored for magnetite [3] and required for most phyllosilicates [4]. Asteroidal formation of fayalite [13] was rejected [2], partly because hydrous minerals are absent in Allende. We suggest that Allende-like CV3 chondrites may have formed in an asteroid by aqueous alteration and dehydration; see Krot et al. [this volume] for details. Higher Na and K concentrations in oxidized CV3 chondrites are not inconsistent with asteroidal alteration, as CM2 chondrites show similar heterogeneities. Acknowledgments: This work was supported by NASA grants NAGW-3281 (K. Keil) and 152-11-40-23 (M.E.Z.). References: [1] Peck J. A. and Wood J. A. (1987) GCA, 51, 1503-1510. [2] Hua X. et al. (1988) GCA, 52, 1389-1408. [3] Blum J. D. et al. (1989) GCA, 53, 543-556. [4] Keller L. P. et al. (1994) GCA, 58, 5589-5598. [5] Hashimoto A. and Grosman L. (1987) GCA, 51, 1685-1704. [6] Kimura M. and Ikeda Y. (1992) Proc. Symp. Antarc. Meteorites, 17, 31-33. [7] Peck J. A. (1983) LPS XIV, 373-374. [8] McSween H. Y. (1977) GCA, 41, 1777-1790. [9] Weinbruch S. et al. (1993) GCA, 57, 2649-2661. [10] Palme et al. (1991) Meteoritics, 25, 383. [11] Palme H. and Fegley B. (1991) EPSL, 101, 180-195. [12] Swindle T. D. et al. (1988) GCA, 52, 2215-2227. [13] Housley R. M. and Cirlin E. H. (1983) in Chondrules and Their Origins (E. A. King, ed.), pp. 145-161.
Krot Alexander N.
Scott Edward R. D.
Zolensky Michael E.
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