Mathematics – Logic
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27q.304w&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 304
Mathematics
Logic
4
Scientific paper
Ca-dialuminate is a rare phase in Ca,Al-rich inclusions from carbonaceous chondrites (e.g., MacPherson et al., 1988; Kimura et al., 1992). Recently, some reports on CaAl4O7 within CAIs in the paired samples of the CR-type chondrite El Djouf 001/Acfer 059 were published (Weber and Bischoff, 1992). Based on the high modal abundance of Ca-dialuminate in three CAIs from the Acfer-El Djouf CR-chondrite these authors defined a new class of CAIs: Ca- dialuminate-rich inclusions (compare Nos. 1,2; Tab. 1). In this paper we present mineralogical and chemical data on Ca- dialuminate-rich inclusions within the unique chondrite Acfer 182, which has been recently described (Bischoff et al., 1992a). The modal abundance of CAIs in Acfer 182 is certainly <1 vol% and the size of the inclusions is below 450 micrometers. Most of the inclusions are irregularly shaped; some are spherical. The most abundant types of CAIs are Ca-dialuminate-rich and melilite-rich inclusions (Type A). Also, some spinel-bearing olivine-rich (AOAs), and unusual spinel-hibonite and perovskite-rich inclusions were observed. In the latter inclusion Y,Zr-rich perovskite has a modal abundance of about 80 vol%. In nine of about 20 Ca,Al-rich inclusions CaAl4O7 is an abundant phase. In five CAIs Ca-dialuminate is by far the dominating mineral. Textures and chemistry: In four of the nine Ca-dialuminate- bearing CAIs the core of the inclusion consists of CaAl4O7 with tiny perovskite and/or melilite inclusions. These cores are either rimmed by layers of melilite (+-perovskite, spinel) and Ca-pyroxene or by layers of spinel (+-hibonite), melilite (+- perovskite, spinel) and Ca-pyroxene (compare Nos. 3,5,6; Tab. 1). In three cases (compare No. 4; Tab. 1) cores consist of large Ca- dialuminate (+-perovskite) areas poikilitically enclosed by melilite (+- perovskite and spinel). These assemblages are surrounded by a very thin layer of Ca- pyroxene. One of the Ca- dialuminate-rich inclusions (No. 7) has a core of perovskite- bearing hibonite enclosed by a mantle of Ca-dialuminate (+perovskite). This mantle follows a layer of an intergrowth of hibonite and spinel. The outermost layers consist of melilite and Ca-pyroxene. Additionally, Ca- dialuminate is present as a minor phase in the perovskite-rich inclusion mentioned above. The Ca- dialuminate-rich inclusions are strongly enriched in Al2O3 (Table 1). Based on an ion probe study the trace element analysis indicates a condensation history for these Ca,Al-rich inclusions (Bischoff et al., 1992b). References: Bischoff A., Palme H., Schultz L., Weber D., Weber H.W., Spettel B. (1992a) Geochim. Cosmochim. Acta (submitted). Bischoff A., Beckerling W., Weber D., and Zinner E. (1992b) Meteoritics, this volume. Kimura M., El Goresy A., Palme H., and Zinner E. (1992) Geochim. Cosmochim. Acta (submitted). MacPherson G.J., Wark D.A., and Armstrong J.T. (1988) In "Meteorites and the Early Solar System" (eds. J.F. Kerridge and M.S. Matthews), 746-807, Univ. Arizona Press, Tucson. Weber D. and Bischoff A. (1992), Lunar Planet. Sci. (abstract) 23, 1505-1506; Table 1: Bulk composition of Ca-dialuminate-rich inclusions from the CR-chondrite Acfer-El Djouf (Nos. 1,2) and from Acfer 182 (Nos. 3-7). .....CAI 10..CAI D...CAI 15..CAI 17..CAI 6...CAI 9...CAI 19 .......1.......2.......3......4**......5.......6.......7 SiO2..11.7.....2.32....1.15....8.1.....1.01....4.6.....2.44 TiO2...1.33....1.66....0.86....2.39....2.09....0.72....5.1 Al2O3.56.9....75.6....73.5....59.3....72.3....69.1....76.1 Cr2O3.<0.02...<0.05....n.d....<0.04...<0.01...<0.01...<0.03 FeO....0.49....0.88....0.48....2.85....0.26....1.80....1.14 MnO...<0.01...<0.02...<0.01...<0.02...<0.03...<0.01...<0.01 MgO....4.0.....2.11....0.10....0.43....0.86....0.31....1.57 CaO...25.5....17.3....22.0....26.8....21.3....23.4....13.0 Na2O..<0.01....0.08....n.d....<0.05...<0.01...<0.02...<0.01 K2O...<0.01...<0.01...<0.01...<0.02...<0.01....n.d....<0.02 Total.99.97*.100.03*..98.11..100.00*..97.88...99.97*..99.42 *Normalized to 100 wt% based on high abundance of pores and fractures. **Contains secondary materials in fractures.
Bischoff Addi
Weber Darrell
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