Other
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27q.204b&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 204
Other
1
Scientific paper
The Renazzo-type carbonaceous chondrites Acfer 087 and El Djouf 001 (Bischoff et al., 1992a) and the unique chondrite Acfer 182 (Bischoff et al., 1992b; Weber and Bischoff, 1992) contain Ca- dialuminate (CaAl(sub)4O(sub)7 or CA2) -bearing inclusions that are generally much larger than those found in ALH85085 (Kimura et al., 1992), a meteorite very similar to Acfer 182. We have studied four such inclusions, two (CAI-6 and CAI-9) from Acfer 182, and one each from Acfer 087 (CAI-D) and El Djouf 001 (CAI- 10). CAI-6 (420 x 220 micrometers) consists of CA2, poikilitically enclosing small (~2-3 micrometers) perovskites, CAI-9 (round, 235 micrometers diameter) of nearly pure CA2. Both inclusions are surrounded by rims of spinel (1-2 micrometers; only in CAI-6), gehlenitic (Angstrom k(sub)1.8) melilite (~10-20 micrometers) and diopside (1-2 micrometers). The other two inclusions (CAI-D 180 x 110 micrometers, CAI-10 180 x 95 micrometers) consist of intergrown CA2 and melilite with minor perovskite, spinel, hibonite, and (in CAI-D) an Al-rich phase (Al-hydroxide?). The Ca-isotopic compositions are normal. There are no detectable ^26Mg excesses in CAI-6 and CAI-9. In contrast, the other two inclusions have ^26Mg excesses; data points lie on a line with the canonical (^26Al/^27Al)(sub)0 ratio of 5 x 10^-5. In this respect these inclusions resemble hibonite-bearing inclusions where hibonite associated with melilite tends to have radiogenic ^26Mg while isolated hibonite has none (Hutcheon et al., 1986; Ireland et al., 1991) although the CA2 inclusions do not display the ^48Ca anomalies typical for the latter. Trace element analysis revealed Group II patterns, indicating a condensation history, in all four inclusions. There are considerable variations, though. Whereas in CAI-6 all refractory REE Gd-Er and Lu are lower than the LREE by a factor of ~20, this difference is smaller in the other inclusions. In CAI-D and CAI- 10 there is a steady decrease in CI-normalized abundances from Tb to Lu. While in CA2 of CAI-9 Tm is the most abundant REE element, the melilite rim shows large excesses in Eu and Yb, the most volatile REE (Fig. 1). This pattern indicates that the rim must have condensed onto the CA2 core from a reservoir with different chemical composition than that from which the core formed. References: Bischoff A., Palme H., Ash R. D., Clayton R. N., Schultz L., Herpers U., Stoffler D., Grady M. M., Pillinger C. T., Spettel B., Weber H., Grund T., Endreb M. and Weber D. (1992a) Geochim. Cosmochim. Acta, submitted. Bischoff A., Palme H., Schultz L., Weber D., Weber H. W. and Spettel B. (1992b) Geochim. Cosmochim. Acta, submitted. Hutcheon I. D., Armstrong J. T. and Wasserburg G. J. (1986) Lunar Planet. Sci. (abstract) 17, 372-373. Ireland T. R., Fahey A. J. and Zinner E. K. (1991) Geochim. Cosmochim. Acta 55, 367-379. Kimura M., El Goresy A., Palme H. and Zinner E. (1992) Geochim. Cosmochim. Acta, submitted. Weber D. and Bischoff A. (1992) Meteoritics, this volume.
Beckerling W.
Bischoff Addi
Weber Darrell
Zinner Emst
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