Other
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.223g&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 223
Other
7
Scientific paper
Among the approximately 400 meteorites recovered in the past three years in the Sahara at least 19 carbonaceous chondrites exist. Ten paired samples belong to the CR- chondrite Acfer 059/El Djouf 001 which has been recently described (Bischoff et al., 1992a). Three samples (Acfer 182, 207, 214) represent a unique (carbonaceous) chondrite (Bischoff et al., 1992b) that has similarities to ALH 85085 (e.g. Scott, 1988). Here, we describe six unpaired carbonaceous chondrites and Acfer 217, an unusual chondritic breccia. Three chondrites probably belong to the CV- chondrites (Acfer 082, 086, 272) and two to the CO- chondrites (Acfer 202, 243). Acfer 094 has trace element characteristics of a CM-chondrite, but has oxygen isotope ratios different from CM-chondrites (Bischoff et al., 1991). CV-chondrites: Based on the petrography and mineral chemistry Acfer 082 and 086 are typical CV3 chondrites. Large coarse-grained CAIs (especially in Acfer 082), fine- grained spinel-rich and large olivine-rich aggregates, chondrules and fragments are embedded in a fine-grained groundmass mainly consisting of small Fe-rich olivine laths. In Acfer 082 and Acfer 086 the average compositions of matrix olivine are Fa 55 and Fa 40, respectively. Acfer 082 is quite fresh concerning the degree of weathering, whereas Acfer 086 is severely weathered as indicated by the occurrence of abundant calcite filling fractures and pores between the olivine laths of the matrix. Acfer 272 has a mineral chemistry similar to Acfer 082, but is in texture, trace element chemistry (Spettel et al., 1992), and oxygen isotopes (Mayeda and Clayton, pers. communication) different to Acfer 082. Acfer 272 has a densely compacted matrix without individual euhedral to subhedral olivine laths. CO-chondrites: Acfer 202 and 243 consist of abundant porphyritic chondrules, CAIs, olivine-rich inclusions, and fragments embedded in abundant fine-grained matrix. The main component of the matrix is Fe-rich olivine. Matrix olivines are quite homogeneous in composition; their average compositions in Acfer 202 and 243 are Fa 45 and Fa 38, respectively. Acfer 094: This chondrite has trace element characteristics of CM-chondrites, but cannot be a CM-chondrite based on the oxygen isotopic composition (Bischoff et al., 1991), which is more closely related to CO-chondrites. The sample consists of abundant chondrules, olivine aggregates, CAIs and fragments embedded in an extremely fine-grained, sulfide-rich groundmass. Based on microprobe analyses of the bulk matrix we rule out significant proportions of phyllosilicates (high totals). The abundance of nonporphyritic chondrules (~8%) is higher than in most CO-chondrites, similar to that of CM-chondrites. Acfer 217: This meteorite is a new interesting chondritic breccia. At the moment it cannot be assigned either to the ordinary or carbonaceous chondrites. Acfer 217 consists of a low percentage of chondrules and larger fragments embedded in a well-compacted olivine-rich matrix. The overwhelming majority of olivines are Fe-rich (Fa ~39); however, Mg-rich olivines of Fa 3-20 are present as well. Ca-rich pyroxenes are more abundant than Ca-poor pyroxenes. The latter are unequilibrated in composition (Fs 3-32; mean: 25.0 +- 10.9). The sample is extremely depleted in Ni; no Fe,Ni-metals were observed. Other constituents include abundant plagioclase with variable composition, large Cr-spinels and clusters that appear to consist of phyllosilicates. We rule out that Acfer 217 is related to CK chondrites because the latter have lower mean Fa-values (Fa 30-33), significant amounts of NiO in olivine, and have equilibrated olivines and pyroxenes. References: Bischoff A., Palme H., Clayton R.N., Mayeda T.K., Grund T., Spettel B., Geiger T., Endress M., Beckerling W., and Metzler K. (1991) Meteoritics (abstract) 26, 318. 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., Endress M., and Weber D. (1992a) Geochim. Cosmochim. Acta. 56 (submitted). Bischoff A., Palme H., Schultz L., Weber D., Weber H.W., and Spettel B. (1992b) Geochim. Cosmochim. Acta. 56 (submitted). Scott E.R.D. (1988) Earth Planet. Sci. Lett. 91. 1-18. Spettel B., Palme H., Wlotzka F., and Bischoff A. (1992) Meteoritics 27 (this volume).
Bischoff Addi
Geiger Tobias
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