Other
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.218e&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 218
Other
2
Scientific paper
Introduction: Dark clasts (DC) occur in almost all known chondrite groups. They have been interpreted mostly as matrix or C1 and C2 chondritic material. Weisberg et al. (1991) recently described DCs in six samples of the CR2 chondrite group. Here we focus on DCs in the paired samples of the Acfer/El Djouf meteorite (Bischoff et al., 1992). Thirty-six DCs in 12 polished thin sections were studied. Results: DCs occur in all investigated thin sections with an average of 50 DCs in each and make up 2.4 to 3.7 vol%. DCs are mostly irregularly shaped and up to 2.4 mm in maximum dimension, with most ranging between 200 and 400 micrometers. They have sharp boundaries to the surrounding matrix of the host meteorite in reflected light as well as in backscatter images of an SEM. Some DCs are light to dark brownish, others are opaque in transmitted light. Both types are heavily fractured and contain only a negligible abundance of weathering products. DCs have abundant fine-grained matrix including chondrule fragments, mineral fragments and, frequently, phyllosilicate clusters. The abundances of these components vary greatly among the DC. Only one CAI and two microchondrules in DCs were detected. Chondrule fragments are mostly relics of former porphyritic chondrules (PO, POP, PP); olivine and pyroxene compositions are Fa1-47 and Fs3-25, respectively. Some olivines show high MnO contents up to 3 wt%. Major components in the matrix of DCs are magnetite, pyrrhotite and pentlandite. Magnetite grains are up to 25 micrometers in size and show the typical features of magnetite from CI chondrites: spherules, framboids and platelets. The sulfides constitute an average of 1 vol% and occur mostly as large laths up to 50 micrometers long. Accessory phases throughout the matrix in DCs are spinel, carbonate (ankerite-like), phosphate, ilmenite, schreibersite and Fe, Ni. Two Os,Mo,Ir-rich particles were detected. The bulk compositions of individual DCs vary, probably due to different abundances of chondrule fragments, phyllosilicates, sulfides and magnetite. DCs are occasionally rich in Fe and/or S, reflecting high sulfide and/or magnetite abundances; others are rich in Mg and/or Mn due to abundant chondrule fragments and/or Mn-rich phases (carbonates, Mn-rich olivine). On average, the abundances of most major elements are similar to those of CI-chondrites (0.8-1.3 x CI). Exceptions are Na and S, which are strongly depleted (0.17 x CI and 0.13 x CI, respectively). The oxygen isotopic compositions of two DCs were determined [sample 1: delta^18O: 3.39, delta^17O: -0.81; sample 2: delta^18O: 5.60, delta^17O: 0.99] (T.K. Mayeda and R.N. Clayton; pers. comm.). Both points lie below the main trend of CR chondrites and chondrules. They show similar compositions to dark inclusions in Allende (Bischoff et al., 1988). Conclusions: Textural and compositional differences between DCs and matrix in the Acfer/El Djouf meteorite strongly suggest a different origin for DCs. They probably existed as lithified material prior to final accretion of the Acfer/El Djouf parent body. The chondrule fragments in DCs are much more altered than chondrules in the host meteorite, indicating a higher degree of aqueous alteration for DCs. This lends support to the suggestion that DCs were formed under oxidizing conditions in a different nebular environment (perhaps on planetesimals that were destroyed), in contrast to the dominating components of Acfer/El Djouf that were formed under reducing conditions. References: Weisberg M.K., Prinz M., Chatterjee N., Clayton R.N., and Mayeda T.K. (1991) Meteoritics (abstract) 26, 407. 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. (1992) Geochim. Cosmochim. Acta. (submitted). Bischoff A., Palme H., Spettel B., Clayton R.N., and Mayeda T.K. (1988) Lunar Planet. Sci. (abstract) 19, 88-89.
Bischoff Addi
Endress Magnus
Keil Klaus
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