Mathematics – Logic
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.308w&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 308
Mathematics
Logic
1
Scientific paper
Acfer 111 is a regolith breccia with exceptionally unfractionated solar noble gases (Pedroni and Weber, 1991). It consists of an unequilibrated host containing lithic clasts of higher petrologic type. Two mm-sized clasts were found that consist of euhedral to skeletal olivine crystals (Fa 18.5) in a K-rich mesostasis, similar to the clasts found before only in LL-chondrites (Wlotzka et al., 1983). The bulk composition of these clasts is close to the mean bulk H-chondrite silicate composition, but with higher K (and Rb, Cs) and lower Na (Table 1). The REE, which are usually enriched together with K in igneous differentiations, are also close to normal, see Fig. 1 (except for a negative Eu anomaly that was also found in the K-rich clasts in LL chondrites). This shows that these clasts were not formed by igneous differentiation. The Na-K relations in the host and equilibrated clasts also show deviations from the average chondritic Na/K ratio of 8.0. In the feldspar and feldspar glasses of the host this ratio varies between 1.0 and 80, mean 13.5. An H5 clast has a low bulk K content of 350 ppm, Na/K is 18; in the feldspars Na/K is 21 +-9. It seems significant that the average Na/K is higher than the chondritic ratio, indicating K loss. Acfer 111 also contains several fragments (about 100 micrometers) of K-rich crystalline feldspar (18 to 33% Or) in the host. The chondritic bulk silicate composition of the K-rich clasts suggests an origin as impact melt that lost metal and sulfide. It remains to explain the differentiation of the alkalies. One possible way is vapour/solid fractionation, where K is preferentially incorporated into cold solid feldspar or feldspar glass, while Na stays in the vapour (Wlotzka et al., 1983). This relation between K and Na in vapour and solid has been established experimentally (Orville, 1963). A location for this process can be found on the cold surface or cold pocklets in a primordial regolith of a parent body, alkali-containing vapours coming from hot spots in the interior. Material like the H5 clast with low K may have lost its K in this process. The K/Ar-ages of the two K-rich clasts deduced from ^40Ar concentrations of (6.4 - 6.8)x10^-4 cc stp/g are 4.5+-0.1 GA and show that the source rocks were formed very early in the history of the parent body. Cosmogenic ^21Ne in the K-rich clasts are comparable (when corrected for the target element abundances) to the average observed for the normal clasts (see Pedroni, this volume). References: Jarosewich E. (1990) Meteoritics 25, 323-337. Kallemeyn G.W., Rubin A.E., Wang D., Wasson J.T. (1989) Geochim. Comochim. Acta 53, 2747-2767. Orville, P.M. (1963) Amer. J. Sci. 261, 201-237. Pedroni, A. and Weber H. (1991) Meteoritics 26, 383. Wlotzka, F., Palme H., Spettel B., Wanke H., Fredriksson K., Noonan A.F. (1983) Geochim. Cosmochim. Acta 47, 743-757. Table 1 SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O K2O Acfer 111 K-rich clast S 49.7 0.16 2.56 0.68 13.40 0.38 29.3 2.02 0.77 1.04 Av. H-chondrite silicate 48.2 0.16 2.82 0.68 13.56 0.41 30.6 2.29 1.13 0.12 Figure 1, which in the hard copy appears here, shows element abundance for K-rich clast S versus mean H-chondrite silicate composition (Jarosewich, 1990; Kallemeyn et al., 1989).
Pedroni Anselmo
Spettel Bernhard
Wlotzka Frank
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