Computer Science
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27q.218e&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 218
Computer Science
2
Scientific paper
Recent Cr isotopic measurements of sphalerites and alabandites in three EL3 chondrites MAC88136, MAC88180, and MAC88184 and of sphalerites in the EH4 chondrite Indarch (El Goresy et al., 1992) revealed ^53Cr excesses (^53Cr*), resulting from the in situ decay of ^53Mn (tau(sub)1/2=3.7 Ma), in most grains analysed. However, the initial ^53Mn/^55Mn ratios calculated for these grains were quite variable, and it was concluded that redistribution of ^53Cr* by diffusional processes was the most likely cause for these variations. In a continuation of the previous work (El Goresy et al., 1992), we report new mineral-chemical and Cr-isotopic data for two EH3 chondrites, Qingzhen and Yamato 69001. The distribution of Fe, Mg, and Mn in niningerites and sphalerites occurring in individual sulfide assemblages was determined by electron microprobe analysis. Among the meteorites of the EH3 subgroup, Qingzhen and Yamato 69001 are unique in that niningerites in both meteorites display normal as well as reversed zoning, indicating complex thermal histories (Ehlers and El Goresy, 1988; Lin et al., 1989; Lin, 1991; Nagel, 1991). Niningerites have different MnS contents (9.2-32.6 mol% MnS in Qingzhen vs. 4.2- 6.3 mol% MnS in Yamato 69001), as do the sphalerites (4.0-9.2 mol% MnS in Qingzhen vs. 2.0-3.5 mol% in Yamato 69001). Sphalerites in both meteorites are normally zoned, with 46.0-49.8 mol% FeS in sphalerites from Qingzhen, and 42.3-49.7 mol% FeS in sphalerites from Yamato 69001. The spatial distributions of Fe and Mg in niningerites and of Fe and Mn in sphalerites indicate complex processes that may have occurred before accretion and/or during later metamorphic events in the parent body (El Goresy and Ehlers, 1989; Lin, 1991; Nagel, 1991). Ion microprobe measurements of 6 sphalerites and 3 niningerites in Qingzhen and of 3 sphalerites and 2 niningerites in Yamato 69001 showed that ^55Mn/^52Cr ratios in these sulfide phases are significantly lower than in sphalerites and alabandites from the three EL3 chondrites previously analysed (El Goresy et al., 1992). Within analytical uncertainties, no ^53Cr excesses were found in any of the analysed Qingzhen and Yamato 69001 grains and only upper limits on the inferred ^53Mn/^55Mn ratios could be obtained. In Qingzhen, the smallest upper limits on the ^53Mn/^55Mn ratio in the niningerites and the sphalerites are 4.3x10^-6 and 2.0x10^-6, respectively; in Yamato 69001, the corresponding values for niningerites and sphalerites are 9.0x10^-6 and 3.0x10^-6. These values are consistent with previous data on EL3 chondrites (El Goresy et al., 1992) but the sphalerite limits are smaller than the highest ^53Mn/^55Mn ratio of (6.5+-1.8)x10^-6 inferred for a sphalerite in Indarch (El Goresy et al., 1992). This does not, however, imply that sphalerites in Qingzhen and Yamato 69001 are younger than their counterparts in Indarch but rather indicates that sulfides in the two EH3 chondrites studied here are more equilibrated than those in Indarch. REFERENCES Ehlers K. and El Goresy A. (1988) Geochim. Cosmochim. Acta 52, 877-888. El Goresy A. and Ehlers K. (1989) Geochim. Cosmochim. Acta 53, 1657-1668. El Goresy A., Wadhwa M., Nagel H.-J., Zinner E.K., Janicke J., and Crozaz G. (1992) Lunar Planet. Sci. (abstract) 23, 331-332. Lin Y.T. (1991) Ph.D. Thesis, Inst. Geochim. Guiyang, PRC, 205 pp. Lin Y.T., El Goresy A., Ouyang Z., and Wang D. (1989) Meteoritics (abstract) 24, 293. Nagel H.-J. (1991) Ph.D. Thesis, Univ. Heidelberg, 117 pp.
Crozaz Ghislaine
El Goresy Ahmed
Janicke J.
Nagel H.-J.
Wadhwa Meenakshi
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