The 842-g Kaidun meteorite fell on 3 December 1980 in South Yemen and is classified as a CM2 chondrite (Graham et al., 1985). However, it differs markedly from the "typical" C2 chondrite in that it contains a mixture of petrographically recognizable components of very different properties (Clayton et al., 1994). We have been provided with a number of 8-30 mg portions that have been characterized petrographically, by INAA and by oxygen isotopes properties (Clayton et al., 1994). We report here on 6 of these samples, 2 classed as enstatite chondrite-like and 4 as carbonaceous chondrite-like.
We irradiated these samples for 17 or 30 days at a flux of 8 x 1013 n/cm2/s and used our RNAA technique to determine 15 trace elements of varying geochemical character and volatilities: U, Au, Co, Sb, Ga, Rb, Ag, Se, Cs, Te, Zn, Cd, Bi, Tl, In (in increasing order of volatility).
The 2 enstatite chondrite-like samples, # 05.05 and
# 01.3.01 are described as EL and melted enstatite material, respectively. Volatiles in these 2 samples are consistent with expectations from their petrography: sample # 01.3.01 contains substantially less (<<0.5x) Sb and 8 more volatile elements (Rb ÆBi), and equivalent levels of Tl and In than sample # 05.05. However, more refractory elements also differ: there is very much less U and more of the siderophiles Au, Co and Ga in sample # 01.3.01 than in # 05.05. Apparently, these 2 samples derive from different reservoirs, each with its characteristic chemical fractionation history. The trace element patterns in these 2 samples bear some resemblance to those of Khairpur (Binz et al., 1974) and Abee treated to 1100-1300°C (Biswas et al., 1974), respectively. However, there are some differences: Ag, Te, Zn, and Cd are richer in # 05.05 than in Khairpur and Bi, Tl, and In are depleted. Most elements (Co, Ag, Cs, Te, Zn, Cd, Bi, Tl, In and Sb(?) in sample # 01.3.01 are at levels similar to those in Abee (EH4) heated for a week at 1100-1300°C (Biswas et al., 1974) but U, Ga, and Se levels in these samples differ very markedly. Undoubtedly, extended heating of # 01.3.01 parent material was responsible for much of the elemental fractionation evident in it but some other process was responsible for U loss, and gain of Ga and Se.
The 4 carbonaceous chondrite-like samples are generally volatile-rich and all but one exhibit a trace element pattern like that of Cl and other carbonaceous chondrites Anders & Grevesse, 1989; Xiao & Lipschutz, 1992). However, the exception, # 01.3.18a, is depleted in Te, Bi, and Tl and enriched in Au and Rb. No evidence suggests loss of volatiles by heating (Paul & Lipschutz, 1989).
The trace element data thus far suggest that Kaidun assembled by gentle accretion of small bits of previously processed chondrite material. How materials as different
in oxidation state as these carbonaceous and enstatite
chondrite-like samples could coexist in the same nebular region and moreover accreted gently is very puzzling. Further study of this meteorite could well prove central to unraveling the chondrite genetic story.
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