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HEAVY-ELEMENT ACTIVATION ANALYSES OF METEORITES George W. Reed, Jr. Argonne National Laboratory During the past few years, a comparatively large number of neutron activation analyses for various heavy elements have been done on meteorites by our group (Hamaguchi, Kigoshi, Turkevich and Reed). The results of this work in general have been published already (Hamaguchi et al., 1957) or are in press (Reed et al., 1960), but it seems worthwhile To- review briefly some of the data, indicating certain of the principal impli- cations they may have. Analyses have been done for Hg, Tl, Pb, Bi and U; Ba has been determined as a by-product of the uranium work. In some cases, infor- mation on isotopic abundances has been obtained. These results have significance for the investigation of the so-called "cosmic" (really mete- oritic, in this case) abundance curve, and for the history of the meteorites. In particular, it has been possible to show the identity of meteoritic and terrestrial ratios of isotopes of Ba, Hg and U (to within 10 percent or better), and to confirm independently some of the mass spectrometric work on meteoritic Pb-isotope ratios. The accuracy of our work is of necessity not as good as can be obtained by mass-spectrometry, but problems of terrestrial contamination are generally less prominent in our methods. A significant point is that Indarch (enstatite chondrite), Mighei, and Orguiel (carbonaceous chondrites) not only contain much more Pb, by a factor of about ten, than the "ordinary" (hypersthene-bronzite) chondrites studied so far, but their Pb208/po204 ratios are the same, within experi- mental errors, as the ratio for primordial Pb in troilite from iron mete- orites, as determined both by Patterson (1956) and by ourselves. Since these chondrites are very different from iron meteorites, it would seem that Patterson's identification of primordial Pb is strongly supported by these results. A very interesting pattern can be inferred from the data on ele- mental abundances in the meteorites studied. "Ordinary" chondrites have rather low and reasonably constant abundances of Tl, Pb, Bi and U, while the first three elements are very much more abundant in enstatite and carbonaceous chondrites (see Table 1). U (and Ba) are quite constant in all chondrites. The elemental abundances of the carbonaceous and 82
TABLE 1 Abundances of Heavy Elements in Chondrites Tl Pb (10-9 g/g) (10-6 g/g) Bi (10-9 g/g) "Ordinary" Chondrites ~0.4 0. 01 - 0.4 ~3 Abee (Enstatite Chondrite) ~87 ~3.5 ~80 Indarch (Enstatite Chondrite) 125 ~2. 0 prob. > 100 Mighei (Carbonaceous Chondrite) 97 -1. 5 180 Orguiel (Carbonaceous Chondrite) 141 ~3.0 130 enstatite Chondrites agree more nearly with the predicted values of Suess and Urey and of Cameron than those observed in "ordinary"chondrites. The small amount of data on Hg abundances appears to fit this pattern, except that the carbonaceous chondrite Orguiel seems to have a remark- ably high content of Hg. It may be quite possible to correlate Hg contents with the thermal history, as indicated by compactness, hardness, content of volatiles, etc., of the meteorites. Further studies of Hg in stone mete- orites are in progress. While the variation in Hg abundances is probably at least in part a reflection of different thermal histories, no such simple explanation seems applicable to the observations for the other elements. A variation in sulfur content among the several classes of chondrites exists, but is too small to account for the differences in elemental abundances without postulating large fractionations of the trace elements relative to sulfur. Analyses of U and Bi in Nuevo Laredo, which is an achondrite strongly enriched in the former element, allow us to infer the time elapsed between the nucleosynthesis of Np237 and the solidification of this meteor- ite. Assuming that 2. 2 x 106 year Np237 (ancestor of Bi2°9) was formed in a single-event nucleosynthesis with abundance equal to that of IT2.", the time elapsed until the solidification of Nuevo Laredo was >2 x 107 years, given that there is ~10-7 g U/g and<2 x 10-9 g Bi/g in Nuevo Laredo. [This lower limit concurs with the I-Xe ages reported elsewhere by Reynolds and with the models proposed by Cameron and Kohman in this volume, in the sense that it is about an order of magnitude less than the time spans indicated.] Anders: The abundance anomalies pointed out by Reed may well be of great significance for considerations of the histories of meteorites. These large fractionations of certain trace elements may be explained 83
by a model recently proposed by Fish. Goles and Anders (1960). The important conclusions reached are that such a fractionatiort is not only possible but that Hg, Tl, Pb, Bi and In should be affected by it and Se, Zn and Cd should not be fractionated. It is suggested that the rare meteorites which are troilite-rich (e. g., Soroti) may represent a reservoir of certain trace elements which are chalcophile at high tem- peratures, including those elements subject to the abundance anomalies discussed by Reed. Weatherill: From your work on Nuevo Laredo, could you state whether the Pb-Pb and U-Pb ages are or are not concordant? Reed: The ages are very close to agreeing with each other. We feel, however, that we do not know the Pb isotopic composition accurately enough to answer this question. Certainly, the way to go at it is to measure, with high sensitivity, U, Th and Pb in the same sample. We are trying to develop a procedure for this now. Turkevich: The discrepancies in the Nuevo Laredo ages are still some- what outside what we believe to be our experimental error, but it is possible that with further work they could be brought into agreement. For every other meteorite on which we have enough information, the ages are definitely discordant, and therefore, in our view, suspect. REFERENCES Fish, R. A., Goles, G. G., and Anders, E. (1960) Astrophys. J. 132, 243. Hamaguchi, H., Reed, G. W., and Turkevich, A. (1957) Geochim. et Cosmochim. Acta 12, 337. Reed, G. W., Kigoshi, K., and Turkevich, A. (1960) Geochim. et Cos- mochim. Acta (in press). Patterson, C. C. (1956) Geochim. et Cosmochim. Acta 10, 230. 84
