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XENON IN STONE METEORITES John H. Reynolds Department of Physics University of California at Berkeley [REYNOLDS reviewed briefly the negative results of attempts to find excess Xe12^ in Beardsley (chondrite) and Nuevo Laredo (achon- drite), and discussed his positive results on Richardton (chondrite) (Reynolds 1960a) and Murray (carbonaceous chondrite) (Reynolds 1960b).] Table 1 gives the relative abundance of isotopes in the anomalous compo- nent of Xe from Richardton, Murray, Mighei and Orgueil. These values are derived by subtracting Xe of terrestrial composition, normalized to Xe . from the total meteoritic Xe as given by peak heights. This nor- malization is arbitrarily chosen so that the anomalous abundances are all positive. It seems likely that a considerable fraction of the so-called terrestrial component is in fact atmospheric Xe adsorbed on the sample and apparatus. No outgassing of the crucible was included in the proce- dure; the extraction system is being redesigned in order to make it pos- sible to include such a step. TABLE 1 Anomalous Components of Meteoritic Xenon (Percent Abundance) Isotope Richardton Murray Mighei Orgueil 124 0. 33±0. 09 0. 64±0. 05 0. 62±0. 11 0. 72±0. 24 126 0. 23±0. 02 0. 56±0. 10 0. 52±0. 10 0. 52±0. 25 128 2. 5 ±0.3 6. 52±0. 25 6. 05±0. 50 6.45±0. 41 129 81. 5±3. 5 46.6±3.4 54. 3±4. 4 49.4±4.4 130 2. 3±0.4 7. 33±0. 81 5.67±0. 60 6. 08±0. 68 131 8. 2±2. 0 25. 12±2. 7 22. 5±2. 9 24.03±3. 0 132 3.6±2. 3 11. 59±3.4 9. 52±3. 37 11.63±2.5 134 1. 4*1.0 1.69±1. 3 0. 83±1. 30 1. 16±1. 31 136 0 0 0 0
In general, there appear to be two types of Xe mass spectra observed in meteorites. The "anomalous" spectra are, except for variable amounts of Xe ', identical within experimental error with that of Murray, and when normalized to terrestrial Xe (choosing Xe as the reference iso- tope) indicate that there may have been a mass fractionation effect in the Earth's atmosphere. The two heaviest Xe isotopes, Xe and Xe* , do not seem to follow such a fractionation curve, however [see especially Reynolds (1960b) for discussion of this point] . The second type of spec- trum, of which Beardsley and Pesyanoe are examples, appears to be ter- restrial in isotopic composition. Both Beardsley and Pesyanoe have very little Xe, so that the observed spectra may be due to the blank. Kohman: If one attributes all this variation to some effect which is a func- tion of mass, then there is a deficiency of Xel29 in Murray [and the other carbonaceous chondrites], rather than an excess. Reynolds: That is correct, but then Xe134 and Xe1 are in substantial excess in the meteorite. There are difficulties in accounting for the excess of these isotopes as fission products, since their amounts do not fit the yield curve for spontaneous fission of U . Anders: Kuroda (1960) has treated the problem by normalizing to Xe130, which is a shielded isotope and therefore should be unaffected by fission, and by suggesting that neutron-induced fission or fission of extinct transuranic elements such as Pu244 could have accounted for most of the observed differences between the Earth's atmosphere and the mete- oritic Xe. [Kuroda's idea is that the U/Xe ratio for the Earth is higher than for the meteorites, so that atmospheric xenon has been enriched in those isotopes of Xe which are fission products--especially by spon- taneous fission of Pu244. ] i ? ft Reed: Is this scheme self-consistent; that is, can one normalize to Xe , which is also shielded, and get similar results ? Anders: Yes, within the accuracy of the data. Kuroda's treatment repro- duces the second-order peak at Xe which one would expect from the data on transuranic spontaneous fission. Also, the secondary anoma- lies in meteoritic Xe are considerably reduced, so that they could possibly be accounted for by spallation. Reynolds: In view of this discussion, it is very important to find out whether all Xe from "xenon-rich" meteorites has an anomalous compo- nent closely similar to that of Murray, which one would certainly expect from Kuroda's arguments, or whether there is any Xe of strictly terres- trial composition in such meteorites. This would be a crucial test of 10
Kuroda's hypothesis. Meteorites such as Beardsley, Pesyanoe and Nuevo Laredo should also be investigated further, in a system with small blanks. Table 2 presents a survey of results on excess Xel29 to date. The Xe129/Xel32 ratios in the table are the maximum observed. Data obtained by other workers at Heidelberg and Minnesota are also included. The ratios sometimes display considerable variability from one sample to another of the same meteorite, which may be due to variance in the blank, or to real variations among samples [the iodine results reported by Goles for some of these meteorites also display large variance among different aliquots] . Sample sizes, while ranging up to~6 g, were normally about 1. 5 g, so that some sampling errors could be expected. TABLE 2 Xe129/Xe132 Ratios For Meteorites Indarch 3.4 (Heidelberg: 3.1) Richardton 1.48 (Minnesota: 1.4) Murray 1.09 (Heidelberg: 1.25) Elenovka 1. 08 (tentative) Mighei 1.08 Orgueil 1.06 Kyushu 1.05 (tentative) Atmosphere 0.98 The absolute amounts of excess Xe 9, using the normalization to Xe13° for subtracting a terrestrial component, were determined by iso- topic dilution with a Xe128 spike made by irradiation of KI. These amounts, in cc STP/g, are: Richardton 0. 13 ± 0. 01 x 10-9 Indarch 2. 54 ± 1. 0 x 10-9 Murray 1.09 ± 0. 11 x 10-9 The Indarch result is provisional; it was hurried to completion, at the ex- pense of accuracy, so as to be reported at this meeting. However, it is clear that Indarch. while exceptionally rich in excess Xe129, has a total Xe content much smaller than Murray (Murray has~40 x 10-9 cc STP/g total Xe, while Indarch has roughly 6 x 10-9 cc STP/g Xe). The Xe in Murray has been shown (Reynolds 1960b) to be primordial. 11
The Ar from Murray and the other carbonaceous chondrites also appears to be primordial. The Ar"*"/Ar ratios in these meteorites are: Murray: 8.5 Orgueil: 5 Mighei: ~17. These ratios are clearly very different from the atmospheric value of ~300. While the isotopic compositions of Ar in these carbonaceous chondrites seem very similar, the total amounts may differ. [Note that Orgueil has a higher K content (730 ppm) than Murray (380 ppm) according to Edwards, and so must have either a much shorter K-Ar age or much more Ar3° than Murray. REYNOLDS reported 260 ppm K in Murray, and a K-Ar age of~3 A. E., which would strengthen the argument above.] Cameron: What information do you have on primordial He and Ne20? These isotopes are very important, He3 because it can only be made in a limited number of ways, and Ne20 because it is made in cool stars and the intergalactic material seems to have been greatly enriched in it since the solar system was formed. Reynolds: The He in Pesyanoe, assuming its U content is low enough so that there was no contribution of radiogenic He , is primordial. (The He3 is about ten times as abundant as Ne2 , implying a negligible amount of cosmogenic He.) The He3/He* ratio in Pesyanoe is~0. 004 according to our measurements, so that this is probably the primordial He3/He^ ratios. Zahringer and Centner (1960) have come to a similar conclusion from their work on Kapoeta. Anders: Some Russian work has been published on Pesyanoe claiming â¢~10 g U/g. However, I put a sample of Pesyanoe directly into a scintillation counter and looked for y-radiation from the U ° decay chain. I would estimate 10-' g U/g as an upper limit: possibly much less is present. Reynolds: Then 0. 0004 for the primordial He /He"* ratio would seem to be a good value. I have results on Murray (0. 0005) and Orgueil (0. 0004) which agree with this, but the agreement may be fortuitous. The Ne21/Ne20 ratios for these carbonaceous chondrites are: Murray: 0. 034 Orgueil: 0.013 Much of this data is summarized in Figure 1. The profiles of primor- dial noble gases in stone meteorites are compared with cosmic (Suess- Urey) and atmospheric noble gases. (The data on Kapoeta and Abee 12
COSMIC 1 X He A Xe Ne Kr Log|Q(cc STP/g) V -1 PESYAN0E MURRAY -2 -3 (MIGHEI) (0RGUEIL) â KA P0ETA ~l -4 - 1 -5 L ABEE -6 !â¢â¢ a,' -7 1 -8 u -9 _r 1â -10 -1 1 He A Xe He A Xe He A Xe He A Xe Ne Kr Ne Kr Ne Kr Ne Kr ATM0S- PHERE He A Xe Ne Kr Figure 1. Primordial noble gases in stone meteorites. are from Zahringer; Pesyanoe data from Russian work. ) There are marked similarities between Pesyanoe, Kapoeta and the cosmic pro- file on the one hand, and Murray, Abee and the atmospheric profile on the other. [Cameron and Suess discussed the Ne/Ar ratios at some length, pointing out possible reasons for variations between the given profiles. Suess suggested that it is important to keep in mind the effects of adsorption, desorption and diffusion in changing the profiles, e. g., by inducing a loss of Ne relative to Ar or Kr relative to Xe, which could account for some of the features of the profiles. Anders indicated that such sepa- ration processes would be critically dependent upon temperature, among other parameters. For further discussion of some of these points, see the paper by Stauffer in this volume.] Finally, it now seems likely that the K-Ar age of Richardton is greater than that of Beardsley. Using the results of recent potassium analyses, Richardton has a K-Ar age of 4. 47 A. E., compared to 4. 30 A. E. for Beardsley according to Geiss and Hess. Within experimental error, these values are the same, but at least it is no longer necessary to try to 13
explain why Beardsley should appear to be older than Richardton and yet have retained no excess REFERENCES Kuroda, P. K. (1960) Nature 187, 36. Reynolds, J. H. (1960a) Phys. Rev. Letters 4, 8. Reynolds. J. H. (1960b) Phys. Rev. Letters 4, 351. Zahringer, J., and Centner, W. (1960) Z. fur Naturfor. 15a, 600. 14
