Problems Related to Interplanetary Matter (1961)

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PRIMORDIAL ARGON AND NEON IN STONE METEORITES Heinz Stauffer School of Science and Engineering University of California at La Jolla The abundance and isotopic composition of argon and neon in carbonaceous chondrites and ureilites (diamond-bearing achondrites) have been measured. A detailed description of the experimental procedure is to be published by Stauffer (1960). The results are given in Table 1. The striking results are the unusually high abundances of the iso- topes of Ar36 and Ar38. The ratios Ar36/Ar are all very close to that of atmospheric argon. However, the low values for the ratios Ar "/Ar show that only a small fraction of the isotopes Ar3° and Ar38 could be due to atmospheric contamination, and therefore their extraterrestrial origin is proved. No nuclear processes are known to produce them in meteorites in such quantities and in such a ratio. It is concluded, therefore, that they are primordial. The isotopic composition of cosmogenic neon in stone meteorites has been found to be about Ne20/Ne21/Ne22 = 0.9/0.93/1.0. The results of Table 1 show in each case an excess of Ne20 and Ne22. Except for Mokoia, an addition of atmospheric neon to the cosmogenic fraction could explain the measured isotopic ratios. However, for the carbonaceous chondrites atmospheric contamination can be excluded by comparing the observed Ar40/Ne20 ratios with the corresponding atmospheric ratio, and therefore the excess neon is considered to be primordial. Table 2 shows the effect of adsorbed atmospheric argon. The large differences in the Ar40/Ar3° ratios between the two different samples of Pesyanoe and between the aliquots of the Goalpara sample are due to dif- ferent amounts of adsorbed atmospheric argon. The abundances of pri- mordial argon agree. Several heating experiments have been carried out. The results are listed in Table 3. For a detailed discussion of the experimental pro- cedure and the results, see Stauffer (1960). Summarizing, we may conclude: 15

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TABLE 2 Variation in Adsorbed Ar-content Sample 40/36 Ar40 Ar36 Ar36/Ar38 Pesyanoe I Sample obtained 28.7 4590 160 from Russia 5. 17 Pesyanoe II Sample obtained 25.1 3880 155 5.25 from Smithsonian Collection Goalpara Two different 11. 1 440 39.6 5. 14 aliquots of the same sample 16. 5 651 39.5 5. 05 Absolute amounts given in 10-8 cc STP/gm 1. Predegassing the samples for several hours at about 100o C under continuous pumping releases the adsorbed atmospheric argon in the case of Lance. We may expect that to be true for all carbonaceous chon- drites. All samples listed in Table 1 have been predegassed. Therefore, the Ar abundances given in Table 1 are equal to the abundances of radiogenic Ar40, except for the sample Goalpara, where a considerable fraction of the adsorbed argon survives the predegassing. 2. Radiogenic argon diffuses out more readily than primordial argon. The diffusion coefficients have been calculated, using the same mathematical model as Goles, Fish and Anders (1960). They are listed in Table 4. 3. The primordial argon is much more strongly bound than the radiogenic. It is enclosed within the matrix of the crystal lattice. 4. The same is true for cosmogenic neon, since its diffusion co- efficient is smaller than that of radiogenic argon. 5. No accurate value for the diffusion coefficient of primordial neon could be given because of high blanks. It seems, however, that there are no large differences between the diffusion coefficients of primordial and cosmogenic neon. 6. Inhomogeneities in the content of primordial rare gases occur. 17

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TABLE 4 Diffusion Coefficients Calculated From Heating Experiments Ar40 Ar36 Ne21 Sample Conditions D -1 D -1 D -1 F — T sec a p _sec p _gec Lance 1 hr 1000o K 41. 3 5. 3xl0-6 14.7 5.6xl0-7 25 1.8x10-6 Goalpara 1 hr 1000o K 76 3x10-5 7.4 1.4X10-7 11 3. 1xl0-7 Chondrites* 1 hr 973o K 14 5. 1Sxl0-7 F = Fraction expelled, in % of total content. * = Value calculated from heating experiments by Geiss and Hess (1958). The only potassium analyses on carbonaceous chondrites are those published by Edwards (1955). Using his values and the present argon analyses, the following Ar/K ages have been calculated: Felix 4. 5 A. E.; Mokoia 3.4 A. E.; Murray 1.9 A. E.; Ivuna 1.4 A. E. The Ar/K age of Pesyanoe is given as 4. 2 A. E. by Gerling and Levskii (1956). No potas- sium content is known for Novo Urei, and the value for Goalpara given by Edwards (1955) is very uncertain as is the content of radiogenic Ar40. From Table 5 it can be seen that a large loss of rare gases com- pared to the silicon, as well as a fractionation between them, must have occurred in the history of meteoritic material. Furthermore, the degree of such fractionation shows differences of several factors of ten between different samples. Comparing the results of the heating experiments with the Ar/K ages and the values in Table 5, it can be concluded: 1. The high Ar/K age of Felix shows that no loss of radiogenic argon and--because of the difference in diffusion coefficients--no loss of primordial argon occurred since the formation of this meteorite. Only minor losses of primordial neon could have occurred. The observed fractionation must have happened before or during the formation of the meteorite. 2. The other carbonaceous chondrites have lost a large fraction of the radiogenic argon, about 50 percent for Mokoia and about 90 percent for Ivuna. Here we expect losses of primordial gases too, though it is difficult to estimate their extent. However, the observed differences between the diffusion coefficients of primordial argon and primordial neon cannot alone account for the large fractionation. 19

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The degree of fractionation may depend on differences in the properties of the rare gases. Processes such as adsorption and, at low temperatures, condensation are known to give large fractionation. Also, a selective trapping of the rare gases in the crystal lattice, or at disloca- tions, may cause a large fractionation. Small isotopic effects are expected in all possible mechanisms of fractionation, between rare gases, due to the differences in the mass of the isotopes. It can be seen from Table 5 that in some cases the primor- dial isotopic ratios differ from the atmospheric values. The errors are rather high, due to the uncertainty in the correction for the cosmogenic rare gases. However, for the samples Mokoia and Pesyanoe, the primor- dial Ne20/Ne22 ratios are undoubtedly higher than the atmospheric value, and in Kapoeta it almost agrees with the cosmic ratio, calculated by Suess (1949) from the distribution of the rare gases in the Earth's atmosphere. Correspondingly, these samples show the smallest fraction- ation, and they have almost a cosmic Ne20/Ar ratio. REFERENCES Edwards, G. (1955) Geochimica et Cosmochimica Acta 8, 285. Geiss, J., and Hess, D. C. (1958) Astrophys. J. 127, 224. Gerling, E., and Levskii, L. (1956) Dokl. Akad. Nauk. S. S.S. R. 110, 750. Goles, G., Fish, R. A., and Anders, E. (1960) Geochimica et Cosmo- chimica Acta 19. 177. Reynolds, J. H. (1960) Phys. Rev. Letters 4. 351. Stauffer, H. (1960) to be published in Geochim. et Cosmochim. Acta. Suess, H. E. (1949) J. Geology 57. 600. Suess. H. E.. and Urey, H. C. (1956) Rev. Mod. Phys. 28, 53. Urey, H. C., and Craig, H. (1953) Geochim. et Cosmochim. Acta 4, 36. Wiik, H. B. (1956) Geochim. et Cosmochim. Acta 9. 279. Zahringer, J., and Gentner, W. (1960) Zeitschr. f. Naturf., in press. 21