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COSMIC RAY AGES OR IRON METEORITES* Oliver A. Schaeffer Chemistry Department Brookhaven National Laboratory By the cosmic ray age of a meteorite is meant the length of the time interval between the formation of the meteorite in its preatmospheric form and its collision with the earth. The knowledge of such ages is of interest in the understanding of the origin of meteorites and the past his- tory of the solar system. To measure the cosmic ray exposure age one needs to measure the time-integrated content and rate of production of a cosmic ray produced nuclide which was not present in the meteorite before being exposed to cosmic rays. The most easily measured products are the stable rare gases. In order to obtain the production rate it is nec- essary to measure the cosmic ray induced activity of a nuclide with a half life short compared to the cosmic ray age and long compared to the time since its arrival at the earth. It is then only necessary to know the relative production rate of the radioactive and stable products. A simple case exists if the stable product is produced predominantly by the decay of the radioactive nuclide, then the two production rates are of course equal. In addition, it is not necessary to make a correction for the de- pendence of the relative production rates on the energy of the cosmic ray particles. There are two cases where a relatively long lived activity decays to a stable rare gas, namely Cl36, Ar36 and T, He3. Of course the produc- tion rates of He3 and T or of Ar3" and Cl3" are not equal as in each case some of the rare gas is produced directly by the action of cosmic rays. Thus, if one computes an exposure age it is necessary to make a correc- tion for the directly produced material. A cosmic ray exposure age can also be computed by measuring a radioactive nuclide of nearly the same mass, but not necessarily decaying to a rare gas nuclide, e.g., Ar39, Ar38 is such a pair. In this case one must have rather complete information on the cosmic ray production rate of the two nuclides. * Research performed under the auspices of the U. S. Atomic Energy Commission. 22
In general, in order to interpret the radioactive and rare gas measurements it is necessary to have knowledge of the cosmic ray pro- duction ratios of various nuclides from meteoritic material. Production ratios can be determined from high energy proton irra- diation of target elements of representative materials. In the case of nickel-iron meteorites, iron itself furnishes a good target material. In the case of the stone meteorites, there is no single element which can reasonably approximate the meteorite, so in the case of stones a number of target materials should be studied as well as stone meteorites them- selves. In comparing the results of thin target irradiations to the products in meteorites one must exercise some caution. In the meteorite, a large number of low energy secondaries are formed which in turn produce radioactive and stable rare gas products. In a thin target, however, the secondary production is, to a large extent, missed. If a given ratio of products depends on the energy of the bombarding particles, a comparison of meteorite results to thin target data can be misleading. For this reason, it is important to determine cosmic ray ages from the ratio of two nearly equal mass nuclides as such a ratio is usually not too sensitive to the energy. Let us turn to some of the bombardment results. It has been found that the production ratios of the argon isotopes in the energy interval 200 Mev to 6 Bev are constant to within 5% and are Ar36:Ar37:Ar38:Ar39: : _1^33.8:4. 2. These ratios are in good agreement with the predictions of nuclear evaporation theories. On the other hand the production ratio He3+T/He4 (which is to be compared to He3/He4 ratios in meteorites) varies considerably with proton energy, from 0. 15 at 200 Mev to 0. 30 at 6 Bev. Finally the ratio He3+T/Ar38 varies from 10 at 400 Mev to 23 at 3 Bev. A cosmic ray age dependent on a ratio of Ar39/Ar38 should be much more reliable than an age dependent on a T/He3 ratio or a T/Ar38 ratio. In Table 1 are listed the rare gas ratios in several iron meteorites. It is seen that the meteorite results are in the same range as the proton production ratios quoted above. The Ar38/Ar " ratio is essentially con- stant at 1. 6 as would be expected from the lack of dependence of this ratio on energy. The corresponding proton production rate is, however, five times as large. This is due to the fact that evidently only 1 / 5 of the Ar36 is directly produced by cosmic rays and 4/5 of the Ar3^ must come from the decay of Cl3 . From this result, one concludes that a cosmic ray age based on the ratio Ar3^/Cl3" should be practically independent of the energy of the bombarding particle and hence the depth of the sample in the original meteorite. 23
TABLE 1 Cosmogenic Nuclides in Typical Iron Meteorites He3 He3/He4 He3/Ar38 Ar38/Ar36 Cl36 Cl36-Ar36 (10-8 cc/g) (dpm/kg) age Meteorite Williams town 480 0. 25 18 1.5 3.3 2200 m. y. Canyon Diablo 95 0. 30 23 1.6 6.8 160 m. y. Carbo* 350 0.24 19 1.6 3.6 1200 m. y. Sikhote-Alin 130 0.27 32 1.5 7.8 170 m. y. * Rare gas analyses from results of Nier et al., University of Minnesota. The He3/He4 ratio in the meteorites listed in Table 1 runs from 0. 24 to 0. 30. This ratio shows the energy dependence of the cosmic ray production ratios. The samples of Carbo are evidently from near the center of the preatmospheric meteorite and that of Canyon Diablo from near the surface. This is also shown by the He3/Ar38 ratios. The C13*> values listed in Table 1 also show the same behavior. For this reason a cosmic ray age calculated from a T/He3 ratio depends on the depth in the original meteorite. In additon to determining cosmic ray exposure ages a study of cosmic ray produced activities can also lead to new knowledge about the cosmic rays themselves. For example, the constancy of cosmic rays in time can be tested by measuring several cosmic ray produced radioactive nuclides with different half-lives. The long lived isotope will yield an average value of cosmic ray flux over a long period, while the short lived isotope will give an average flux for more recent time. Such a convenient set of isotopes is Ar39, ti/2 - 325 y; Cl , ti/2 = 308, 000 y and K40, ti/2 = 1- 3*109 y. The Ar39 activity will give a mean value of the flux for the last 103 years, the Cl36 for the last 106 years and the K40 (as cosmic ray exposure ages are shorter or comparable to the K40 half life), for the exposure age of the meteorite. In order to make a comparison the rela- tive production rates must be known. One way of comparing is to use the above cited results and compute exposure ages for the three radioactiv- ities. The comparison of the three ages will then furnish a test for the constancy of cosmic ray flux in time. Such a comparison is afforded by comparing the Ar3^-Ar38 and Ar -Cl exposure ages of Sikhote-Alin and the Ar36-Cl36 and K40-K41 exposure ages of Carbo. The results are listed in Table 2. It is seen that cosmic ray flux appears to be constant in time when such averages are compared. This observation then removes one of the assumptions implicit in the determination of cosmic ray exposure ages. 24
TABLE 2 Constancy of Cosmic Rays From Cosmic Ray Exposure Ages Cl36-Ar36 age m. y. Ar39-Ar38 age m. y. K40_K41 age m. y. Meteorite Sikhote-Alin Carbo 170 1200 150 1360* * Determination at Max-Planck-Institut fur Chemie by Hintenberger et aL In order to measure the exposure age of a number of meteorites it would be of interest if an age could be obtained from the rare gas content of the meteorite; then, a large number of meteorites could be measured because of the small sample size (the order of 0. 1 g) required for a mass spectrometric measurement compared to the relatively large sample size (the order of 100 g) required for a measurement of the low level activities. It would be possible to use the rare gases alone if one had some way to make a correction for the change in cosmic ray flux with depth inside a meteorite. One way of accomplishing this is to make use of the obser- vation that some of the rare gas ratios vary with depth in the meteorite. From Table 1 it is seen that He3/He4 or He3/Ar3s would be a suitable ratio. Table 3 presents data on rare gases in several meteorites. There are a few cases where the ratios observed are very different from the general trend. These variations are most likely due to impurities in the irons, e. g., the low He3/Ne21 ratio for Tucson is probably due to pro- duction of Ne21 from Mg, which is known to be present in minute fosterite inclusions. If one assumes that corrections due to such effects are not important for rough age calculations, and assumes that the He production rate implied by the data in Table 1 is applicable, estimates of He3 ages may be made. Figure 1 represents a histogram of these ages. A peak in the distribution near 200 million years is apparent, as well as a long tail out to 2 billion years. This pattern may be interpreted as a reflection of the collision probabilities for meteoroids (so that fresh surfaces are ex- posed) and of the mean lifetimes for collision with the Earth. 25
TABLE 3 Rare Gas Contents of Iron Meteorites He3 (10-8 cc/g) He3/Ne21 He3/Ar3S He3/He4 Ne21/Ar38 Meteorite Toluca 100 125 42 0. 34 0. 33 Casas Grande 130 130 36 0. 35 0.28 Arispe 270 117 30 0.29 0.26 Sikhote-Alin 130 87 32 0. 27 0.36 Washington Co. 193 94 26 0.07 0.28 Williamstown 480 80 18 0.25 0. 23 Odessa 220 96 18 0. 31 0. 19 Tucson 13 16 14 0. 09 0.88 Smithland 58 94 13 0. 28 0. 14 Canyon Diablo* 95 95 13 0. 30 0. 13 Forsyth 48 83 13 0. 32 0. 16 Santa Catherina 57 111 12 0. 25 0. 11 Santa Rosa 47 107 12 0.26 0. 11 Tombigbee 4.7 61 7.3 0. 14 0. 12 Canyon Diablo* 4. 2 38 9.5 0. 10 0.25 * Two individual pieces of Canyon Diablo, one obtained from B. Mason, New York Museum of Natural History and the other from Wards Natural Science Foundation, Rochester, New York. 3 NUMBER OF METEORITES q O ro .& . CD O r\, - â h . , n ^^ , ,n, , JT ) 200 400 600 800 1000 1200 I4OO 1600 1800 AGE IN my e 1. Cosmic ray exposure ages of iron meteorites 26
Cameron: The age distribution could also reflect differing erosion rates, due to variations in the semi-major axes of the orbits of these meteor- ites. If erosion is principally by dust swept from the asteroid belt to ward the Sun (by the Poynting-Robertson effect), one would expect approximately a r dependence of the dust concentration on distance from the Sun, and perhaps essentially no dust beyond the asteroids. Fireman: The age of greatest interest is the oldest one, because it at least puts an upper limit on the amount of dust in the regions where the meteorite resided. It appears that this upper limit is significantly lower than the dust concentration in the region of the inner planets estimated from the intensity of the zodiacal light. Anders: In this connection, it is important to remember that a critical parameter is the orbital inclination. An orbit with high inclination will result in lengthening both the erosional lifetime, since the dust is presumably strongly concentrated in the plane of the ecliptic, and the lifetimes for collision with other meteoroids and with the Earth. 27
