We have measured significant concentrations of 3c'c1, 4•Ca, 36Ar from decay of 3c'c1, and •5øSm produced from the capture of thermalized neutrons in the large Chico L6 chondrite. Activities of 3r121 and 4•Ca, corrected for a high-energy spallogenic component and a terrestrial age of ~50 ka, give average neutron-capture production rates of 208 atoms/min/g-Cl and 1525 atoms/min/kg-Ca, which correspond to thermal neutron (n) fluxes of 6.2 n/cm2/s and 4.3 n/cm2/s, respectively. If sustained for the -65 Ma single-stage, cosmic ray exposure age of Chico, these values correspond to thermal neutron fluences of ~l.3x1016 and 0.8 x1016 n/cm 2 for 3r'CI and n•Ca, respectively. Stepwise temperature extraction of Ar in Chico impact melt shows 3•Ar/3SAr ratios as large as ~9. The correlation of high 3aAr/3SAr with high C1/Ca phases in neutronirradiated Chico indicates that the excess 3•Ar above that expected from spallation is due to decay of neutron-produced 3c'c1. Excess 3•Ar in Chico requires a thermal neutron fluence of 0.9-1.7x10 • n/cm 2. Decreases in a49Sm/aS2Sm due to neutron-capture by •49Sm correlate with increases in 15øSm/152Sm for three samples of Chico, and one of the Torino H-chondrite. The 0.08% decrease in 149Sm/152Sm shown by Chico corresponds to a neutron fluence of 1.23x10 la n/cm 2. This fluence derived from Sm considers capture of epithermal neutrons and effects of chemical composition on the neutron energy distribution. Excess 3•Ar identified in the Arapahoe, Bmderheim, and Torino chondrites and the Shallowater aubrite suggest exposure to neutron fluences of --0.2-0.6x10 la n/cm 2. Depletion of 149Sm in Torino and the LEW86010 angrite suggest neutron fluences of 0.8x1016 n/cm 2 and 0.25x10 la n/cm 2, respectively. Neutron fluences of ~10 • n/cm 2 in Chico are almost as large as those previously observed for some lunar soils.Consideration of exposure ages suggests that the neutron flux in Chico may have been greater than that in many lunar soils. Neutron-capture effects, although seldom reported, may be common for large meteorites and could affect calculation of exposure ages based on cosmogenic Ar. Combining measurements of radioactive and stable species produced from neutron-capture has the potential for identifying large meteorites with complex exposure histories.Copyfight 1995 by the American Geophysical Union.Paper number 95JE00663. 0148-0227/95/95JE-00663505.00The rates at which these high-energy or spallation reactions proceed vary with depth in the meteorold, but all of them reach maxima at shielding depths less than 150 g/cm 2, some much less. The exact depth of maximum production depends on the size of the body and the specific product [e.g., Reedy, 1985; Graf et al., 1990; Bhandari et al., 1993]. Some of the neutrons produced by cosmic rays do not interact, however, until they are slowed to thermal energies. In large meteoroids (radius >200 g/cm 2) these neutron-capture products are predicted to reach maxima in their production rams at shielding depths of 100-300 g/cm 2 [e.g., Eberhardt et al.,...
The nuclides made in extraterrestrial materials by cosmic rays help reveal the histories both of the irradiated objects and of the cosmic rays. Improvements in measurement techniques, especially in accelerator mass spectrometry, have greatly reduced detection limits.Thanks to several extensive series of measurements in meteorites, a comprehensive picture has taken shape of how cosmogenic nuclide production depends on size, shape, and composition. Complementary to this work, (1) the irradiation in space of meteoroids has been simulated by means of accelerator experiments both with very thick and with spherical targets, and (2) various models for calculating production rates of cosmogenic nuclides have been developed or refined. New classes of material-meteorites recovered in Antarctica and tiny meteorites from the stratosphere and deep-sea sediments, for example--have become more widely available for analysis. From their cosmogenic nuclide contents we have learned about the exposure histories of lunar meteorites, of SNC meteorites, which may come from Mars, and of interplanetary dust particles. Cosmogenic nuclides have been put to use in unfolding increasingly complex exposure histories. Some grains in gas-rich meteorites were irradiated in at least two episodes, and others perhaps even before the solar system formed. Continuing measurements of lunar samples reveal the past behavior of energetic solar particles. We now know from cosmogenic nuclide measurements that many meteorites found in Antarctica have been there for-0.1-1 million years and that the age distributions vary from site to site.Among the more important isotopes we may list 3He, 2•Ne 10 26 (both stable), Be, A1, 36C1, 41Ca, 53Mn, and 8•Kr ("long-lived," 100,000 years to 3.7 million years, cosmogenic radionuclides), and 22Na and COCo ("short-lived," less than 10 years). Before 1969, work on cosmogenic nuclides focused on meteorites: There were no other suitable extraterrestrial materials available and, except for •4C, the concentrations of longer-lived cosmogenic nuclides in terrestrial materials were then too small to measure without heroic efforts. The emphasis switched in the 1970s to lunar samples, when a flood of samples was brought back from the Moon. During the last decade, studies of cosmogenic nuclides in extraterrestrial materials have increased in number and breadth in response to (1) the development of new sources of materials, e.g., interplanetary dust particles; (2) the identification of objects of special interest, e.g., lunar meteorites in the collections from the Antarctic ice fields; (3) the need to plan for the analysis of materials from elsewhere in the solar system, e.g., Mars and comets; and (4) the improvement of analytical methods. Among the technical advances, we would single out the pages 253-275 Paper number 90RG00850 253 ß 254 ß Vogt et al.: COSMOGENIC NUCLIDES 28,3 / REVIEWS OF GEOPHYSICS use of accelerators for high-energy mass spectrometry as having had the most profound impact to date. With accelerator mass spectrometry...
Abstract— We determined He, Ne, Ar, 10Be, 26Al, 36Cl, and 14C concentrations, as well as cosmic‐ray track densities and halogen concentrations in different specimens of the H6 chondrite Torino, in order to constrain its exposure history to cosmic radiation. The Torino meteoroid had a radius of ∼20 cm and travelled in interplanetary space for 2.5–10 Ma. Earlier, Torino was part of a larger body. The smallest possible precursor had a radius of 55 cm and a journey through space longer than ∼65 Ma. If the first‐stage exposure took place in a body with a radius of >3 m or in the parent asteroid, then it lasted nearly 300 Ma. The example of Torino shows that it is easy to underestimate first‐stage exposure ages when constructing two‐stage histories.
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