Cosmic-Ray Exposure Ages

Nishiizumi, K., Regnier, S., and Marti, K., "Cosmic ray exposure ages of chondrites, pre-irradiation and constancy of cosmic ray flux in the past, 1980, Earth Planet. Sci. Lett., 50, 156-170. 

Morgan, C. J., "Cosmic ray exposure ages of features and events at the Apollo landing sites", 1975, The Moon, 1 3,... 

Wetheril 1, G. W., Multiple Cosmic Ray Exposure Ages, paper presented at 43rd meeting of Meteoritical Society, La Jolla, Calif., 1980, Meteoritics (in press). 

Aluminum-26 is produced by stellar nucleosynthesis in a wide variety of stellar sites. Its abundance relative to other short-lived nuclides provides information about the stellar source(s) for short-lived nuclides and the environment in which the Sun formed. Aluminum-26 is also produced by interactions between heavier nuclei such as silicon atoms and cosmic rays. Aluminum-26 is one of several nuclides used to estimate the cosmic-ray exposure ages of meteorites as they traveled from their parent asteroids to the solar system. 

Turner, G., Huneke, J. C., Podosek, F. A. and Wasserburg, G. J. (1971) 40Ar-39Ar ages and cosmic-ray exposure ages of Apollo 14 samples. Earth and Planetary Science Letters, 12,... 

Table 9.1 Nuclides used for cosmic-ray exposure ages ...

The fraction of incoming cosmic rays that generate nuclear reactions is quite low. In a meteorite traveling in space, about one in 108 of the target atoms undergoes a nuclear reaction in a 10-Myr period. However, the cosmogenic nuclides that they produce can be measured to estimate the time that an object has been exposed to cosmic rays. Table 9.1 shows some of the nuclides that are used to estimate cosmic-ray exposure ages in meteorites and in materials from planetary surfaces. 

Cosmic-ray exposure ages of Cl and CM chondrites. Modified from Eugster et al. (2006). 

Ejection ages (sum of cosmic-ray exposure age + terrestrial age) for Martian meteorites. The ages cluster by meteorite type, suggesting that each cluster represents a distinct impact (ejection) event. The only outliers are the EETA 79001 and Dhofar 019 shergottites and ALHA84001. Modified from McSween (2008). 

Cosmic-ray exposure ages are determined from spallation-produced radioactive nuclides. Cosmic-ray irradiation normally occurs while a meteoroid is in space, but surface rocks unshielded by an atmosphere may also have cosmogenic nuclides. These measurements provide information on orbital lifetimes of meteorites and constrain orbital calculations. Terrestrial ages can be estimated from the relative abundances of radioactive cosmogenic nuclides with different half-lives as they decay from the equilibrium values established in space. These ages provide information on meteorite survival relative to weathering. 

Describe cosmic-ray exposure ages of meteorites and summarize what they tell us about the history of meteorites. 

Eugster, O., Herzog, G. F., Marti, K. and Caffee, M. W. (2006) Irradiation records, cosmic-ray exposure ages, and transfer time of meteorites. In Meteorites and the Early Solar System II, eds. Lauretta, D. S. and McSween, H. Y., Jr. Tucson University of Arizona Press, pp. 829-851. A good summary of what is known about cosmic-ray exposure ages and the transfer of meteorites from the asteroid belt to Earth. 

Herzog, G. F. (2005) Cosmic-ray exposure ages of meteorites. In Treatise on Geochemistry, Vol. 1. Meteorites, Comets, and Planets, ed. Davis, A. M. Oxford Elsevier, pp. 347-380. 

Noble gas isotopes are also produced through irradiation by cosmic rays. These rays are mostly high-energy protons that produce a cascade of secondary particles when they bombard other target nuclei, in a process called spallation. Neon produced by spallation reactions has similar abundances of all three isotopes (Fig. 10.8). Cosmic-ray irradiation occurs on the surfaces of airless bodies like the Moon and asteroids, as well as on small chunks of rock orbiting in space. Using these isotopes, it is possible to calculate cosmic-ray exposure ages, as described in Chapter 9. 

Cosmic-ray exposure age The retention of 38Ar by solids enables a measure of the duration for which cosmic solids have been exposed to cosmic rays. This is useful in studies of meteorites and of lunar samples. Cosmic rays striking calcium in the rocks leave fragments of 3 Ar and 38Ar in the ratio about 1.5 1, which is 38Ar-rich in comparison with solar Ar. Since the rate of arrival of cosmic rays is known, the time required for the buildup of the excess 38 Ar can be computed. For the lunar soils the... 

Table 8. Isotope ratios of trapped noble gases, cosmic ray exposure ages (in 106 years) of soil samples and breccias (all data from Hintenberger et al.52 Isotope ratios were obtained by various methods (see original paper52 ...

Cosmic-ray exposure ages are between —5.5 Ma and 7 Ma for all acapulcoites and most of the lodranites, possibly indicating sampling from a single impact event on a common parent body (McCoy et al., 1996, 1997a,b). 

Crabb J. and Schultz L. (1981) Cosmic-ray exposure ages of the ordinary chondrites and their significance for parent body stratigraphy. Geochim. Cosmochim. Acta 45, 2151-2160. 

Cosmic-ray exposure ages suggest that chondrites in many groups were exposed to space as... 

Okazaki R., Takaoka N., Nakamura T., and Nagao K. (2000) Cosmic-ray exposure ages of enstatite chondrites. Antarct. Meteorit. Res. 13, 153—169. 

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