Cosmic-Ray-Produced Noble Gases in Meteorites

Abstract

Energetic particles of the galactic cosmic radiation (GCR) have a mean penetration depth in rock of about 50 cm, comparable to the typical size of a meteorite. GCR-induced effects therefore provide a means to study the history of meteorites as small objects in space or in the top few meters of their parent body. These effects include cosmic ray tracks, i.e., the radiation damage trails in a crystal lattice produced by heavy ions in the GCR (Fleischer et al. 1975), and thermoluminescence, i.e., the light emitted by a heated sample which had been irradiated by energetic particles (Benoit and Sears 1997). By far most important, however, are “cosmogenic” nuclides, produced by interactions of primary and secondary cosmic ray particles with target atoms. This chapter concentrates on the cosmogenic noble gas nuclides in meteorites. A separate chapter in this book by Niedermann (2002) discusses cosmogenic noble gases in terrestrial rocks, which are by now a major tool in quantitative geomorphology. Cosmogenic noble gases in lunar samples are briefly presented in the chapter by Wieler (2002) on noble gases in the solar system. Here and in the other chapters mentioned, cosmogenic radionuclides such as 10Be, 26Al, or 36Cl are often also considered, as comprehensive studies usually require combining noble gas and radionuclide analyses. Not further discussed here are cosmic-ray-induced shifts of the abundances of stable isotopes of a few elements besides noble gases. The most important of these are Gd, Sm, and Cd, which have isotopes with extremely high cross sections for the capture of slow (thermal or epithermal) neutrons produced as secondary cosmic ray particles by interactions of the primary GCR protons with target atoms (Eugster et al. 1970; Hidaka et al. 1999; Sands et al. 2001). Because the flux of thermal and epithermal neutrons peaks …