Cosmic-Ray-Produced Noble Gases in Terrestrial Rocks: Dating Tools for Surface Processes

Abstract

The production of noble gas isotopes by interactions of high-energy cosmic ray particles with rocks was first recognized half a century ago when Paneth et al. (1952) showed that the high 3He/4He ratios in iron meteorites must be due to cosmic ray interactions and provide information about their ages. Other investigations in the late 1950s and the 1960s revealed the presence of a whole spectrum of cosmic-ray-produced noble gases in meteorites, showing characteristic isotopic abundances deviating substantially from those of all other known noble gas reservoirs (e.g., Marti et al. 1966). Likewise, a number of radionuclides with half-lives of some 105 to 107 years were also detected, including the unstable noble gas isotope 81Kr. As cosmic ray particles can penetrate a few meters at most into rock material, an obvious application of these “cosmogenic” nuclides was to determine the time during which meteorites had traveled through space as small objects, which of course required the knowledge of production rates. After the return of lunar samples by the Apollo and Luna missions, the new technique was successfully applied to date the surface exposure of lunar rocks and, thereby, determine for instance the ages of lunar craters (e.g., Eugster et al. 1977). A review of the basics, methods, and results of cosmogenic nuclide studies in extraterrestrial material is given in another chapter of this book (Wieler 2002). On Earth, most interactions of cosmic rays with matter occur in the upper layers of the atmosphere, whose shielding depth down to sea level is 1033 g/cm2, corresponding to about 3.5 m of rock. Therefore it is not surprising that interactions with terrestrial rock material are several orders of magnitude less abundant than in space. Nevertheless, the first observation of a cosmogenic nuclide in a terrestrial …