A MODEL FOR THE PRODUCTION OF COSMOGENIC NUCLIDES IN CHONDRITES

A model is presented which permits the calculation of production rates of cosmic-ray-produced light noble gases He, Ne and Ar as well as 10Be, 26Al and 53Mn in chondrites of variable size and shape The production of a given nuclide is described by a semi-empirical equation used by Signer and Nier (1960) to describe the production of the light noble gases in the iron meteorite Grant. For a given nuclide, this equation contains two free parameters that are fitted to the depth profiles measured in the Knyahinya chondrite. Model predictions are tested by comparison with literature data for depth profiles in other meteorites (ALH78084, St. Severin, Keyes). The agreement between model predictions and experimental data is found to be 5̃% for the concentrations of 10Be, 21Ne, 22Ne, 38Ar and 53Mn, respectively and to be better than 1% for the 22Ne 21Ne ratios. The model predicts that 3-nuclide correlations that use the same reference nuclide are linear. The correlation between P(10Be) P(21Ne) and 22Ne 21Ne ratios is experimentally verified over a wide range of irradiation conditions. With this relation, shielding and size corrected exposure ages can be derived from the measurements of 10Be and of the Ne isotopes in a single sample. The calculated exposure ages are believed to have a precision of 5̃%. © 1990.