Ejection ages from krypton-81-krypton-83 dating and pre-atmospheric sizes of martian meteorites

Cosmic-ray exposure (CRE) ages and Mars ejection times were calculated from the radionuclide Kr-81 and stable Kr isotopes for seven martian meteorites'. The following Kr-81-Kr CRE ages were obtained: Los Angeles=3.35+/-0.70 Ma; Queen Alexandra Range 94201=2.22+/-0.35 Ma; Shergotty=3.05+/-0.50 Ma; Zagarm=2.98+/-0.30 Ma; Nakhla=10.8+/-0.8 Ma; Chassigny=10.6+/-2.0 Ma; and Allan Hills 84001=15.4+/-5.0 Ma. Comparison of these ages with previously obtained CRE ages from the stable noble gas nuclei He-3, Ne-21, and Ar-38 shows excellent agreement. This indicates that the method for the production rate calculation for the stable nuclei is reliable. In all martian meteorites we observe effects induced by secondary cosmic-ray produced epithermal neutrons. Epithermal neutron fluxes, phi(n) (30-300 eV), are calculated based on the reaction Br-79(n, gammabeta)Kr-8O. We show that the neutron capture effects were induced in free space during Mars-Earth transfer of the meteoroids and that they are not due to a pre-exposure on Mars before ejection of the meteoritic material. Neutron fluxes and slowing down densities experienced by the meteoroids are calculated and pre-atmospheric sizes are estimated. We obtain minimum radii in the range of 22-25 cm and minimum masses of 150-220 kg. These results are in good agreement with the mean sizes reported for model calculations using current semiempirical data.