Cosmogenic Radionuclides in Hot Desert Chondrites with Low C-14 Activities: A Progress Report

Terrestrial ages of meteorites from hot deserts provide an important tool to estimate meteorite fluxes to the Earth. Most desert meteorites have terrestrial ages less than 40 ky, but a few achondrites from the Sahara region were recently shown to have significantly higher ages, up to approx. 100 ky. In general, C-14 (half-life = 5730 y) is the most suited radionuclide to determine terrestrial ages for desert meteorites. However for meteorites with ages greater than 35 ky, the concentration of cosmogenic C-14 has decreased to a level at which it becomes difficult to distinguish between cosmogenic C-14 and terrestrial contamination. These meteorites may therefore be much older than 35 ky. We selected chondrites with low C-14 activities (less than or equal to 2 dpm/kg) for measurements of the concentrations of cosmogenic Cl-36 (half-life= 3.01 x 10 (exp 5) y) and Ca-41 (half-life= 1.04 x 10 (exp 5) y) in the metal phase. Since the ratio of Ca-14/Cl-36 in the metal phase of chondrites is relatively constant and well known, the measured ratio is a direct measure of the terrestrial age. A major problem is that most or sometimes all of the metal in these old "hot desert" meteorites has been oxidized to hydrated Fe-Ni-oxides. Therefore, we also measured the concentrations of Be-10, Al-26 and Cl-36 in the stony phase in order to constrain the terrestrial age as much as possible.