STABLE MAGNESIUM ISOTOPES, 26AL, 10BE, AND 26MG26AL EXPOSURE AGES OF IRON METEORITES

Abstract

Concentrations of magnesium isotopes in the iron meteorites Charlotte, Picacho, Tlacotepec, and Grant between 0.15 and 5 ppbw have been measured by glow discharge mass spectrometry, typically with a precision of 15%. The25Mg24Mg and26Mg24Mg ratios are well above terrestrial values in most samples indicating the presence of a component produced by cosmic rays. Inferred concentrations of cosmogenic 26Mg range from barely detectable to 0.255 ppb by mass. 26Al and10 Be activities were measured in the same four and other iron meteorites by accelerator mass spectrometry. Cosmic ray exposure ages based on the cosmogenic 26Mg26Al ratios are comparable to, but not precise enough to distinguish between discordant, published ages obtained from other isotopes. Measured primordial or common magnesium contents are in the range between 0.2 and 4.6 ppb.