Noble gases and K-Ar ages in Aouelloul, Zhamanshin, and Libyan Desert impact glasses

Abstract

Elemental abundances of noble gases and isotopic compositions of Ne and Ar have been determined in seven Aouelloul impact glasses, four Zhamanshin impact glasses, and one Libyan Desert glass sample. Similar to most other natural terrestrial glasses, a high Ne Ar ratio was observed in all impact glasses. The Ne concentrations in the impact glasses are of the same order of magnitude as those in tektites, but the concentrations of Ar (and the heavy noble gases Kr and Xe) are significantly higher than those in the tektites. As a result, the Ne Ar ratios in the impact glasses are lower than in tektites. The isotopic composition of Ne in the impact glasses is identical to that of air, as also observed in tektites, suggesting that the Ne is atmospheric in origin. From the K content of the glasses, we calculated K-Ar ages of 10–15 Ma for Aouelloul and 0.7–1.0 Ma for Zhamanshin impact glasses. For Zhamanshin glasses, these ages are in good agreement with other radiogenic and fission track ages, while for Aouelloul, our K-Ar ages agree with older K-Ar ages reported before, but not with the younger (~3.2 Ma) fission track ages.