Anomalously old ⁴⁰Ar–³⁹Ar ages of Antarctic meteorites due to weathering

Abstract

In 1969, the Japanese Antarctic Research Expedition found nine meteorites on bare ice near the Yamato mountains, East Antarctica¹. Since then, >5,000 meteorites have been found in Antarctica by meteorite search teams^2,3. They are generally estimated to have been exposed on the ice sheet surface for >10³–10⁴ yr (ref. 4), and have been shown to have been subject to weathering effects^5–8. By comparing the ⁴⁰Ar–³⁹Ar age patterns for the inner and outer portions of two Antarctic meteorites, we have found that the oxidized outer portions show anomalously old ⁴⁰Ar–³⁹Ar ages which exceed 4,600 Myr in the intermediate temperature fractions of around 800–1,000 °C. This is contrary to the generally expected tendency as a result of Ar loss caused by weathering of a sample. Because old ⁴⁰Ar–³⁹Ar ages are accompanied by an increase of ³⁶Ar, we attribute the anomalous ages to the incorporation of terrestrial atmospheric Ar in some phase like haematite which might be changed from goethite during neutron irradiation.