Leaching procedure used in dating young potassic volcanic rocks by the 226Ra/230Th method

A method for determining the ages of shoshonitic volcanics erupted within the last 5000 years is presented and assessed. This method is based on 226Ra/230Th radioactive disequilibrium and on a leaching procedure and γ-counting of volcanics containing Th-enriched, Rainitial-depleted mineral phases detected by microprobe analyses. K is used as a Ra-stable chemical analogue. The data are plotted on (226Ra/228Ra)− K(228Ra) Osmond-type diagrams in order to evaluate the 226Raradiogenic230Th activity ratio of the Th-enriched, Rainitial-depleted phases. Four samples of volcanics from Vulcano Island (southern Italy) gave ages consistent with historical reports andK/Ar chronology.