Leaching procedure used in dating young potassic volcanic rocks by the Ra-226/Th-230 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 Ra-226/Th-230 radioactive disequilibrium and on a leaching procedure and gamma-counting of volcanics containing Th-enriched, Ra-initial-depleted mineral phases detected by microprobe analyses. K is used as a Ra-stable chemical analogue. The data are plotted on (Ra-226/Ra-228)-K/(Ra-228) Osmond-type diagrams in order to evaluate the Ra-226(radiogenic)/Th-230 activity ratio of the Th-enriched, Ra-initial-depleted phases. Four samples of volcanics from Vulcano Island (southern Italy) gave ages consistent with historical reports and K/Ar chronology.