The K/Ar Dating Method:Principle, Analytical Techniques, and Application to Holocene Volcanic Eruptions in Southern Italy

The K/Ar geochronometer is appropriate to date geological materials ranging in age from several thousands to several billions of years. Two complementary techniques of the K/Ar method are classically used: 1. the K/Ar conventional technique, in which potassium and argon are measured separately from a homogeneous preparation of the sample, and 2. the 40Ar/39Ar technique, which consists in the combined measurement of radiogenic argon (40Ar*), and argon produced from potassium (39ArK) during irradiation of the sample in a fast neutron nuclear reactor. We here review the principle, the analytical procedures, the advantages and the limitations of the two techniques. More specifically, we focuss on the unspiked K/Ar Casignol-Gillot technique, which allows to detect tiny amounts of radiogenic argon (as low as 0.1%) and is thus especially suitable to date very recent volcanic products, even basic in composition. Applications of this technique to young volcanic eruptions in southern Italy are presented, including the Avelino sub-historical explosive event from Monte Somma-Vesuvius. Sanidines separated from a grey pumice layer sampled independently in the sites of Nola and Cava Novasche yield a mean age of 3840 ± 180 yr, in perfect agreemeent with calibrated ams radiocarbon ages achieved recently on charcoal fragments (Albore-Livadie et alii 1998). This demonstrates the effectiveness of the K/Ar method to date accurately volcanic episodes up to historical periods, with absolute uncertainties presently reaching a few centuries.