DATING RECENT VOLCANIC-ROCKS THROUGH TH-230-U-238 DISEQUILIBRIUM IN ACCESSORY MINERALS - EXAMPLE OF THE PUY-DE-DOME (FRENCH MASSIF-CENTRAL)

Precise dating of very young (less than or equal to 10 000 yr) silicic volcanic rocks can be achieved by measurements of Th-230-U-238 disequ ilibria in accessory minerals such as apatite and zircon. It is illust rated here by the isochron age of 12 000 +/- 1000 yr obtained on a tra chyte from the Puy de Dame (French Massif Central). The crystallizatio n age is virtually the same as the eruption age determined by thermolu minescence and C-14 dating. This indicates a short time interval (a fe w centuries at most) between crystallization and eruption in such smal l volume silicic magmas.