MAGMA DYNAMICS AT MT ETNA - CONSTRAINTS FROM U-TH-RA-PB RADIOACTIVE DISEQUILIBRIA AND SR ISOTOPES IN HISTORICAL LAVAS

U-238-Th-230-Ra-226-Pb-210 disequilibria and Sr-87/Sr-86 ratios have b een measured in a suite of prehistoric and historical lava flows from Mt. Etna. Whereas Th isotope ratios remain nearly constant during the whole period studied, Ra-226 and Sr-87/Sr-86 data allow us to distingu ish two main episodes in the volcanic history. For most of the past tw o millenia until 1970 A.D., Ra and Ba exhibit a similar behaviour (dec reasing (Ra-226/Th-230) and Ba/Th ratios with increasing Th contents), which is mainly controlled by plagioclase fractionation during differ entiation from hawaiites to mugearites. This differentiation occurs on a short timescale (less than or equal to 200 yr) and the parental haw aiitic magma seems to have maintained a constant (Ra-226/Th-230) ratio during the whole period.The Ra-226-Th-230 data are interpreted in the context of a steady-state deep reservoir: an upper limit of 1500 yr i s derived for the residence time of the magma in this reservoir, which could have a maximum volume of 150-300 km(3), much smaller than the v olume inferred from seismic studies. The present period of activity, a fter 1970 A.D., is characterized by the injection in the shallow plumb ing system of a new basic magma enriched in Ra, K, Rb and Cs, with a h igher Sr isotope ratio (0.70359 compared to 0.70340 just before 1970 A .D.). These peculiarities probably result from selective crustal conta mination. Mixing of this magma with that of the pre-1970 period in the upper plumbing system explains the geochemical variability of the pre sent lavas. Ra-228 and Pb-210 data and consideration of effusion rates suggest that the injection of the contaminated magma in the shallow p lumbing system took place in 1950 A.D. The volume of this shallow syst em is rather small (similar to 0.5 km(3)), with a transfer and residen ce time of the magma of the order of a few tens of years. Finally, Ra- 226-Th-230 disequilibria provide a new method for dating prehistoric ( < 8000 yr) and historical flows of unknown ages on Mt. Etna.