Magma dynamics at Mt Etna: Constraints from U-Th-Ra-Pb radioactive disequilibria and Sr isotopes in historical lavas

Abstract

238U- 230Th- 226Ra- 210Pb disequilibria and 87Sr/ 86Sr ratios have been 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, 226Ra and 87/Sr 86Sr data allow us to distinguish two mainepisodes in the volcanic history. For most of the past two millenia until 1970 A.D., Ra and Ba exhibit a similar behaviour (decreasing ( 226Ra 230Th) andBa/Th ratios with increasing Th contents), which is mainly controlled by plagioclase fractionation during differentiation from hawaiites to mugearites. This differentiation occurs on a short timescale (≤ 200 yr) and the parental hawaiitic magma seems to have maintained a constaant ( 226Ra 230Th) ratio during the whole period. The 226Ra- 230Th data are interpreted in the context of a steady-state deep reservoir: an upper limit of 1500 yr is 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 volume inferred from seismic studies. The present period of activity, after 1970 A.D., is characterized by the injection in the shallow plumbing system of a new basic magma enriched in Ra, K, Rb and Cs, with a higher Sr isotope ratio (0.70359 compared to 0.70340 just before 1970 A.D.). These peculiarities probably result from selective crustal contamination. Mixing of this magma with that of the pre-1970 period in the upper plumbing system explains the geochemical variability of the present lavas. 228Ra and 210Pb data and consideration of effusion rates suggest that the injection of the contaminated magma in the shallow plumbing system took place in 1950 A.D. The volume of this shallow system is rather small (∼ 0.5 km 3), with a transfer and residence time of the magma of the order of a few tens of years. Finally, 226Ra- 230Th disequilibria provide a new method for dating prehistoric (< 8000 yr) and historical flows of unknown ages on Mt. Etna.