Short-lived radioactive disequilibria and magma dynamics in Etna volcano

Abstract

The interest of radioactive disequilibria for the study of magmatic processes has long been recognised1. The use of 230Th-238U disequilibria has been growing in recent years2–4; but, since the pioneering work by Capaldi et al.5, very few studies have been concerned with a systematic use of short-lived isotopes of the radioactive series (for example, 226Ra, 210Pb, 210Po in the 228U series, and 228Ra, 228Th in the 228Th series). Here we present new precise results on the short-lived disequilibria in some recent lavas from Etna volcano. Our data show that a Ra enrichment took place in the Etna magma between 100 and 8,000 yr ago. Moreover, there is a clear correlation between (226Ra/230Th) activity ratios and the degree of differentiation of the magma, as indicated by its Th content. This correlation is interpreted as the result of mixing of a differentiated magma and a deep basic magma with a high (226Ra/230Th) ratio. The Ra and U enrichments relative to Th in recent volcanics could be a consequence of fluid-transfers in the mantle sources or in crustal magma reservoirs.