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 years^2–4; 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.