A New Degassing Model to Infer Magma Dynamics from Radioactive Disequilibria in Volcanic Plumes

Abstract

Mount Etna volcano (Sicily, Italy) is the place where short-lived radioactive disequilibrium measurements in volcanic gases were initiated more than 40 years ago. Almost two decades after the last measurements in Mount Etna plume, we carried out in 2015 a new survey of 210Pb-210Bi-210Po radioactive disequilibria in gaseous emanations from the volcano. These new results [(210Po/210Pb) = 42 and (210Bi/210Pb) = 7.5] are in fair agreement with those previously reported. Previously published degassing models fail to explain satisfactorily measured activity ratios. We present here a new degassing model, which accounts for 222Rn enrichment in volcanic gases and its subsequent decay into 210Pb within gas bubbles en route to the surface. Theoretical short-lived radioactive disequilibria in volcanic gases predicted by this new model differ from those produced by the former models and better match the values we measured in the plume during the 2015 campaign. A Monte Carlo-like simulation based on variable parameters characterising the degassing process (magma residence time in the degassing reservoir, gas transfer time, Rn-Pb-Bi-Po volatilities, magma volatile content) suggests that short-lived disequilibria in volcanic gases may be of use to infer both magma dynamics and degassing kinetics beneath Mount Etna, and in general at basaltic volcanoes. However, this simulation emphasizes the need for accurately determined input parameters in order to produce unambiguous results, allowing sharp characterisation of degassing processes.