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 [ ( 210 Po / 210 Pb ) = 42 and ( 210 Bi / 210 Pb ) = 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.
Highlights
Many active basaltic open-conduit volcanoes emit a persistent gas plume, even during quiescence stages without eruptive activity at the surface
IN20 counter, both 210Pb) L by ( (210Pb) and 210Pb)θG − λBi ( (210Bi) detection limit can be quantified at 0.8 mBq/m3 and that of 210Po at
That low values of ( 210Bi/ 210Pb)G can be produced for gas transfer time θ as short as a few days
Summary
Many active basaltic open-conduit volcanoes emit a persistent gas plume, even during quiescence stages without eruptive activity at the surface. This behaviour provides evidence for ongoing magma degassing beneath volcanic centers. At subduction-zone-related volcanoes, the amount of degassing magma usually exceeds by one or several orders of magnitude the volume of lava erupting during the same period [3], which is notably the case of Mount Etna [4]. It emphasizes the fact that degassing and erupted volumes of magma can be significantly unbalanced, a feature that is not observed at non-subduction-related volcanoes [3,6]. At a volcano like Mount Etna (subduction-zone-related stratovolcano fed with volatile-rich-alkali basalts, e.g., [8]), it could be explained by a dynamic regime of magma redistribution beneath the volcano during which degassed magma is continuously removed by convection from the degassing reservoir and replaced by fresh undegassed magma [9,10,11]
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