Basaltic volcanoes as large-scale aerators: the example of Mt. Etna

The recognition and simulation of the patterns of gas release from active volcanoes in relation to those of magma supply and transfer are a major geochemical goal. At basaltic volcanoes such as Mt. Etna (Sicily, Italy) this knowledge would greatly assist our comprehension of the mechanisms of magma rise and injection at different storage levels, from depth up to the shallow systems feeding lava fountains and flows. In this contribution we investigate the H2O-CO2-SO2-H2S-silicate melt system by integrating theoretical models on volcanic degassing with data from plume chemistry, fumarole sampling, chemistry and volatile contents of melt inclusions (MIs). Given an initial bulk composition, we show that the degassing processes behind this ensemble of data can be quantitatively assessed by carefully evaluating the interplay of 1) crystallization, hence phase proportions, 2) redox variables, 3) gas addition (flushing) occurring at different steps along the magmatic column. Because of pervasive CO2-flushing through the magma, we picture Mt. Etna as a big aerator, by analogy with gas absorption techniques in chemical process engineering. CO2-flushing is particularly efficient at P > 140 MPa, where the volatile influx generates a family of degassing paths that embrace the range of variability displayed by H2O, CO2 and S contents dissolved in MIs. The flushing mechanism can work under two extreme scenarios: in one case the rising volatile phase is completely blocked by the shallower magmas, whereas in the other one at each addition of deep gas the pre-existing gas phase is completely separated from the flushed magma. This study shows that equilibrium thermodynamics provides reasonable physico-chemical constraints to interpret the ensemble of data observed, without invoking diffusive regimes acting far from the equilibrium. This is a strong argument for the joint adoption of MI-based volatile contents and volatile-melt saturation algorithms. The results of this study are expected to shed light on the close association between basaltic volcanism and the release of large amounts of carbon dioxide.