Magmatic degassing of multicomponent vapors and assessment of magma depth: application to Vulcano Island (Italy)

Abstract

Degassing of magmatic H 2O, CO 2 and rare gases plays a major role in understanding large-scale Earth processes and in the assessment of volcanic activity. Here we describe a quantitative model for magmatic degassing of H 2O–CO 2–noble gas–N 2 mixtures. Our modeling takes into account non-ideal behaviors by adopting recently developed approaches for the solubility of H 2O–CO 2 mixtures in silicate liquids and for noble gas partitioning in H 2O–CO 2 bearing magmas. This new approach allows quantitative treatment of inert gas fractionation throughout the degassing of any H 2O–CO 2 bearing natural magma in a wide range of thermo-baric conditions. Magma degassing simulations performed by our model have clearly displayed that dissolved H 2O and CO 2 in the melt strongly affect inert gas degassing. Due to their modest solubility differences in H 2O-rich magmas, all the inert gases are strongly partitioned into vapor at early degassing extents, after the quick exhaustion of CO 2. In contrast, CO 2-rich melts retain dissolved helium longer because CO 2 has firstly to be released from magma, whereas nitrogen and heavy noble gases undergo a similar or higher exsolution than CO 2 at early magma degassing extents. We have successfully applied the degassing model to the active volcanic system of Vulcano Island (Italy), where several geochemical parameters have been monitored over the last decade. By quantitatively assessing magma pressure over time, the model has allowed us to reconstruct the rise of magma towards the surface. The results clearly show that the magma has reached low pressure and depth beneath Vulcano, thus interaction with the hydrothermal system, as well catastrophic magma rises may result in hazardous future scenarios.