Three years (2011-2013) of eruptive activity at Mt. Etna: Working modes and timescales of the modern volcano plumbing system from micro-analytical studies of crystals

Chemical zoning of olivine crystals highlights processes of multi-step magma transfer and residence at different levels of the plumbing system. The upward migration of magmas occurred primarily through multiple episodes of injection and mixing between five compositionally distinct magmatic environments (Mi), whose P-T-ƒO2 characteristics and concentrations in dissolved volatiles were constrained by thermodynamic modeling on the basis of the forsterite contents found at the olivine cores. From a deepest reservoir, located at depth of ~ 600 MPa, the most primitive magma M00 (Fo84) moved along dominant pathways, intercepting the M0 (Fo80-82) at ~ 390 MPa and/or M1a (Fo78; 250 MPa), M1b (Fo75; ~ 140 MPa) and finally the shallow M2 (Fo70-73; ~ 40 MPa) storage zones. For some eruptive episodes, olivine zoning highlights a preferential route of transfer, connecting the M00 and M2 storage zones that facilitated the fast migration of primitive magma at shallow depth. Fe-Mg diffusion modeling on olivine normal and reverse zoning defines the timescales of magma transfer and storage across these magmatic environments, which vary from ~ 1 to 18 months, whereas intrusion and mixing by more basic magma into the shallowest reservoir occurred always within 5 months before the eruption. Relevance of this study mainly relies on the quantification of volcanic processes at depth that may have considerable consequences in development of unusual, high-energy eruptions at basaltic volcanoes, generally acknowledged for their weak to mild explosive activity.