Pre-eruptive conditions and triggering mechanism of the ~ 16 ka Santa Bárbara explosive eruption of Sete Cidades Volcano (São Miguel, Azores)

The Santa Bárbara trachytic eruption (~ 16 ka) was one of the major eruptions of the Sete Cidades Volcano (São Miguel Island, Azores), recording the last phase of caldera formation. Here, we report and combine geochemical, mineralogical, and petrological constraints on natural samples with time-series experiments to describe the pre-eruptive conditions of the Santa Bárbara plumbing system. The trachytic pumice clasts are notably characterized by the presence of hawaiitic enclaves, banded textures (~ 60–67 wt% SiO2) and high variability in mineral phases, occasionally rounded and partially resorbed. The hawaiitic enclaves contain quench textures such as sharp contacts with the trachytic groundmass, as well as acicular and skeletal growth of several minerals, pointing to a high-temperature gradient between the hot hawaiitic magma and the colder trachytic reservoir. Distinct phenocryst rim compositions in both magmas exclude significant chemical diffusion. We suggest that the hawaiitic enclaves represent an intrusion that triggered the eruption, but was only partially involved in the mixing process that generated the banded groundmass textures. These textural heterogeneities are interpreted to be related to a self-mixing event induced by convection within a compositionally zoned reservoir, with trachytic and mugearitic magmas at the top and the bottom, respectively. In detail, the model requires the likely arrival of hawaiitic magma to the base of the reservoir, inducing mixing by reheating of the resident mugearitic magma and volatile transfer. These processes produced a thermo-chemical destabilization (i.e., convection) of the shallow reservoir and mixing between the mugearitic and the trachytic magmas. We reproduced the observed chemical signatures performing time-series mixing experiments and calculated the concentration variance decay during mixing. Estimated timescales indicate that the hawaiitic intrusion took place ~ 41 h before the onset of the eruption.