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The El Abrigo member, belonging to the Diego Hernández Formation (Tenerife, Canary Islands), represents the final (0.179 Ma) and most voluminous eruption (>20 km3 DRE) of the last cycle of explosive volcanism of the Las Cañadas volcano. Its products are predominantly phonolitic, but both mafic scoriae and banded pumices are commonly found, suggesting that El Abrigo eruption was triggered via the input of fresh, more mafic magma into a shallow, phonolitic magma chamber. A petrological and geochemical study was carried out on a variety of pumices sampled in the El Abrigo member, ranging from dominantly mafic to banded, and in turn to phonolitic. Banded pumices show a diversity of textures varying from coarsely to finely banded. Mafic scoriae host numerous phonolitic enclaves, sometimes grading to banded within a hand sample. In contrast, phonolitic pumices are homogeneous, almost crystal-free and highly vesicular. Small-scale, fluidal mixing structures are widely present in SEM BSE images and involve the entire compositional spectrum from a partially-crystallized tephriphonolitic composition (SiO2 ~52 wt.%; MgO ~3 wt.%) to a phonolitic melt (SiO2 ~61 wt.%; MgO ~0.3 wt.%). The distribution of fast-diffusing versus slow-diffusing elements indicates that diffusive hybridization has played a significant role. The presence of such small-scale structures and high compositional gradients at scales in the order of tens to hundreds of microns suggest that mixing was of limited efficiency and that the eruption likely started shortly after the beginning of this process, as longer timescales would erase such heterogeneities by advection and chemical diffusion. Several mineral and mineral-melt thermobarometric methods show good agreement and suggest that the mafic end-member had a tephritic composition and was stored prior to the mixing event at ~400 MPa (lowermost oceanic crust) and 1050ºC, and both endmembers contained ~3 wt.% dissolved H2O.

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% SiO ) 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.

In this work we use fractal statistics in order to decipher the mechanisms acting during explosive volcanic eruptions by studying the grain size distribution (GSD) of natural pyroclastic-fall deposits. The method was applied to lithic-rich proximal deposits from a stratigraphic section of the Cretaio Tephra eruption (Ischia Island, Italy). Analyses were performed separately on bulk material, juvenile, and lithic fraction from each pyroclastic layer. Results highlight that the bulk material is characterized by a single scaling regime whereas two scaling regimes, with contrasting power-law exponents, are observed for the juvenile and the lithic fractions. On the basis of these results, we infer that the bulk material cannot be considered as a good proxy for deducing eruption dynamics because it is the result of mixing of fragments belonging to the lithic and juvenile fraction, both of which underwent different events of fragmentation governed by different mechanisms. In addition, results from fractal analyses of the lithic fraction suggest that it likely experienced a fragmentation event in which the efficiency of fragmentation was larger for the coarser fragments relative to the finer ones. On the contrary, we interpret the different scaling regimes observed for the juvenile fraction to be due to sequential events of fragmentation in the conduit, possibly enhanced by the presence of lithic fragments in the eruptive mixture. In particular, collisional events generated increasing amounts of finer particles modifying the original juvenile GSDs and determining the development of two scaling regimes in which the finer fragments record a higher efficiency of fragmentation relative to the coarser ones. We further suggest that in lithic-rich proximal fall deposits possible indications about the original GSDs of the juvenile fraction might still reside in the coarser particles fraction.