Pre-eruptive magma hybridization and storage conditions recorded in banded pumices from the caldera-forming El Abrigo eruption (Tenerife)

González-García, Diego

Pre-eruptive magma hybridization and storage conditions recorded in banded pumices from the caldera-forming El Abrigo eruption (Tenerife)

Tuesday, 15 December 2020: 09:30

Diego González-García, Leibniz Universität Hannover, Institut für Mineralogie, Hannover, Germany, Maurizio Petrelli, University of Perugia, Perugia, Italy, Daniele Giordano, Università di Torino, Dipartimento di Scienze della Terra, Torino, Italy, Diego Perugini, University of Perugia, Department of Physics and Geology, Perugia, Italy, Joali Paredes-Mariño, University of Perugia, Department of Physics and Geology,, Perugia, Italy, Joan Marti, Institute of Earth Sciences Jaume Almera-CSIC, Barcelona, Barcelona, Spain, Jeremie Vasseur, LMU Munich, Munich, Germany and Donald B Dingwell, Ludwig Maximilian University of Munich, Earth & Environmental Sciences, Munich, Germany

Recorded Presentation

Abstract:

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.