Experimental constraints on pre-eruptive conditions of phonolitic magma from the caldera-forming El Abrigo eruption, Tenerife (Canary Islands)

We have performed phase equilibrium experiments to determine the pre-eruptive conditions of the largest caldera-forming eruption (∼ 20 km3 of DRE) that has occurred on Tenerife (Canary Islands). The Abrigo ignimbrite is phonolitic and was erupted during the last caldera-forming episode (ca. 190 ka), prior to the construction of the Teide–Pico Viejo active central complex. The Abrigo phonolite contains low amounts of phenocrysts (ca. 7 vol.%), mainly sanidine, biotite, clinopyroxene, and magnetite, with minor amounts of titanite, apatite, and haüyne. We have investigated pressures from 50 to 250 MPa, temperatures of 700–900 °C, water contents in the melt from 1.5 to ca. 10 wt.%, and oxygen fugacity (fO2) from 1 log unit above the Ni–NiO solid buffer to Quartz–Magnetite–Fayalite. Comparison of the natural and experimental phase proportions and compositions indicates that the phonolite at the roof of the Abrigo magma reservoir was at 130 ± 50 MPa (corresponding to ca. 4–5 km below the surface), 825 ± 25 °C, with 3 ± 1 wt.% dissolved H2O and fO2 at the Ni–NiO buffer ± 1 log unit. This shows that the magma that produced the largest ignimbrite on Tenerife was stored at relatively shallow depths but was water-undersaturated, and its eruption was probably triggered by input of fresh mafic magma. Comparison of the Abrigo phase equilibrium results with those obtained in other phonolitic magmas (e.g., Eifel, Kerguelen, Somma-Vesuvius) shows that the major element and probably fluid composition (H2O, Cl and S) may have a strong effect on the presence or absence of phases and their compositions. Extrapolating phase equilibria results obtained on a specific phonolite to constrain the pre-eruptive conditions of other phonolites should be done with care.