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

We have performed phase equilibrium experiments to determine the pre-eruptive conditions of the largest caldera-forming eruption (similar to 20 km(3) 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 cent tal 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 hauyne. We have investigated pressures from 50 to 250 MPa, temperatures of 700-900 degrees C, water contents in the melt from 1.5 to ca. 10 wt.%, and Oxygen fugacity (fO(2)) 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 degrees C, with 3 +/- 1 wt.% dissolved H2O and fO(2) 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 oil a specific phonolite to constrain the pre-eruptive conditions of other phonolites should be done with care. (C) 2008 Elsevier B.V. All rights reserved.