Experimental Constraints on Parameters Controlling the Difference in the Eruptive Dynamics of Phonolitic Magmas: the Case of Tenerife (Canary Islands)

Abstract

We have performed phase equilibrium experiments to determine the pre-eruptive conditions of the explosive eruption of Montana Blanca (2020 BP) that occurred from a satellite vent located in the east flank of Teide volcano (Tenerife Island). Crystallization experiments were performed using a phonolitic obsidian from the fall out deposit that contains 5 wt% anorthoclase, diopside and magnetite with minor amounts of biotite and ilmenite, set in a glassy matrix that contains microlites of Ca-rich alkali feldspars. Temperature was varied between 850oC and 800oC, and pressure between 200MPa to 50 MPa. The oxygen fugacity (fO2) was varied between NNO+0.2 (0.2 log units above to the Ni-NiO solid buffer) to NNO-2, whilst dissolved water contents varied from 7 to 1.5 wt%. Comparison between natural and experimental phase proportions and compositions indicates that the main body of phonolite magma was stored at 850±15oC, 50±20MPa, 2.5±0.5wt% H2O at an fO2 around NNO-0.5 prior to eruption, equivalent to depths between 1 to 2 km below the surface. Some clinopyroxene hosting H2O-rich melt inclusions possibly originates from intermittent supply of phonolitic magma stored at somewhat deeper levels (100 MPa). The Ca and Fe-rich composition of alkali feldspar phenocrysts rims and microlites attests to the intrusion of a mafic magma in the reservoir just prior to eruption, as borne out by banded pumices appearing in the later products of the eruptive sequence. The comparison with other phonolitic magmas from Tenerife and elsewhere (e.g., Vesuvius, Laacher See) shows that differences in the eruptive dynamic of phonolitic magmas can be correlated to differences in storage depths, along with variation in pre-eruptive volatile contents.