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Volatile degassing, petrology, and magma dynamics of the Villarrica Lava Lake, Southern Chile

par Witter, Jeffrey J.B.;Kress, Victor V.C.;Delmelle, Pierre ;Stix, John
Référence Journal of volcanology and geothermal research, 134, 4, page (303-337)
Publication Publié, 2004-07

Article révisé par les pairs

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Résumé :

Villarrica is a 2847-m-high, subduction zone stratovolcano located in the southern Chilean Andes (39.5°S). This volcano has been characterized by continuous, open-vent, passive degassing from a summit lava lake, with negligible eruption of magmatic material, from the end of the last eruption in 1985 to the time of writing. The major- and trace-element bulk composition of Villarrica magma (basaltic andesite) has been identical from 1984 to the present. Samples of Villarrica magma from the lava lake contain 33 wt.% plagioclase (An58-74), 7 wt.% olivine (Fo75-78), and trace amounts of chromian spinel. Glass inclusions in olivine and plagioclase are basaltic andesite in composition, are mostly degassed, and contain 30-440 ppm S, 170-360 ppm Cl, 340-600 ppm F, 0.1±0.6 wt.% H2O, and 20-70 ppm CO2. One gas-rich glass inclusion contains 920 ppm S, 530 ppm Cl, and 1.4 wt.% H2O. Matrix glasses are basaltic andesite and contain 80-150 ppm S, 190-250 ppm Cl, 440-630 ppm F, and 0.0±0.7 wt.% H2O. MELTS modeling suggests that all crystallization occurred at P <170 bar, T =1130-1140 °C, fO2=NNO under water-saturated conditions. The upper bound for the initial amount of dissolved H2O in Villarrica melt is unconstrained. In 2001, Villarrica emitted 460±260 tons/day SO2, 102±42 tons/day HCl, and 13±3 tons/day HF. On average, ∼2.2 m3/s of magma is degassed (or 5800 kg/s) which implies ∼1.2 km3 of magma has degassed since 1985. Intrusion of 1.2 km3 of degassed magma into the volcanic edifice would cause >50 m of vertical deformation, but there is no visible bulging of the edifice or mass wasting of the cone caused by deformation-induced gravitational instability. Instead, convection of magma in a volcanic conduit with ∼5 m diameter is the most likely mechanism for transporting both the volatiles and heat to produce the observed gas emissions and prevent the lava lake from solidifying. © 2004 Elsevier B.V. All rights reserved.