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

Abstract

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 (An 58–74), 7 wt.% olivine (Fo 75–78), 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.% H 2O, and 20–70 ppm CO 2. One gas-rich glass inclusion contains 920 ppm S, 530 ppm Cl, and 1.4 wt.% H 2O. 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.% H 2O. MELTS modeling suggests that all crystallization occurred at P<170 bar, T=1130–1140 °C, f O 2 =NNO under water-saturated conditions. The upper bound for the initial amount of dissolved H 2O in Villarrica melt is unconstrained. In 2001, Villarrica emitted 460±260 tons/day SO 2, 102±42 tons/day HCl, and 13±3 tons/day HF. On average, ∼2.2 m 3/s of magma is degassed (or 5800 kg/s) which implies ∼1.2 km 3 of magma has degassed since 1985. Intrusion of 1.2 km 3 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.