Quantifying the pre-eruptive conditions of the Mocho-Choshuenco volcano (Chile)

Abstract

Volcanism along the Andes results from the subduction of the Nazca and Antarctic plates under the South American plate. Mocho-Choshuenco is a Chilean volcano located in the Central Southern Volcanic Zone (CSVZ). Formed during the late Pleistocene to early Holocene Period, this volcano has been highly explosive ever since. The combination of this explosivity and the rapidly increasing population in the region makes Mocho-Choshuenco a hazardous volcano. Therefore, tighter constraints on its magmatic system are needed. To achieve this, a selection of tephra from different eruptive events of Mocho-Choshuenco was analyzed to determine the pre-eruptive conditions (P, T, % H2O and fO2) of the magma reservoir. Various geothermobarometers based on mineral–liquid equilibria were used to estimate these parameters. Pressure was calculated using different geobarometers (clinopyroxene-liquid, clinopyroxene only and amphibole only) with average values ranging from 1.4 to 2.8 kbar (approximately 7 km depth). Temperatures were estimated to range from 1001 to 1094 °C using models based on clinopyroxene- and olivine-liquid equilibria. Amphibole appears to be a late crystallizing phase, as lower temperatures were deduced from its composition and the calculated composition of melts in equilibrium with amphibole are SiO2-rich. The amphibole composition also allowed for the estimation of an oxygen fugacity of about NNO+1.6 and a water content of 5.8 wt. % in the evolved melts. The water content of the parental magma was further calculated with the Rayleigh’s distillation law at about 3–3.5 wt. %. This water content is higher than those estimated for nearby volcanoes such as Osorno and La Picada but is similar to the value obtained for the parent magma of Calbuco.