Phase equilibrium experiments establish a depth and temperature for the magma chamber at Volcán Quizapu, Chile.
The end-member dacite lavas erupted at Quizapu contain ~20 vol% phenocrysts, mainly plagioclase, with subordinate amphibole, orthopyroxene, and rarer clinopyroxene. Accessory minerals include apatite and an iron sulfide phase.
The bulk end-member materials erupted at Quizapu are slightly alkalic, mainly due to their high K content. Glass from both lava and pumice samples has evolved to a rhyolitic composition.
Working in the experimental petrology lab of Julia Hammer at the University of Hawaii, along with field and petrological input from Quizapu expert Philipp Ruprecht, I used the capabilities of the cold-seal pressure line (CSPV) to simulate the magmatic environment below an Andean volcano. By covering a grid in pressure and temperature space, I was able to narrow down the storage depth and temperature of the dacite magma that erupted in both 1932 and 1846-7.
To end an experiment, the sample must be quickly cooled to ambient temperature in order to freeze in the high temperature (and pressure) phase assemblage. Here, I am about to cool the vessel with compressed air before plunging into a bucket of water.
The completed phase diagram indicates the stability fields of various phases, the isopleths of constant plagioclase composition, and a field showing where experimental glass compositions are a good match to the natural glass. The darker shaded region marks the best estimate of the magma chamber depth and temperature.
Emily First 