Temperature of the Villarrica Lava Lake from 1963 to 2015 Constrained by Phase-Equilibrium and a New Glass Geothermometer for Basaltic Andesites

Magma/lava temperature is one of the most basic and critical parameters in volcanology. Yet at many active volcanoes, including during eruptions, this parameter remains poorly constrained due to vent accessibility, with estimates from different geothermometers commonly varying by several tens to more than a hundred degrees. One such volcano is Villarrica, one of the most active and hazardous volcanoes in South America and whose crater hosts a persistent lava lake. Here, we use experimentally determined phase equilibria to constrain the temperature of the Villarrica lava lake. The experiments were all performed at ambient pressure (i.e. 0.1 MPa), ranged in temperature from 1000°C to 1250°C and in oxygen fugacity from QFM + 1 to QFM–1 (where QFM stands for the quartz-fayalite-magnetite redox buffer). Within the investigated range, fO 2 was found to have little to no effect on the phase equilibria and hence the lava lake fO 2 could not be constrained. The temperature range explored, however, did bracket the entire liquidus to solidus assemblage evolution. On the basis of these experiments, two empirical geothermometers calibrated strictly between 1050°C and 1200°C (i.e. not to be used outside this range) were developed for the Villarrica lava lake based on its matrix glass composition and modal proportion. The 2015 state of the natural system was experimentally reproduced at temperatures around 1141°C. In detail, the lava lake melt composition in the period 1963 to 2015 indicates a cooling trend from 1186 ± 19°C in 1963 to 1153 ± 8°C in 1999–2000 and 1149 ± 28°C in 2015. A generic glass geothermometer was also developed for any Basaltic Andesite composition from the compilation of 175 phase equilibria experiments giving the relation: $Melt\ Temperature\ \left({}^{\circ}C\right)=12.43\times {\mathrm{Al}}_2{\mathrm{O}}_3+10.73\times \mathrm{MgO}+8.51\times \mathrm{CaO}+5.91\times \left({Na}_2\mathrm{O}+{K}_2\mathrm{O}\right)+814.08$ with all oxides in wt.% normalized. The geothermometers we have developed can be used to follow future temperature evolutions of the Villarrica lava lake or to estimate the lava temperature at any basaltic andesite eruption. Their application is simple, requiring only petrographic observations (to quantify the melt proportion) or analysis of the melt composition.