CYCLES OF MAGMA RECHARGE AND ERUPTION TRIGGERING AT A HAZARDOUS ANDEAN ARC VOLCANO: EL MISTI, SOUTHERN PERU

Preliminary tephrostratigraphy indicates that explosive activity in the last ~21 ky at Misti volcano (Peru) produced at least 11 tephra-fall deposits (TFDs). These were dispersed to the S-SW of the edifice toward the present-day city of Arequipa (population >1 million). We investigate the oldest 9 of these TFDs (in stratigraphic order, informally named: Autopista, Espuma Iridescente, Espuma Gris, Sandwich Inferior, Sandwich Superior, Unnamed 1, La Rosada, Unnamed 2, Unnamed 3), which are overlain by the 2 ka Plinian eruption deposits and 15^th century ash. Our goal is to use petrologic indicators to track the variation in magma recharge, storage conditions, remobilization, and eruption triggering prior to each of these explosive eruptions. These TFDs are typically intermediate in composition and contain ~10–25 volume % phenocrysts with an assemblage of plagioclase + amphibole + clinopyroxene + orthopyroxene + Fe-Ti oxides. However, differences exist between these eruptions in the degree of chemical and textural homogeneity, and these differences do not correlate with explosivity or size of a given eruption. The Espuma Iridescente, Sandwich Inferior and Superior, and La Rosada TFDs show evidence of magma mingling (the incomplete homogenization of two disparate magmas), including the presence of mingled textures in hand sample, bimodal glass chemistry, amphibole or pyroxene with reaction rims, and multiple plagioclase populations based on BSE imaging and compositions. In contrast, the interim deposits have unimodal glass compositions (Espuma Gris, Unnamed 1, Unnamed 2). This homogeneity likely results from complete mixing of the bimodal magmas that supplied the respective preceding eruption. As such, we propose that Misti’s activity during this time is driven by cycles of magma recharge followed by mingling and/or mixing; the combination of magma properties, storage conditions, and timescales that favor one outcome over the other is the subject of ongoing and future work. This includes thermobarometry to characterize storage conditions, diffusion chronometry to measure recharge and remobilization timescales, and ²²⁶Ra/²³⁰Th and ⁴⁰Ar/³⁹Ar geochronology to quantify repose intervals. Finally, Magma Chamber Simulator (MCS) will be used to quantify mass and energy constraints on magma recharge at Misti.