DECIPHERING THE PETROLOGY OF A HAZARDOUS VOLCANO: MAGMA STORAGE CONDITIONS, REMOBILIZATION, AND TIMESCALES INFERRED FROM TEPHRA-FALL DEPOSITS FROM MISTI VOLCANO, PERU

Misti, an active volcano in southern Peru, has a history of Plinian and sub-Plinian explosive eruptions over the past 30 ky. Despite the hazard to the adjacent city of Arequipa (population >1 million), few of Misti’s eruptions are understood in a detailed petrological context. This study investigates a stratigraphic section of ten tephra-fall deposits (TFDs) spanning ~30–2 ka to characterize Misti’s magma storage system, eruption triggers, mingling, hybridization, and eruption timescales. The TFDs’ glasses range from ~59–79 wt% SiO₂, including andesite and rhyolite end-members present in five TFDs as separate clasts, small mingled blebs, or compositional banding. Fe-Ti oxide and amphibole-melt thermometry record magma temperatures of 800–1000 °C. Amphibole-only and amphibole-melt barometry indicate a magma storage zone at ~3–4 kbar, though reacted and non-reacted amphibole textures in mingled units indicate a second shallower storage zone above the amphibole stability field. These data suggest a deeper storage system (~3–4 kbar) for an andesitic magma with plagioclase + amphibole + two-pyroxenes + Fe-Ti oxides stored at ≥1000 °C, and a shallower zone above the amphibole stability field (perhaps ~1 kbar) for an ~800 °C rhyolitic magma containing only plagioclase + Fe-Ti oxides. In mingled magmas, temperatures correlate with inferred proportions of end-members and represent a given magma body’s post-mingling thermal equilibration. Silica content of glass through time indicates periodic recharge of the shallower rhyolite by the deeper andesite which initiates a cycle of 1) short-lived mingling, 2) eruption of mingled products, 3) hybridization of end-members, and 4) eruption of hybridized products. Preliminary diffusion chronometry on plagioclase in mingled TFDs indicates that recharge occurs on the order of days to weeks prior to eruption. Initial evaluation of amphibole single crystal total fusion ⁴⁰Ar/³⁹Ar ages from two TFDs yield dates that are consistent with the units’ stratigraphic relation with each other and among other units of known age. Additional amphibole-melt and two-pyroxene thermobarometry, plagioclase diffusion chronometry, and ⁴⁰Ar/³⁹Ar geochronology are ongoing to further elucidate the details of magma storage, remobilization, and timescales at Misti.