HOLOCENE EVOLUTION OF MAGMA STORAGE AND DYNAMICS BENEATH PLANCHON-PETEROA, CHILE

The Planchon-Peteroa Volcanic Complex (PPVC) is in the Andean Southern Volcanic Zone at 35°S with active vents that produced explosive eruptions in 1991, 2010, and 2018-19. We aim to track how the trans-crustal magma system fueling these eruptions evolved from 7 ka to present. The andesitic Los Ciegos eruption occurred 7 ka BP with a volume ~0.1 km³. The Pomez de los Baños eruption (PLB) produced a 0.05 km³ mingled dacite and andesite pumice deposit at 1.5 ka. Most recently, PPVC sourced an explosive phreatomagmatic eruption in 2018-19. Melt inclusion-bearing crystals of plagioclase, olivine, and clinopyroxene were separated from fine lapilli in each of these deposits to determine pre-eruptive storage conditions. Major elements of melt inclusions were measured on a Cameca SXFive electron probe; trace elements and volatiles (H₂O, CO₂, F, Cl, and S) on a CAMECA IMS 1280 ion probe in the WiscSIMS laboratory. Magnetite and ilmenite from each eruption are used to constrain temperature and fO₂ conditions. PLB oxide pairs separated from silicic and mafic components record temperatures of 840 and 850 °C respectively, and both record fO₂ of NNO +1. Oxides from the other eruptions recorded a wider spread in temperatures and may have re-equilibrated. Melt inclusions analyzed from the Los Ciegos tephra range from 52 - 60 wt. % SiO₂ and contain 0.5 - 2.5 wt. % H₂O. Melt inclusions from the PLB eruption are bimodal with distinct populations with 51 to 57 and 65 to 71 wt. % SiO₂. Melt trapped in PLB crystals contains up to 4.3 wt. % H₂O. Plagioclase separated from the mafic scoria of PLB interacted with, and trapped, more silicic melt indicating a role for magma mixing in the buildup to eruption. The lack of a clear hybridized melt composition suggests that mixing may not have been complete. Thus, chemical evidence of mixing may be more cryptically recorded in trace elements and in the transfer of volatiles. Melt inclusions from the 2019 tephra also record bimodality in composition. This suggests that the modern magma reservoir operates similar to when the PLB eruption occurred. Initial saturation pressure calculations from melt inclusions indicate magma storage ranging from 0.5 to 2.0 kbar at PPVC. Saturation pressures will be used for all three eruptions to track how the depth of magma storage and saturation of phases has evolved throughout the Holocene.