UNRAVELING MAGMA RECHARGE AND STORAGE HISTORIES WITH GEOCHEMISTRY, CRYSTAL POPULATIONS, AND THERMOBAROMETRY IN THE PACHECO SEQUENCE OF MISTI VOLCANO, PERU

Misti is a historically active Andean arc volcano with a large volcaniclastic fan upon which Arequipa, the second most populated city in Peru (>1.1 million) is situated. Since the late Pleistocene, Misti produced dozens of explosive eruptions associated with tephra-fall deposits (TFDs). The youngest TFD series, the Pacheco sequence (<30 ka), consists of ten andesite to basaltic-andesite TFDs produced by VEI 3–5 eruptions. Whole rock textures, glass compositions, and crystal compositions and textures indicate the presence of at least three endmember magmas (mafic, intermediate, and felsic) during this period that cyclically interacted mechanically and chemically. Broadly, three populations of plagioclase persist throughout the Pacheco sequence: low-An crystals or zones (An_25–49); moderate-An crystals or zones (An_50–73); and high-An cores (An_74–84). We use plagioclase crystal exchange, interpreted from their compositions and textures, to track magma recharge events prior to eruption. Disequilibrium textures of felsic endmember-hosted amphiboles and pyroxenes along with their geochemistry also suggest they originated in an intermediate magma composition and reacted upon mingling. We can thus define a) an intermediate magma endmember with relatively calcic plagioclase + pargasite + augite + enstatite + oxides and b) a felsic endmember with relatively sodic plagioclase + oxides. The high-An cores are likely the only remnant of a deep cryptic mafic endmember. Furthermore, we apply several models for phase equilibria to estimate intensive parameters and crystallization conditions of Misti’s magmas during this period, including plagioclase-melt, amphibole-only, amphibole-melt, clinopyroxene-only, two-pyroxene, and Fe-Ti oxide pairs for each eruption. The temperatures and pressures estimated for the intermediate endmember are remarkably consistent over the Pacheco sequence with crystallization at pressures of 2–6 kbar and pre-eruption temperatures of ~1025 °C, while the felsic endmember is consistently cooler and likely at ~800–850 °C. This work suggests Misti’s magma system has had a constant upper crustal magmatic storage configuration since the late Pleistocene marked by recurrent magma recharge events prior to eruption, which we infer from our data acts as a key eruption trigger at Misti.