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The origin of gaps or zoning in the composition of erupted products is critical to understanding how sub-volcanic reservoirs operate. We characterize the compositionally zoned magma that produced the 2053 ± 50 cal. yr BP Paso Puyehue Tephra from the Antillanca Volcanic Complex in the Andean Southern Volcanic Zone (SVZ). The 3.7 km3 Paso Puyehue Tephra is zoned from dacite (69 wt% SiO2) lapilli and ash comprising the lowermost 80% of the deposit that abruptly transitions upward into basaltic andesite scoria (54 wt% SiO2) making up the remaining ~20%. Variations in whole-rock, matrix glass, and mineral compositions through the deposit allow us to estimate pre-eruptive magma storage conditions and to develop a model of how this magma body was generated. Our findings suggest that amphibole-bearing basaltic andesitic magma stored at ~8.0 ± 1.3 km depth fractionally crystallized and cooled from 1048 ± 1.1 to 811 ± 28.6 ◦C under highly oxidizing conditions to produce silicic a melt that upon extraction and rise, pooled at ~6.4 ± 1.2 km depth at temperatures as low as 810 ◦C before eruption. MELTS models suggest that crystallization of a basaltic andesite parent magma with 4 wt% dissolved H2O can produce the dacite under conditions predicted by mineral thermobarometers with phase compositions comparable to those measured in minerals. Pervasive normal zoning at the rims of plagioclase crystals—most pronounced at the transition between dacite and basaltic andesite, and compatible vs. incompatible trace element concentrations, suggest that magma mixing was limited and likely occurred at the interface between the dacitic and basaltic andesitic magmas during ascent within the conduit upon eruption. Compositionally bimodal tephras are increasingly recognized throughout the SVZ with several interpreted to reflect basaltic recharge and mixing into extant rhyolitic reservoirs. In further contrast to other SVZ rhyolitic products, e.g., from the nearby Cord´on Callue and Mocho Choshuenco volcanoes, the Paso Puyehue magma was highly oxidized. This may reflect enhanced delivery of H2O from the subducting plate into the mantle wedge, which in turn may facilitate efficient extraction and separation of buoyant, low-viscosity rhyolitic melt from crystal-rich basaltic andesitic parent magmas and the co-eruption of both end members. 

Abstract Glacial loading and unloading could impact eruptive outputs at mid‐ to high‐latitude arc volcanoes, yet the influence on magma storage conditions remains poorly understood. Mocho‐Choshuenco volcano in the Andean Southern Volcanic Zone has been impacted by the advance and retreat of the Patagonian ice sheet (PIS). We present stratigraphic observations, conceptual and numerical models of ice (un)loading during the most recent glacial‐interglacial cycle, and use mineral‐based thermobarometry and thermodynamic modeling to explore the evolution of the magma plumbing system. Crustal assimilation was evident during glaciation when incompatible element‐rich basaltic andesitic lavas at 26 to 18 ka crystallized at depths of 11–15 km. During this period a >5 km³silicic reservoir began to grow at 10–15 km, similar to the depth of maximum mean stress change by ∼5.5 MPa caused by PIS loading. Volatile exsolution within the reservoir might have generated overpressure that initiated diking and propelled two caldera‐forming, amphibole‐bearing rhyolites at 13.5 and 11.5 ka. We propose that increased compressional stresses associated with ice loading during the Last Glacial Maximum inhibited eruption and promoted growth of the deep silicic reservoir. Explosive eruptions from this reservoir 3–5 kyr following rapid ice unloading likely reflect the interplay of viscous relaxation of crustal wallrocks and delayed, but abrupt, volatile exsolution from rhyolitic melt to generate overpressure and dike formation. This interdisciplinary approach can be applied to other glaciated volcanoes and arcs to examine how ice loading and unloading modulate upper crustal magma fluxes that are governed at greater depths by rates of mantle melting.