No abstract

Arc volcanoes produce most of the Earth's subaerial volcanic activity and are responsible for some of the largest historical eruptions (Siebert et al., 2015). However the structure of the magmatic systems that feed these volcanoes is still largely unknown. The traditional view that the magmas erupted from arc volcanoes reside within a melt-dominated subvolcanic "magma chamber" has been superseded in recent years by a more nuanced view of magma storage and supply, whereby melts ascend through a vertically extensive series of melt-rich zones, termed a "transcrustal magma system" (TCMS, Cashman et al., 2017). This conceptual model describes the complex processing of primary magmas throughout the entire crust by crystallization, assimilation, and mixing (e.g., Annen et al., 2006Annen et al., , 2015. This combination of processes provides a theoretical framework to investigate the origins of the variety of mineral compositions and textures found in a single eruption's crystal cargo.

Tsuchiyama, A. (1985). Dissolution kinetics of plagioclase in the melt of the system diopsidealbite-anorthite, and origin of dusty plagioclase in andesites. Contributions to Mineralogy

Calbuco volcano is a Late Pleistocene composite stratovolcano and member of the Southern Volcanic Zone of the Chilean Andes (41°19′S, 72°36′W). It lies~20 km west of the Liquiñe-Ofqui Fault Zone, but is not located directly upon any major regional structures. During April 2015, a sub-Plinian eruption occurred, with a bulk erupted volume of~0.3-0.6 km 3 (~0.1-0.2 km 3 DRE). The eruption was a rapid-onset event that produced highly crystalline products (from 40 to 60 vol.%) including the mineral phases: plagioclase, clinopyroxene, orthopyroxene, amphibole, olivine, apatite, ilmenite, titanomagnetite and chalcopyrite. An upper-crustal reservoir is inferred using available geophysical data combined with amphibole geobarometry. Consideration of textural features, including high crystallinity, complex mineral zonation, crystal clots and interstitial glass between crystals from clots, suggests the presence of a mush zone within this reservoir. From the nine collected samples, whole-rock chemistry and an array of geothermometers (amphibole, amphibole-plagioclase, two-pyroxenes and Fe-Ti oxides) gave similar results for all samples possessing~40 vol.% of crystals, with the exception of the sample Cal-160 (~60 vol.% crystallinity), which is slightly more evolved and yields lower temperatures for all geothermometers. By comparing temperatures calculated in sample Cal-160 using pairs of ilmenite-titanomagnetite core compositions with those calculated using rim compositions, we observe a late-stage temperature increase of between 70 and 200°C. We suggest that this local-scale heating event was at least partly responsible for triggering the eruption. Our data suggest that the bulk of the erupted magma was derived from a relatively uniform (970-1000°C), crystal-rich magma mass. Sample Cal-160 was derived from a cooler environment (910-970°C), where it was subjected to pre-eruptive heating to temperatures considerably higher than those observed in associated, erupted magmas (up to 1070°C). This requires the involvement of a hot, presumably mafic magma injection at the base of a shallow, crystal-rich reservoir, though the mafic magma was not itself erupted. The localised nature of interaction and rapidity of eruption onset have implications for potential future hazards at Calbuco volcano.