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.

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In April 2015, an unpredicted rapid-onset eruption occurred at Calbuco Volcano, Southern Andes of Chile. This event consisted of two, sub-Plinian eruptions separated by a few hours. By analysis of Fe-Ti exchange between ilmenite and titanomagnetite crystals in samples of erupted material, we determine timescales of pre-eruptive heating experienced at the partially solidified chamber base and constrain the magma residence time for the bulk of the carrier magma. Analysis of the Fe-Ti oxide pairs from a sample retrieved from a pyroclastic density current deposit (Cal-160) shows that it was affected by a significant heating event (recording 70-220 °C of heating), while other collected samples did not record this late heating. This sample is interpreted to represent a piece of crystal mush located at the bottom of a prolate, ellipsoidal mush reservoir, mobilised < 4 days before the eruption by a triggering pulse of mafic magma considerably hotter than the typical magmatic temperature of the reservoir. Another two fall deposit samples (lapillus, Cal-149Tb and Cal-155) of the eruption are interpreted to represent resident, eruptible magmas that did not interact with any magma recharge immediately prior to or during the eruption. We infer that these magmas had been at eruption temperature for some years based on their extensively equilibrated Fe-Ti oxides.

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