Abstract
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 km3 (~ 0.1–0.2 km3 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.
Highlights
Within the continental crust, magma reservoirs are considered to be represented by both magma chambers, these being defined as continuous, melt-rich, lenticular regions where the magma is fluid and eruptible and within which magma crystallinity is < 45 vol.%, and crystal mushes, i.e. a body with a rigid crystal framework and crystallinity > 45% (Marsh 1989, 1996; Hildreth 2004; Bachmann and Bergantz 2008)
This study reports whole-rock chemistry, mineral chemistry, textural features and calculations of pre-eruptive intensive conditions derived from fall deposits of the sub-Plinian eruption of Calbuco volcano (Southern Chile), which occurred on 22–23 April 2015
The initial melt composition used in MELTS (Ghiorso and Sack 1995) modelling corresponds to the anhydrous groundmass composition of sample Cal-159 (Table 4) because it shows the highest Mg# (0.59), and possesses representative mineral assemblages and textural features observed in all April 2015 Calbuco products
Summary
Magma reservoirs are considered to be represented by both magma chambers, these being defined as continuous, melt-rich, lenticular regions where the magma is fluid and eruptible and within which magma crystallinity is < 45 vol.%, and crystal mushes, i.e. a body with a rigid crystal framework and crystallinity > 45% (Marsh 1989, 1996; Hildreth 2004; Bachmann and Bergantz 2008). This study reports whole-rock chemistry, mineral chemistry, textural features and calculations of pre-eruptive intensive conditions (geothermometry, geobarometry and oxybarometry) derived from fall deposits of the sub-Plinian eruption of Calbuco volcano (Southern Chile), which occurred on 22–23 April 2015. During 22–23 April 2015, two sub-Plinian eruptive events occurred at Calbuco volcano. The first eruptive pulse started on April at 17:50 local time (20:50 UTC) and the second started on April at 01:00 local time (04:00 UTC) (SERNAGEOMIN 2015a, b, c, d). In terms of precursory activity, Valderrama et al (2015) reported 147 seismic events occurring from 1 January to 21 April 2015, of which 142 were volcano-tectonic (VT, related to fracturing of the wall rock) and 5 were long-period (LP, related to collapse of gas bubbles in rising magma or fluid movement events).
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