Petrology of the April 2015 Eruption of Calbuco Volcano, Southern Chile

Abstract

Understanding the origin of intermediate magmas that commonly erupt from subduction zone volcanoes is important to better constraining the mechanisms of continental crust formation. We performed a detailed mineralogical and petrological study of the eruptive products from April 2015 eruption of Calbuco volcano, Chile, a three-phase sub-Plinian eruption that produced pyroclastic deposits of andesitic composition. The eruptive products comprise a glass phase and a high but variable proportion of minerals dominated by plagioclase, clinopyroxene, and orthopyroxene, with minor olivine, amphibole, and magnetite. Plagioclase is very strongly zoned with highly anorthitic cores surrounded by more albitic rims, and no intermediate compositions between them. Based on thermodynamic calculations and published experimental data, we estimate that the anorthitic cores crystallized from a basaltic andesite melt containing 3.5–4.5 wt.% H2O. The bulk-rock major and trace element variability at Calbuco is best explained by the accumulation of minerals (72% plagioclase, 28% pyroxene) in a dacitic melt. These minerals most likely formed in the crystal mush zone of the magma chamber, at 200–300 MPa (8–11 km depth) according to pyroxene and amphibole compositions. A few weeks to months before the eruption, the crystal mush was disaggregated, perhaps due to magmatic underplating, and a crystal-bearing dacitic melt migrated into a sub-surface storage region where the albitic plagioclase rims crystallized. The eruption was probably internally triggered by over-pressurization in the shallow magma chamber.

The petrology of a hazardous volcano: Calbuco (Central Southern Volcanic Zone, Chile)

“…Glomerocryst designates a monomineralic or polymineralic assemblage of macrocrysts. Our petrographic observations agree with those previously described by Lopez-Escobar et al (1992), Lopez-Escobar et al (1995), Moreno (2011), Romero et al (2016), Arzilli et al (2019), Morgado et al (2019b) and Namur et al (2020). There is a gradual evolution in the mineralogy from the basalts to the andesites: olivine and plagioclase are the sole macrocrysts in the least differentiated sample (basalt CL119), the proportion of olivine relics surrounded by a peritectical rim apparently decreases with increasing SiO 2 , augite macrocrysts are rare or absent in the basalts, abundant in the basaltic andesites and again less abundant in the andesites, amphibole has not been observed in the basalt and is much more abundant in the andesites than in the basaltic andesites.…”

“…In order to avoid genetic terms, macrocryst and microcryst will refer to crystals that are respectively larger and smaller than 100 μm. Locally, some magnetite macrocrysts can be smaller than this threshold as observed by Namur et al (2020) but still much larger than oxides in the fine-grained matrix. Glomerocryst designates a monomineralic or polymineralic assemblage of macrocrysts.…”

“…2). Samples of the 2015 eruption (Namur et al 2020) are used here for comparison. Lavas were preferentially sampled in the inner part of individual flows where vesicles are less abundant and the texture is more massive.…”

“…Additional details on sample preparation can be found in Vander Auwera et al (2019). Repeat analyses of 12 standards were used to estimate the accuracy and precision that are respectively between ± 2.0 and ± 5.7 and between ± 1.5% and ± 5.2% (Namur et al 2020) (Supplementary Table 1).…”

“…The tephra (0.27 ± 0.007 km 3 : Romero et al (2016); 0.38 ± 0.1 km 3 : Castruccio et al (2016); 0.58 ± 0.28 km 3 : Van Eaton et al ( 2016)) were essentially deposited on the northeastern part of the volcano and the finest ash dispersed across Chile, Argentina and Uruguay. They mainly consist of a basaltic andesite (Castruccio et al 2016;Romero et al 2016;Morgado et al 2019b;Namur et al 2020) with a high amount of crystals (35 to 65 vol. %), plagioclase, orthopyroxene and clinopyroxene being dominant.…”