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Lookup NU author(s): Dr Joaquin Cortes Carrillo
This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License (CC BY-NC 4.0).
On 1 January 2008 Llaima volcano, a basaltic andesite stratocone in southern Chile, entered a phase of violent Strombolian eruption. Llaima, like many passively degassing systems, has experienced prolonged (decades-long) periods of persistent summit degassing from its open vent. The rapid tran- sition from long-lived passive degassing to violent explosive eruption occurred with limited precur- sory monitoring signals. This study is motivated by the desire to understand what occurs in these systems when that switch takes place. To this end, we study the products of the 2008 violent Strombolian eruption of Llaima volcano. We present new textural analyses of scoria and geochem- ical data for five whole-rock samples, troctolite glomerocrysts with and without Cr-spinel, and 182 olivine-hosted melt inclusions from tephra samples. Two populations of scoria (‘brown’ and ‘black’) are distinguished by their variable crystallinity and vesicularity, but are geochemically indistinguish- able. Black scoria contains abundant microlites with tabular to acicular morphologies and convolute vesicles up to 1 75 mm in effective diameter. The brown scoria tends to have fewer, acicular micro- lites, abundant matrix glass, and round vesicles with a narrower size distribution, constrained to < 0 4 mm in diameter. Overall, the textures of the black and brown scoria provide evidence for a textural maturation process in which shallow system magma becomes more crystal rich and prob- ably rheologically stiffer as a result of prolonged passive degassing. The Cr-spinel-bearing and Cr- spinel-free troctolite glomerocrysts have plagioclase and olivine compositions of An65–92 and Fo81, respectively. The Cr# in the Cr-spinel ranges from 26 to 37, consistent with magma originating from the deeper plumbing system. Whole-rock compositions for the tephra average 51 wt % SiO2, 18 wt % Al2O3, and 6 wt % MgO. The major element compositions of olivine-hosted melt inclusions range from 49 to 56 wt % SiO2 and 3 72 to 7 76 wt % MgO; there is no distinct compositional difference be- tween olivine-hosted melt inclusions sourced from the different scoria. Melt inclusion volatile con- tents range from below detection to 2 95 wt % H2O and 1973 ppm CO2 (though not in the same melt inclusion). H2O and CO2 concentrations are consistent with open-system degassing and, when com- pared with differentiation indices (e.g. K2O), indicate coupled degassing and crystallization through- out the system. The majority of melt inclusions define a single liquid line of descent indicative of plagioclase and olivine fractionation. Entrapment pressures range from 8 to 342MPa and fall into two groups: 8–100MPa (300 m to 4km depth) and >100MPa (4–14km depth), revealing that this eruption tapped a deep plumbing system. We propose here that passive degassing at Llaima is maintained by periodic, small-batch magma injections. Consequently, owing to extensive degassing the upper plumbing system magma crystallized and increased in viscosity. Before the 2008 eruption, some volatiles sourced from the repeatedly injected magmas exsolved from the inferred crystal mush and ascending from deeply sourced degassing magmas, and gradually accumulated within the crystal mush and beneath the stiffened conduit magma. Our results support a model in which eruption triggering occurred when magma injection remobilized the mush and, importantly, un- locked the accumulated gases, which ascended rapidly and generated the observed violent Strombolian explosive activity. Our proposed model contrasts with those models for explosive mafic volcanism that require rapid magma ascent under closed-system degassing conditions. Importantly, our proposed mechanism provides a means for systems with dominantly open-system degassing behavior to switch from passive degassing to explosive eruptions.
Author(s): Ruth DC, Cottrell E, Cortes JA, Kelley KA, Calder ES
Publication type: Article
Publication status: Published
Journal: Journal of Petrology
Year: 2016
Volume: 57
Issue: 9
Pages: 1833-1864
Online publication date: 19/09/2016
Acceptance date: 19/09/2016
Date deposited: 09/01/2017
ISSN (print): 0022-3530
ISSN (electronic): 1460-2415
Publisher: Oxford University Press
URL: http://dx.doi.org/10.1093/petrology/egw063
DOI: 10.1093/petrology/egw063
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