ALBERT

Description: 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 transition from long-lived passive degassing to violent explosive eruption occurred with limited precursory 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 geochemical 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 indistinguishable. 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 microlites, 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 probably 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 An 65–92 and Fo 81 , 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 % SiO 2 , 18 wt % Al 2 O 3 , and ~6 wt % MgO. The major element compositions of olivine-hosted melt inclusions range from 49 to 56 wt % SiO 2 and 3·72 to 7·76 wt % MgO; there is no distinct compositional difference between olivine-hosted melt inclusions sourced from the different scoria. Melt inclusion volatile contents range from below detection to 2·95 wt % H 2 O and 1973 ppm CO 2 (though not in the same melt inclusion). H 2 O and CO 2 concentrations are consistent with open-system degassing and, when compared with differentiation indices (e.g. K 2 O), indicate coupled degassing and crystallization throughout 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 342 MPa and fall into two groups: 8–100 MPa (300 m to ~4 km depth) and 〉100 MPa (4–14 km 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, unlocked 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.