Silicic conduits as supersized tuffisites:Clastogenic influences on shifting eruption styles at Cordón Caulle volcano (Chile)

Understanding the processes that drive explosive-effusive transitions during large silicic eruptions is crucial to hazard mitigation. Conduit models usually treat magma ascent and degassing as a gradual, unidirectional progression from bubble nucleation through magmatic fragmentation. However, there is growing evidence for the importance of bi-directional clastogenic processes that sinter fragmented materials into coherent clastogenic magmas. Bombs that were ejected immediately before the first emergence of lava in the 2011–2012 eruption at Cordón Caulle volcano (Chile) are texturally heterogeneous composite assemblages of welded pyroclastic material. Although diverse in density and appearance, SEM and X-ray tomographic analysis show them all to have been formed by multi-generational viscous sintering of fine ash. Sintering created discrete clasts ranging from obsidian to pumice and formed a pervasive clast-supporting matrix that assembled these clasts into a conduit-sealing plug. An evaluation of sintering timescales reveals texturally disparate bomb components to represent only minutes of difference in residence time within the conduit. Permeability modelling indicates that the plug was an effective conduit seal, with outgassing potential—even from high-porosity regions—being limited by the inability of gas to flow across tendrils of densely sintered inter-clast matrix. Contrary to traditional perspectives, declining expressions of explosivity at the surface need not be preceded or accompanied by a decline in fragmentation efficiency. Instead, they result from tips in balance between the opposing processes of fragmentation and sintering that occur in countless cycles within volcanic conduits. These processes may be particularly enhanced at silicic fissure volcanoes, which have laterally extensive subsurface plumbing systems that require complex magma ascent pathways. The textures investigated here reveal the processes occurring within silicic fissures to be phenomenologically identical to those that have been inferred to occur in tuffisite veins: silicic conduits are essentially supersized examples of edifice-penetrating tuffisites

Correlating weathered, microphenocryst-rich, intermediate tephra: An approach combining bulk and single shard analyses from the Lepu\ue9 Tephra, Chile and Argentina

Chemical correlation of intermediate tephra deposits using microanalytical data is problematic because (i) the phenocryst content of their component glass shards affects major and trace element analyses (ii) bulk chemistry can be affected by variations in mineral/lithic components across the fall-out, and (iii) weathering readily alters their composition. All of these problems affect the Lepu\ue9 Tephra, a prominent marker horizon extensively distributed across the Los Lagos Region of Chile and the Chile-Argentina frontier in north-western Patagonia, which was erupted from Volc\ue1n Michinmahuida at c. 11000 cal a BP. Weathering of terrestrial cover-bed deposits in this hyper-humid depositional environment leaves only a few occurrences of the tephra which contain fresh glass shards for microbeam analysis, but their highly phenocrystic nature makes data interpretation difficult. Equally, leaching of mobile elements during weathering causes considerable compositional changes across the fall-out region and is evident in bulk sample analyses. Elements such as the REE and Y, generally regarded as immobile, show marked mobility. Within the REE, the development of \u201cM-type\u201d tetrad effects and positive Ce-anomalies result from a combination of dissolution/leaching of the REE from the bulk sample and retention by co-precipitation of Ce4+ on Fe-oxyhydroxides in this high-rainfall, hyper-humid, oxic environment. Chemical correlation of the Lepu\ue9 Tephra is thus not straightforward. However, by careful consideration of the data for a limited range of elements, chemical correlation can be achieved using elements which (i) are incompatible in magmatic systems (and thus their ratios are unaffected by the presence of phenocrysts in single glass shard microbeam analysis) and (ii) are not mobilised in these weathering conditions. These elements are Zr, Hf, Nb, Ta and Th. Their ratios (i) allow for the comparison of single grain and bulk sample analyses, extending the geographic range over which data can be compared for the Lepu\ue9 Tephra, (ii) provide a robust chemical correlation of this weathered, intermediate tephra deposit, enabling correlation even where elements traditionally considered immobile (REE, Y, and U) have been significantly mobilised, and (iii) allow the Lepu\ue9 Tephra to be distinguished from other local tephra deposits. This combined analytical approach enables tephras that have been variably weathered to become useful marker beds over much wider geographical areas than previously feasible, thereby enhancing their tephrochronological application in Quaternary research

Multiple melt bodies fed the AD 2011 eruption of Puyehue-Cord\uf3n Caulle, Chile

Within the volcanological community there is a growing awareness that many large- to small-scale, point-source eruptive events can be fed by multiple melt bodies rather than from a single magma reservoir. In this study, glass shard major- and trace-element compositions were determined from tephra systematically sampled from the outset of the Puyehue-Cord\uf3n Caulle (PCC) eruption ( 3c1 km3) in southern Chile which commenced on June 4th, 2011. Three distinct but cogenetic magma bodies were simultaneously tapped during the paroxysmal phase of this eruption. These are readily identified by clear compositional gaps in CaO, and by Sr/Zr and Sr/Y ratios, resulting from dominantly plagioclase extraction at slightly different pressures, with incompatible elements controlled by zircon crystallisation. Our results clearly demonstrate the utility of glass shard major- and trace-element data in defining the contribution of multiple magma bodies to an explosive eruption. The complex spatial association of the PCC fissure zone with the Liqui\uf1e-Ofqui Fault zone was likely an influential factor that impeded the ascent of the parent magma and allowed the formation of discrete melt bodies within the sub-volcanic system that continued to independently fractionate

A past-millennium maximum in postglacial activity from Volc\ue1n Chait\ue9n, southern Chile

Volc\ue1n Chait\ue9n (southern Chile, ~43\ub0S) initiated an historically unprecedented eruption in A.D. 2008, surprising the local inhabitants, Chilean and Argentine authorities, and the geologic community. Available data at the time indicated an absence of explosive eruptions from this rhyolitic volcano since a large-magnitude eruptive event dated at ca. 10,500 yr B.P. We present lake-sediment data from Lago Teo, a small closed-basin lake located in the immediate vicinity of both Chait\ue9n township and the volcano, that spans the past ~10,000 yr and contains 26 pyroclastic fallout deposits. Glass-shard electron microprobe analyses revealed ten rhyolitic tephras indistinguishable in composition from the 2008 Volc\ue1n Chait\ue9n eruption, and ten others potentially derived from the Michinmahuida volcanic complex. Among the rhyolites, we detected three closely spaced tephras deposited between ca. 9460 and 9680 yr B.P., followed by two thick tephras dated at ca. 7700 and ca. 5080 yr B.P. Three other closely spaced tephras occur between ca. 600 and 850 yr B.P., the most recent prehistoric event at ca. 420 yr B.P., and a 3-cm-thick tephra deposited during the 2008 event. We calculate a median recurrence of ~310 yr between eruptive events from all sources over the past ~10,000 yr, and ~200 yr between Volc\ue1n Chait\ue9n events over the past millennium. Our results not only challenge the notion of an ~10,500-yr-long quiescence for Volc\ue1n Chait\ue9n activity, but also suggest that the 2008 eruption was an overdue phenomenon in the context of its postglacial eruptive history, illustrating the advantage and absolute necessity of utilizing lake-sediment archives for developing continuous well-dated time series inventories of explosive volcanic events

An 18,000 year-long eruptive record from Volc\ue1n Chait\ue9n, northwestern Patagonia: Paleoenvironmental and hazard-assessment implications

The 2008 eruption of Volc\ue1n Chait\ue9n (VCha) in northwestern Patagonia was the first explosive rhyolitic eruption to have occurred within a century and provided an unprecedented scientific opportunity to examine all facets of the eruption ranging from magma rheology/ascent rates to ash-fall effects on biota and infrastructure. Up to very recently it was thought that the latest eruption prior to the 2008 event occurred c. 9750 cal. a BP. Although a number of researchers have recognised additional eruptive products, but their stratigraphy, age, and geochemical attributes have not been systematically described and/or recorded. In this study, we provide a detailed examination of andic cover-beds and tephra-bearing lake sequences located both proximally and distally to VCha, which record a series of hitherto unknown rhyolitic eruptive products and place all previous observations firmly within a coherent stratigraphic framework. Through major- and trace-element glass shard geochemistry we are able to confidently verify eruptive source. A total of 20 discrete tephra beds are recognised, with at least 10 having widespread areal distributions and/or depositional imprints broadly comparable to, or greater than, the 2008-tephra event. This record indicates that VCha has been continuously but intermittently active as far back as the end of the Last Glacial Maximum (c. 18,000 cal a BP) with two dominant, genetically related magma types and an intermediary \u2018mixed\u2019 type. Before this the eruptive record has been largely obscured and/or erased by widespread Andean piedmont glaciation. However, based on the tempo of VCha activity over the last c. 18,000 years, older VCha eruptives can be anticipated to occur as well as future hazardous explosive events. The new eruptive inventory will ultimately be useful for correlating equivalent-aged sequences and refining long-term eruptive tempo as well as corresponding temporal changes in magmatic evolution

A widespread compositionally bimodal tephra sourced from Volc\ue1n Melimoyu (44\ub0S, Northern Patagonian Andes): Insights into magmatic reservoir processes and opportunities for regional correlation

We describe the stratigraphy, age, constituent geochemistry and phenocryst thermobarometry of a closely spaced Holocene tephra couplet from Volc\ue1n Melimoyu (VMm), located in the Northern Patagonian Andes. The lower tephra unit (La Junta Tephra, Mm-1) is distinctly banded comprising a dominant lower layer of rhyodacitic ( 3c70% SiO2) pumiceous ash and lapilli (Mm-1p) that abruptly transitions to a subordinate upper scoriaceous layer (Mm-1s) of basaltic andesite composition ( 3c53% SiO2). This bimodality within Mm-1 contrasts significantly with the closely overlying Santa Ana Tephra (Mm-2) that has a homogeneous trachyte-dacite ( 3c63% SiO2) composition and is intermediate between the two magmatic end members of Mm-1. We propose a genetic affiliation between Mm-1 and Mm-2, and that the latter event likely represents a hybridised-remnant of those discrete magmas involved in the earlier Mm-1 eruption. To test this hypothesis we applied whole rock elemental mixing and fractional crystallisation model to reproduce the composition and crystallinity of Mm-2. Results indicate that Mm-2 can be reproduced by mixing 3c70% Mm-1p with 3c30% Mm-1s, with subsequent 3c13% fractional crystallisation of plagioclase, and minor amphibole, orthopyroxene, magnetite and biotite. Equilibrium P-T conditions calculated from Mm-1p phenocrysts point towards magma residency at moderately shallow depths (200\u2013290 MPa, 3c7\u201310-km depth, 850\u20131000 \ub0C), whereas Mm-1s phenocrysts indicate higher overall P-T conditions (240\u2013480 MPa, 3c8.5\u201317-km depth, 1080\u20131150 \ub0C). P-T conditions determined for Mm-2 ( 3c290 MPa, 3c10-km depth, 930\u20131000 \ub0C) are similar to those of Mm-1p. There is no physical and/or geochemical evidence of mafic magma involvement in the Mm-2 eruption. Similar compositionally bimodal tephra are known from other Northern Patagonian Andean centres (i.e. Playas Blanca-Negra Tephra, Antillanca; Lepue Tephra, Michimahuida; Ho and H3 eruptions of Hudson) suggests that the intrusion of mafic magma into more silicic magma bodies is a common occurrence throughout this Andean sector. These widely dispersed, compositionally bimodal tephra not only provide key insights into pre-eruptive magmatic conditions and triggering processes, but can also be readily identified geochemically, and thereby be more fully utilised within future hazard- and paleoenvironmental-related studies

Correction to: Silicic conduits as supersized tuffisites: Clastogenic influences on shifting eruption styles at Cordón Caulle volcano (Chile) (Bulletin of Volcanology, (2021), 83, 2, (11), 10.1007/s00445-020-01432-1)

The original version of this article unfortunately contained a mistake. The presentation of Table 1 was incorrect. The corrected table is given below. © 2021, International Association of Volcanology & Chemistry of the Earth's Interior