What modulates eruptive styles at Villarrica and Osorno volcanoes (Chile)?

Villarrica and Osorno are two active stratovolcanoes in the Central Southern Volcanic Zone (CSVZ) of the Chilean Andes that share several geochemical characteristics: near-primary, tholeiitic parent magmas (50-53 wt. % SiO2), overlapping major/trace element differentiation trends, and comparable storage conditions [1-4]. Yet, their eruptive styles contrast each other significantly. Villarrica is a steady-state, open-vent stratovolcano with a lava lake since 1985, which produced ~100 low to moderate intensity Strombolian eruptions and lava flows since 1579 CE. Osorno is a closed-vent stratovolcano with 10x less eruptions for the same period. We initially proposed that differences in eruptive style and frequency could be due to a relatively higher degree of crustal permeability under Villarrica than Osorno due to the Liquiñe-Ofqui Fault Zone [5]. Preliminary analyses show that both volcanoes have broadly similar olivine chemistry ranges and multimodal distributions, with minor differences in olivine and melt chemistry/textures between Villarrica (Fo72-87) and Osorno (Fo66-82) [4,5]. Diffusion timescales for both volcanoes are mostly 250 days. This suggests the degree of crustal permeability underneath the volcanoes are likely comparable, prompting consideration of other factors. In this contribution, we evaluate the role of magma supply rate, storage conditions, and slab input in modulating eruptive styles at Osorno and Villarrica based on an updated dataset of magma storage conditions, diffusion timescales, geochemical data compilations, and inferences drawn from published literature. We find that magma storage conditions of both volcanoes are similar to each other at T~1100°C, P~200 MPa, along with comparable input of fluids released from the down-going slab. The multimodality in olivine chemistry, diversity in types of olivine growth zones and textures, timescale ranges, coupled with the relatively high magma supply rate estimates for Villarrica from the literature suggest magma supply rate could modulate eruptive style at Villarrica and Osorno. With this contribution, we aim to further current understanding of subduction zone magmatism and geodynamics, with implications on volcanic hazard reduction. 1. Vergara et al. (2004). J. S. Am. Earth Sci. 17: 227-238. 2. Morgado et al. (2015). JVGR, 306: 1-16. 3. Pizarro et al. (2019). JVGR. 384: 48-63. 4. Bechon et al. (2022). Lithos. 106777. 5. Romero et al. (2022). Bull. Volc. 85 (2)

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

peer reviewedThe recurrent explosive eruptions of Calbuco (Andean Southern Volcanic Zone (SVZ)) threat a rapidly expanding tour- istic and economic region of Chile. Providing tighter constraints on its magmatic system is therefore important for better monitoring its activity. Calbuco is also distinguished by hornblende-bearing assemblages that contrast with the anhydrous parageneses of most Central SVZ volcanoes. Here we build on previous work to propose a detailed petrological model of the magmatic system beneath Calbuco. Geochemical data acquired on a hundred samples collected in the four units of the volcano show no secular compositional change indicating a steady magmatic system since ~ 300 ka. A tholeiitic Al2O3-rich (20 wt. %) basalt (Mg# = 0.59) is the parent magma of a differentiation trend straddling the tholeiitic/calc-alkaline fields and displaying a narrow compositional Daly gap. Amphibole crystallization was enabled by the higher H2O content of the basalt (3–3.5 wt. % H2O at 50 wt. % SiO2) compared to neighboring volcanoes. This characteristic is inherited from the primary mantle melt and possibly results from a lower degree of partial melting induced by the mantle wedge thermal structure. Although macrocrysts are not all in chemical equilibrium with their host rocks and were thus presumably unlocked from the zoned crystal mush and transported in the carrier melt, the bulk-rock trend follows both experimental liquid lines of descent and the chemical trend of calculated melts in equilibrium with amphibole (AEMs). These contradictory observations can be reconciled if minerals are transported in near cotectic proportions. The AEMs overlap the Daly gap revealing that the missing liquid compositions were present in the storage region. Geothermobarometers all indicate that the chemical diversity from basalt to dacite was acquired at a shallow depth (210–460 MPa). We suggest that differentiation from the primary magma to the parental basalt took place either in the same storage region or at the MOHO.CDR J.00066.14, PDR T.0079.18, Odysseus grant (ON

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

CDR J.00066.14, PDR T.0079.18, Odysseus grant (ON

Dissection du système magmatique d'Osorno, un volcan d'arc pauvre en H2O - De la source mantellique aux produits eruptifs rhyodacitiques

Petrological studies of volcanic systems and magmatic processes provide small scale information that are inaccessible to the current resolution of geophysical tools. In the Southern Volcanic Zone (SVZ) of the Chilean arc, Osorno ranks sixth in the official risk classication in a region of increasing vulnerability due to soaring human activities coupled with lahar hazard and the threat of a tsunami induced by a flank collapse. However, its magmatic system remained poorly investigated. Here, we present and integrate to existing geophysical models a comprehensive set of petrographic and geochemical data including whole-rock and mineral major and trace element analyses, associated with detailed numerical modelling to constrain the storage conditions below Osorno. To capture the full complexity of the system, over 154 samples of all known major units of the volcano have been collected. They cover a large range from tholeiitic primary basalt (Mg# = 0.72 and 50 wt% SiO2) to rhyodacite (Mg# = 0.18 and 70 wt% SiO2), displaying a compositional gap within andesite (59-63 wt% SiO2). This gap results either from andesitic melt thermal instability or from the interstitial melt extraction of the crystal mush. Basalt and basaltic andesite lava with plagioclase and olivine (±clinopyroxene) are dominant. Their crystallinity largely varies from 1 to 53 area % with the lowest values in the most evolved basaltic andesites. Dacites are limited to three small domes with low crystallinities between 7-13 area %. The presence of diktytaxitic enclaves within the dacites indicate minor mingling with a less differentiated melt. Water-bearing phases are generally absent, except for one dacite sample where few small amphibole crystals occur. Petrology, chemical data and thermobarometric results imply shallow fractional crystallization of troctolitic, gabbroic and gabbronoritic cumulates. Differentiation dominantly takes place between 2-3 kbar (6-10 km) and results from a water-poor (≈1 wt% H2O), tholeiitic parental melt. No evidence of high-pressure fractionation was observed. We interpret this differentiation depth as Osorno's main storage zone. It correlates with the depth of the intracrustal discontinuity and seismic reports below the volcano. Only the upper half of the storage zone, imaged with geophysical methods, was erupting. We suggest a comparable behavior for a potential future event. The magma volatile content (especially H2O) is widely recognized as a marker of eruptive explosivity. Measured high water content at Calbuco contrasts with the low one inferred for Osorno volcano that is only a few km away. Understanding the differences between these two closely related volcanic centers is crucial to better assess volcanic explosivity within the Central SVZ. An additional comparative study on olivine-hosted melt inclusions (MIs) was undertaken to constrain the H2O, Cl, S and B supplies of three volcanic centers (Villarrica, Calbuco, Osorno). The dataset includes major, trace (including B) and some volatile (Cl, S, H2O) elements. In agreement with published predictions (Bechon et al., 2022; Vander Auwera et al., 2021), the results confirm the respective high and low water contents of Calbuco (ca. 2 wt% in the parent basalt) and Osorno (ca. 1 wt% in the parent basalt). The water content within Villarrica melt inclusions are strongly scattered and cover a large range from outgassed melts to melts as rich as the Calbuco's ones. The S MIs contents are similar for the three studied volcanoes (200-1600 ppm) whereas the Cl content is globally greater in Calbuco's MIs (950-3160 ppm) than in the Osorno's and Villarrica's ones (ca. <1070 ppm, except for 1 outlier at 2280 ppm). Unexpectedly, the MIs B contents and trace elements evidenced lower fluids inputs and melting rate at Calbuco relatively to Osorno and Villarrica. These results were used to constrain or reject the proposed explications of for higher water contents at Calbuco that were: (i) mixing and assimilation of metapelite partial melts (Lopez- Escobar et al., 1995); (ii) additional slab fluid release along fracture zones (e.g.: Selles et al., 2022); lower melting rate below Calbuco (Vander Auwera et al., 2021). Considering the mechanism proposed to explain lava lake outgassing (e.g.: Pansino et al., 2019; Pioli et al., 2017), we additionally proposed that variable extents of outgassing is a possible solution. The strengths and weaknesses of each propositions are discussed. The primary magma (PM; Mg# 0.7, MgO 8 wt%, Ni 150 ppm, Cr 1000 ppm) compositions are characteristic of their source melting conditions (P, T, H2O, chemical composition). The study of the PMs genesis is crucial to constrain these parameters, however they hardly reach the surface without undergoing significant chemical modifications. The existence of near-PM (NPM) at Osorno (Bechon et al., 2022; Moreno Roa et al., 2010) was used to estimate mantle melting conditions below the volcano. Last equilibration with the mantle occurred between 10.5 to 13.5 kbar at high temperatures (1228-1327°C). These conditions are partially above the dry mantle solidus which implies that beside hydrous flux melting, an additional adiabatic melting component may exist. Mantle melting estimates converge around the partial melting of ca. 19 (±3) wt% of a depleted peridotite source that contains around 0.16-0.22 wt% H2O. The PM and NPM calculated densities indicate that they are buoyant enough to erupt in the Central SVZ

Profondeur de différenciation sous le volcan Osorno (Chili)

Brève présentation des profondeurs de différenciations sous le volcan Osorno (Chili) à travers l'étude de la géochimie de ses roches

Constraining the conditions of magmatic differentiation under Villarrica stratovolcano (Central Southern Volcanic Zone, Chile)

peer reviewe

What is the magma storage depth under Osorno volcano (41°S, CSVZ, Chile)

The depth at which magma chamber processes take place below magmatic arcs and the parameters controlling them are highly debated. These questions are fundamental for our understanding of the global magma differentiation as well as the formation of the continental crust at convergent margins, but also for evaluating the risks associated with volcanic eruptions. In the Central Southern Volcanic Zone (Central-SVZ) of the Chilean Andes, a thin continental crust (30-40 km) and the occurrence of a major fault zone (Linquiñe-Ofqui) likely favor rapid magma ascent. This segment of the arc is as a consequence one of the most active in Chile with several recent eruptions (e.g. Llaima 2009, Cordon Caulle 2011, Calbuco 2015, Villarrica 2015 & 2019). The Central-SVZ is characterized by dominant mafic lavas (basalts, basaltic andesites), few rhyodacitic lavas, a noticeable compositional (Daly) gap in the intermediate compositions (andesites). Noteworthy, amphibole is usually absent, except in a few volcanoes (e.g. Calbuco) or only occurs as microliths in enclaves, which suggests rather low water contents. These observations contrast sharply with the Northern-SVZ where andesitic lavas are dominant and hydrous phases common. We focused our research on the eruptive products of Osorno volcano (41°S, CSVZ) located between two volcanoes (Calbuco and Cordon Caulle) which recently showed very explosive eruptions and partly overlies an older Pleistocene eroded volcanic edifice (La Picada). A large series of samples were collected in four units spanning 200 kyr. They define a differentiation trend ranging from tholeiitic basalts to calk-alkaline dacites with a Daly Gap between 58 wt. % and 63 wt. % SiO2. Plagioclase and olivine are dominant before the gap while plagioclase and clino- and orthopyroxene dominate afterwards. The use of recent thermobarometric models revealed two main storage regions: (1) at the MOHO interface (1-1.2GPa) and (2), at the upper/lower crust interface with rather low pressures (likely ≤0.3 Gpa). While at (1) primary magmas differentiate, (2) is interpreted as the depth of major differentiation and volatile exsolution. Thermodynamic simulations (Gualda et al., 2012; Ghiorso & Gualda, 2015) support these (2) depth estimates and reproduce the main paragenesis by simple fractional crystallization at 0.1-0.2 GPa. Our results may explain the recent seismic unrest below Osorno (from 2015 to 2019) with earthquakes mostly taking place between 0.1-0.3 GPa (4-10km below the summit). We suggest that Osorno is an important target to perform a comprehensive petrological study aiming at characterizing the Central-SVZ magmatic arc and the magmatic storage depths. How to cite: Bechon, T., Vander Auwera, J., Namur, O., Fugmann, P., Bolle, O., and Lara, L.: What is the magma storage depth under Osorno Volcano (Southern Volcanic Zone, Chile)?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4848, https://doi.org/10.5194/egusphere-egu2020-4848, 202

Conduit processes during the February 11, 2010 Vulcanian eruption of Soufrière Hills, Montserrat

International audienceWe analyzed pumice from the February 11, 2010 Vulcanian explosion that immediately followed a large dome collapse at Soufrière Hills volcano. We obtained pre-explosive values of porosity, pressure, and depth by combining textural analyses and glass water content determinations. Our data suggest that the February 2010 explosion evacuated the upper 3 km of the conduit from the dense magma (≤10 vol% porosity) it contained. The low porosity distribution in the volcanic conduit implies that the magma rising from the reservoir had time to extensively degas during emplacement. We use a conduit flow model to characterize the effects of permeability on ascent conditions. Model input parameters were fitted so as to match our pre-explosive porosity data, which yielded first-order constraints on conduit radius, mass flux, outgassing efficiency, and permeability. This parametric study suggests that efficient lateral gas escape is necessary to explain the low pre-explosive porosities. Steady-state solutions fitting the observed range of dome extrusion rate in the month preceding the February 11 event suggest permeabilities < 10−13 m2 deeper than 500 m, which are values typical of crack-supported permeability. Conversely, solutions with parameters consistent with bubble-supported permeability imply transient flow conditions prior to the February 11 event. The transient conditions imply that our data represent a snapshot of the porosity distribution within the conduit that does not preclude the temporary presence of much higher porosities in the conduit. Such unsteady conduit flow conditions are consistent with the irregular but active dome growth in the month prior to the February 11 event

Insights on mantle melting below Osorno Volcano (Southern Volcanic Zone, Chile)

Knowing the extent of mantle wedge melting below volcanic arcs is critical to improve magma genesis models. During the last decades, experimental petrology provided significant data and ready to be used models to retrieve the melting conditions producing primary magmas (Mg# >0.7, Ni > 150ppm, Cr > 1000 ppm after Baker et al. (1994) and Grove et al. (2012)). The Central Southern Volcanic zone (CSVZ, southern Chile) of the Andean arc lies on a thin continental crust (50-30 km : Tassara and Echaurren, 2012) and is crosscut by a major transcrustal fault (LOFZ : Cembrano and Lara, 2009) which speed up magma ascent. As a consequence, (near-) primary magmas have been sampled in the area avoiding the blurring of its original characteristics by deep (MASH) (Hildreth and Moorbath, 1988) to shallower (differentiation) magmatic processes. Osorno is one of the CSVZ volcano that possess the most primitive recorded rocks (Mg# =0.72, MgO: 10.23-10.53 wt%, Cr: 584-745 ppm, Ni: 171-179 ppm, Fo# of olivines up to Fo89) in the area. Using lherzolite melting experiments (Hirose and Kushiro, 1993) as well as numerical (Lee et al., 2009), empirical (Wood, 2004) and chemical models (Kelley et al., 2006) together with modal batch melting equations (Hickey-Vargas et al., 2016a, 2016b), we retrieved temperature, pressure, mantle water content and mantle melting rate (F) below Osorno. Temperatures range from 1303 to 1327 °C, pressures from 10.5 to 13.6 kbar (around MOHO depth, ca. 35-44 km), mantle water content from 0.08 to 0.33 wt% and F from 0.12 to 0.22. The uncertainty on F values reflects the difficulty to precisely estimate this parameter. However, this range allows better constraining geophysical models. Values estimated using a global arc numerical model (Turner et al., 2016; F ≈ 0.11 at ca. 60 km and T≈1100°C, mantle H2O =0.6 wt%) or using Arc basalt simulator, isotopes and trace elements (Jacques et al., 2014; F = 1.6-5.5, P=19 kbar, T = 1240°C) vary significantly from our data emphasizing the remaining gap of knowledge of the mantle melting conditions. Our results nonetheless agree with those calculated at La Picada (Vander Auwera et al., 2019) or southwards (Watt et al., 2013; Weller and Stern, 2018) in the southern SVZ