Dissection of Osorno's magmatic system, a low H2O arc volcano - From mantle source to rhyodacitic products

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.