Petrology of the magmatic system beneath Osorno volcano (Central Southern Volcanic Zone, Chile)

Abstract

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 of the Chilean arc, Osorno ranks sixth in the official risk classification 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% SiO 2 ) to rhyodacite (Mg# = 0.18 and ≥ 70 wt% SiO2), displaying a compositional gap within andesite (59–63 wt% SiO 2 ). 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 and 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 and 3 kbar (6–10 km) and results from a water-poor (≈1 wt% H 2 O), 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. • Basalt evolves to dacite at ≈2 kbar near the depth of the intracrustal discontinuity. • Fractional crystallization dominates under water undersaturated conditions. • Petrographic observations and numerical models were integrated with geophysical data.