Modeling magma recharge dynamics during the 2016 Nevados de Chillan eruption: An interacting two-chamber system evidenced by petrology and geodesy

Abstract

The Nevados de Chillan is a large composite stratovolcanic complex located in the Southern Andean Volcanic Zone, at the south of Chile. Its last eruption spanned six years from January 8, 2016 until January 2023. After three years without any deformation, exhibiting phreatic and phreatomagmatic activity, an uplift episode was reported in June 2019, marking the start of its magmatic phase. How this geodetic activity evolved in the following three years is still not clear, neither how it correlates with its superficial activity. Here we analyze InSAR time series, as well as daily GNSS time series from 2015 to early 2022, highlighting the wide range of ground surface displacements observed during the eruption. In-depth analysis of these displacements together with recent petrological and geochemical results leads us to consider a recharge mechanism involving a double-reservoir model to explain the observed geodetic activity. We develop an analytical model of dynamic magma flow, which we coupled with a boundary element method to account for any geometry of the reservoirs and the topography of the volcano. The model consists of a shallow elongated source located at 5.8 km depth below the volcanic edifice connected by an incompressible magma-filled hydraulic pipe to a deeper sill like source centered at 15 km depth. We propose that the activation of the system started with a small magma intrusion of one month of duration, which was sufficient to overheat the hydrothermal system and re-mobilize magma in the shallow chamber, thus explaining the non-deformation during its phreatic phase and the slight subsidence observed during its phreato-magmatic stage. Then, we propose that a new and larger magma intrusion occurred in June 2019, explaining the uplift episode observed, which continued for the following three years of the eruption, decaying exponentially. Our model indicates that this intrusion was triggered by magma coming from the crust-mantle boundary to the deep reservoir at constant rate of 0.016 km³y⁻¹ from June 2019 to Jan. 2022, with small changes to this rate that would explain the small fluctuations observed during this uplift episode. We show that the conduit-dominated magma transfer between both reservoirs controls the dynamics of the system. A deep mafic reservoir recharging an evolved shallow reservoir would explain the mafic enclaves found in the dacites in the latter eruption and thus offers a physical model to jointly explain the observations we get from petrology, geochemistry and geophysics, bridging the disciplines. The model presented here can be used to study potential recharge mechanism occurring in the different stages of an eruptive cycle including pre-eruptive, co-eruptive and post-eruptive stages.