Uturuncu volcano in southern Bolivia is something of a “zombie” volcano – presumed dead, but showing signs of life. The volcano has not erupted in 250 kyr, but is exhibiting unrest in the form of ground deformation, seismicity, and active fumaroles. Elucidating the subsurface structure of the volcano is key for interpreting this recent unrest. Magnetotelluric measurements revealed alternating high and low resistivity anomalies at depths <10 km beneath the volcano, with a low-resistivity anomaly directly beneath Uturuncu. A key question is, what is the nature of this anomaly? To what extent is it partial melt, a hydrothermal brine reservoir, or a mature ore body? Knowing the density of this anomaly could distinguish between these scenarios, but existing density models of the area lack sufficient resolution. To address this issue, we collected additional gravity measurements on the Uturuncu edifice with 1.5 km spacing in November 2018. Gradient analysis and geophysical inversion of these data revealed several features: a 5 km diameter, high density anomaly beneath the summit of Uturuncu (1 – 3 km elev.), a 20 km diameter ring-shaped negative density anomaly around the volcano (-3 – 4 km elev.), a NNE trending, positive density anomaly northwest of the volcano (0 – 4 km elev.), and a NW trending, negative density anomaly to the southeast. These structures often (but not always) align with resistivity anomalies, features in new seismic tomography models, and relocated earthquake hypocenters. Based on a joint analysis of these data, we interpret the positive density anomaly as a crystallizing dacite pluton, and the negative density ring anomaly as a zone of hydrothermal alteration. Earthquakes around the edges of the crystallizing pluton may represent escaping fluids as the magma cools. The high density anomaly to the northwest likely represents a solidified pluton, and the low density anomaly to the southeast may represent a fractured fault zone. We posit that the alternating zones of high and low resistivity anomalies represent zones of low and high fluid/brine content, respectively. Based on this analysis we suggest that the unrest at Uturuncu is unlikely to be pre-eruptive. This study shows the value of joint analysis of multiple types of geophysical data in evaluating volcanic subsurface structure.

Uturuncu volcano in southern Bolivia is a member of a distinctive class of volcanoes – systems that show unrest despite not having erupted in the Holocene. Uturuncu has not erupted in 250 kyr, but has been deforming (uplift with a moat of subsidence) for several decades, along with seismic swarms and active, sulfur-encrusted fumaroles. Our work builds on previous geophysical imaging at Uturuncu by jointly analyzing multidisciplinary datasets, focusing on imaging the shallow (<15 km depth below surface) structure of the system with geophysical and geochemical data. Whereas previous research pointed to andesite melt at depths >15 km depth, results were ambiguous as to what proportions of melts vs. brines are present at shallower depths. Identifying fluids (melt, brine, etc.) and structures at shallow depths is key for evaluating the hazard potential of the volcano and understanding the source of the unrest. We present new results from gravimetry, seismology (hypocenter relocation, seismic velocity and attenuation tomography), gas geochemistry, and InSAR observations. The results point to an extensive and active hydrothermal system extending ~20 km laterally and ~10 km vertically from Uturuncu, with possible connections at depth to the deeper magmatic system. A combined view of the new density, seismic velocity and attenuation models, and the existing resistivity model is crucial for revealing key features of the hydrothermal system: a vapour-rich conduit beneath Uturuncu (low resistivity/high attenuation column extending from 1.5 to 12.5 km depth), an extensive alteration zone surrounding Uturuncu (complex zone of annular shaped anomalies surrounding Uturuncu from 1.5 to 12.5 km depth), and a possible zone of sulfide deposition just below the western flank of Uturuncu at 1.5 km depth (high density/low resistivity/high attenuation). High fluxes of diffuse CO2 degassing at sub-magmatic temperatures and a small area directly above a low resistivity anomaly subsiding from 2014 to 2017 show that the hydrothermal system is currently active. Analyzed jointly, this multidisciplinary data set suggests that current activity within the shallow structure at Uturuncu is dominated by hydrothermal, rather than magmatic processes.

Estimating the melt fraction and volatile content of regions of partial melt beneath volcanoes has important implications for volcanic hazards since higher melt fraction, volatile-rich magmas are more buoyant and have a lower viscosity, and thus are more susceptible to mobilization and possibly eruption. Magnetotelluric (MT) data can be used to model subsurface bulk resistivity structures through inversion algorithms and can provide information on the distribution and amount of melt and volatiles contained in the residing magma by converting bulk resistivity to estimates of melt fraction, temperature, and water content. These are often treated as independent variables but, in reality, they are thermodynamically correlated. Thermodynamic models such as MELTS can be used to constrain the possible combinations of melt fraction, temperature, and water content such that MT interpretations are petrologically consistent. Probabilistic Bayesian inversion that incorporates these constraints can be used to find a distribution of models and interpretations which fit the MT data and provide a better understanding of the uncertainty in MT-derived estimates of melt fraction. In this study, we apply MELTS-coupled 1-D Bayesian inversions of MT data at Uturuncu Volcano to evaluate the constraints that MT data can provide on melt fraction estimates. Uturuncu Volcano is a large composite volcano in southern Bolivia at the center of the Altiplano Puna Volcanic Complex (APVC), the result of a large ignimbrite flare-up during the past 10 Ma. Previous geophysical studies have shown that the APVC is underlain by the voluminous, laterally-extensive Altiplano Puna Magma Body (APMB) at approximately 15-20 km depth below surface. The APMB has previously been interpreted to have a wide range of melt fractions anywhere from 4% to 45%, but MT results suggest anomalously high water contents of up to 10 wt%. Initial results from petrologically-consistent MT inversion modelling suggests that the resulting low resistivity of the APMB beneath Uturuncu requires high melt fractions (e.g. >90%) in near-saturated conditions. This suggests that either high melt fraction near-saturated magma reservoirs exist at depth or that a significant phase of saline fluids in over-saturated low melt fraction conditions is present.