Investigating hydrothermal alteration in Copahue volcano (Argentina/Chile) using muography and laboratory measurements on rock samples

Abstract

Muography is a non-invasive geophysical method that relies on the detection of muons, which are subatomic particles generated by the interaction of cosmic rays with the Earth's atmosphere. The physical quantity estimated by this method is the opacity, which represents the amount of matter traversed by muons along their trajectories, resulting in energy loss and scattering for the particles. Thus, absorption muography consists of deploying a muon detector targeting the volcano and registering the muons traversing it per unit of time and trajectory. From these data, radiographs of average density of extensive rock volumes can be obtained using a single measuring instrument and from a singular measurement position. Copahue volcano is located in the Andes mountain range and is considered the highest-risk volcano in Argentina due to its proximity to two towns situated within an 8 km radius of the volcano's crater. Additionally, the region attracts a significant number of tourists, leading to a substantial increase in the population of both localities. The latest eruptive cycle, initiated in 2012, has maintained a near-continuous state of activity, marked by ash emissions, crater explosions, and seismic activity. In this work, we study the hydrothermal alteration at Copahue volcano through a combination of muography and laboratory measurements of the chemical and physical properties of rock samples. The muography dataset was acquired by installing a muon detector on the eastern flank of Copahue volcano, situated at an altitude of approximately 2500 meters above sea level. For the laboratory analyses, we collected rock blocks with the objective of representing a diverse spectrum of alteration stages within Copahue volcano. Through this selection process, we captured variations in mineralogical composition, geochemical signatures, and physical properties that correspond to different stages of hydrothermal alteration. We carried out a series of examinations on the rock samples extracted from the targeted flank, such as X-ray diffraction (XRD) and inductively coupled plasma mass spectrometry (ICP-MS) analyses that identified mineralogical compositions and geochemical signatures associated with hydrothermal processes. We also carried out additional measurements, including density, porosity, permeability, thermal properties, and uniaxial compressive strength, contributing to a comprehensive understanding of the physical properties of the samples. In addition, we performed microscopic examinations using a scanning electron microscope (SEM) to study the microstructural changes induced by hydrothermal alteration. This integrative approach, between muography and detailed laboratory measurements on rock samples, aims to reveal correlations between subsurface density variations and hydrothermal alteration observed at the microscopic and macroscopic scales.