Evolution of the magmatic-hydrothermal system at Lastarria volcano (Northern Chile) between 2006 and 2019: Insights from fluid geochemistry

Abstract

One of the major problems in the volcanic surveillance is how data from several techniques can be correlated and used to discriminate between possible precursors of volcanic eruptions and changes related to non-eruptive processes. Gas chemical surveys and measurements of SO2 emission rates performed in the past (2006–2019) at Lastarria volcano in Northern Chile have revealed a persistent increment of magmatic sourced gas emissions since late November 2012, following a 13 years period of intense ground uplift. In this work, we provide new insights into the gas-chemical evolution of Lastarria’s fumarolic discharges obtained from direct sampling (2006–2019) and SO2 emission rates using UV camera and DOAS instruments (2018–2019) and link these to pre-existing information on ground deformation (1998–2016) in order to determine the origin of observed degassing and ground deformation processes. We revise the four mechanisms originally proposed as alternatives by Lopez et al. (Geosphere, 2018, 14 (3), 983–1007) to explain the changes observed in the fluid geochemistry and ground deformation between 2009 and 2012, in order to explain major changes in gas-geochemistry over an extended period between 1998 and 2019. We hypothesize that a continuous sequence of processes explains the evolution in the fluid geochemistry of fumarolic discharges. Two mechanisms are responsible of the changes in the gas composition during the studied period, corresponding to a 1) deep magma chamber (7–15 km depth) pressurized by volatile exsolution (1998–2020), which is responsible of the large-scale deformation; followed by 2) a crystallization-induced degassing (2001–2020) and pressurization of the hydrothermal system (2003-early November 2012), where the former process induced the changes in the gas composition from hydrothermal-dominated to magmatic-dominated, whereas the last produced the small-scale deformation at Lastarria volcano. The changes in the gas composition since late November 2012, which were strongly dominated by magmatic volatiles, produced two consecutive processes: 1) acidification (late November 2012–2020) and 2) depletion (2019–2020) of the hydrothermal system. In this work we have shown that a long-term surveillance of the chemistry of fluid discharges provides valuable insights into underlying magmatic/volcanic processes, and consequently, for forecasting future eruptions.

Physical and chemical characteristics of active sulfur flows observed at Lastarria volcano (northern Chile) in January 2019

“…Gas discharges reach temperatures of up to 408 °C, emitting considerable amounts of acid magmatic species, such as SO 2 , HCl, and HF, in addition to hydrothermal-related species, such as H 2 S and CH 4 (Aguilera et al, 2012). Magmatic emissions most likely originate from at least two magma chambers located at depths of 3-6 and 7-15 km, respectively, whereas hydrothermal emissions correlate well with the presence of a hydrothermal reservoir at a depth <1 km below the summit (Froger et al, 2007;Aguilera et al, 2012;Spica et al, 2015;Robidoux et al, 2020;Layana et al, 2023). According to Aguilera et al (2012), variable scrubbing within the volcanic edifice explains temperature variations and fluctuating contributions of magmatic and hydrothermal compounds to fumarolic emissions.…”

“…Persistent and vigorous fumarolic activity indicates magmatic and hydrothermal fluids feeding surface emissions, placing Lastarria as one of the most important gas suppliers within the last decade of northern Chilean volcanoes, with typical SO 2 fluxes approximately Frontiers in Earth Science frontiersin.org 800 t/d (Tamburello et al, 2014;Layana et al, 2023). Gas discharges reach temperatures of up to 408 °C, emitting considerable amounts of acid magmatic species, such as SO 2 , HCl, and HF, in addition to hydrothermal-related species, such as H 2 S and CH 4 (Aguilera et al, 2012).…”

“…The hydrothermal system comprises a discontinuous hydrothermal aquifer fed with condensed steam and occasional meteoric water inputs. Since late 2012, the chemical composition of the discharged gases has evolved into a more magmatic signature (Tamburello et al, 2014;Lopez et al, 2018;Layana et al, 2023), likely owing to the acidification of the hydrothermal system triggered by a substantial input of volatiles from a pressurized and volatile-rich magma chamber (Layana et al, 2023).…”

“…Inspection of historical data on fumarolic gases discharged at the Lastarria volcano (Aguilera et al, 2012;Layana et al, 2023) permits a better understanding of the physicochemical processes behind the 2019-flows and the feasibility of reaction 3) to form solid/liquid sulfur. Gas species involved in reaction 3) had the following behavior in 2006 and 2022 (excluding 2019 samples; all concentrations expressed in mmol/mol): SO 2 = 5.9 ± 0.9; H 2 S = 2.5 ± 0.4; H 2 O = 876 ± 4.6; SO 2 /H 2 S = ~2.3 (Layana et al, 2023). However, fumarolic gases collected during the 2019-flows contained the following concentrations: SO 2 = 38 ± 2; H 2 S = 4.6 ± 0.6; H 2 O = 695 ± 5.2; SO 2 /H 2 S = ~8.4 (Table 3).…”

“…This phenomenon is important because molten sulfur has been observed prior to or during eruptive events, mainly of a phreatic nature, suggesting a potential correlation with volcanic unrest (e.g., González et al, 2015;Daga et al, 2017;Salvage et al, 2018;Mora-Amador et al, 2019). In January 2019, active pools and flows of molten sulfur were observed and described scientifically for the first time at the Lastarria volcano in northern Chile (Figure 1A), a volcano affected by ground deformation (e.g., Pritchard and Simonds, 2002;Henderson et al, 2017) and changes in the chemical composition of volcanic gases (López et al, 2018;Layana et al, 2023). This phenomenon offers a unique opportunity to investigate the rheological, mineral, and chemical properties of these materials.…”