Response of a hydrothermal system to escalating phreatic unrest: the case of Turrialba and Irazú in Costa Rica (2007–2012)

Abstract

This study presents the first hydrogeochemical model of the hydrothermal systems of Turrialba and Irazú volcanoes in central Costa Rica, manifested as thermal springs, summit crater lakes, and fumarolic degassing at both volcanoes. Our period of observations (2007–2012) coincides with the pre- and early syn-phreatic eruption stages of Turrialba volcano that resumed volcanic unrest since 2004, after almost 140 years of quiescence. Peculiarly, the generally stable Irazú crater lake dropped its level during this reawakening of Turrialba. The isotopic composition of all the discharged fluids reveals their Caribbean meteoric origin. Four groups of thermal springs drain the northern flanks of Turrialba and Irazú volcanoes into two main rivers. Río Sucio (i.e. “dirty river”) is a major rock remover on the North flank of Irazú, mainly fed by the San Cayetano spring group. Instead, one group of thermal springs discharges towards the south of Irazú. All thermal spring waters are of SO4-type (i.e. steam-heated waters), none of the springs has, however, a common hydrothermal end-member. A water mass budget for thermal springs results in an estimated total output flux of 187 ± 37 L/s, with 100 ± 20 L/s accounted for by the San Cayetano springs. Thermal energy release is estimated at 110 ± 22 MW (83.9 ± 16.8 MW by San Cayetano), whereas the total rock mass removal rate by chemical leaching is ~ 3000 m3/year (~ 2400 m3/year by San Cayetano-Río Sucio). Despite Irazú being the currently less active volcano, it is a highly efficient rock remover, which, on the long term can have effects on the stability of the volcanic edifice with potentially hazardous consequences (e.g. flank collapse, landslides, phreatic eruptions). Moreover, the vapor output flux from the Turrialba fumaroles after the onset of phreatic eruptions on 5 January 2010 showed an increase of at least ~ 260 L/s above pre-eruptive background fumarolic vapor fluxes. This extra vapor loss implies that the drying of the summit hydrothermal system of Turrialba could tap deeper than previously thought, and could explain the coincidental disappearance of Irazú’s crater lake in April 2010.

Sulfur origin and flux variations in fumarolic fluids of Vulcano Island, Italy

“…The total geothermal potential estimated at Vulcano (179 MW) has to be compared with other geothermal systems, as shown in Figure 8. As we see, the Tacaná hydrothermal system, Mexico-Guatemala, shows a geothermal potential of approximately 98 MW (Collard et al, 2014;; El Chichón, Mexico, near 171 MW (Taran and Peiffer, 2009); the Flegrean Fields, Italy, approximately 100 MW (Chiodini et al, 2010); Puracè, Colombia, approximately 360 MW; and Irazú-Turrialba, Costa Rica, near 110 MW (Rouwet et al, 2021), while for Vulcano it is approximately 50 MW (Inguaggiato et al, 2012a). Domuyo volcano, Argentina, shows a geothermal potential of 1,000 MW (Chiodini et al, 2014), whereas the Yellowstone hydrothermal system shows a geothermal potential of 5,300-6,100 MW (Fournier, 1989;Ingebritsen et al, 2001), which are both about one to two orders of magnitude higher than the thermal output of Vulcano Island.…”

“…The role of elemental sulfur and sulfate minerals (alunite, jarosite, anhydrite, and gypsum) as sealers in volcanichydrothermal systems has been a key focus during the quest for precursory signals for phreatic eruptions (Takano et al, 1994;Christenson et al, 2010, Christenson et al, 2017Montanaro et al, 2017, Montanaro et al, 2021, Montanaro et al, 2023Rouwet et al, 2017;Rouwet, 2021;Pappaterra et al, 2022). Variations in gas chemistry often turned out to be indicative of the source and mechanism of phreatic eruptions at several volcanoes.…”

Phreatic and Hydrothermal Eruptions: From Overlooked to Looking Over

“…Poás, Kawah Ijen, Ruapehu, Kusatsu-Shirane), they can act as traps for high temperature gases, allowing the formation of molten elemental sulphur (> 114 °C) within the aquifer (Christenson et al 2017;Yamamoto et al 2017). Because sulphur viscosity drastically increases above ~ 160 °C, pore sealing and consequent permeability loss can occur when temperature rises above this threshold value (Christenson et al 2010;Sutawidjaja et al 2013;Manville et al 2015;Scolamacchia and Cronin 2016;Rouwet et al 2021).…”

“…Injection of magmatic fluids prior to phreatic and hydrothermal eruptions has been detected by geophysical, geochemical and petrological investigations (Table 1). Cyclical pressurisation and fracturing enhance circulation of hot fluids within shallow portions of the hydrothermal systems and may indicate a greater instability and risk of eruptions (Christenson et al 2010;Rouwet et al 2014Rouwet et al , 2021Jolly et al 2018;Heap et al 2019;Kennedy et al 2020;Moretti et al 2020). Such cyclical unrest periods are observed at Whakaari/White Island where short explosive paroxysms alternate with periods of pressure-induced fracturing resulting in "failed eruptions" (Dempsey et al 2020).…”

Special issue “Understanding phreatic eruptions - recent observations of Kusatsu-Shirane volcano and equivalents -”

“…They could discriminate the tremor signals from the noise by the lower permutation entropies and higher degrees of polarizations even if the signal-to-noise level is low. Rouwet (2021) presented the first geochemical model of the Turrialba and the Irazú volcanoes in Costa Rica. The Turrialba volcano became active in 2004 after 140 years of dormancy.…”