A decade (2013−2023) of direct sampling from high-temperature fumaroles at Avacha Volcano, Kamchatka: Gas geochemistry, seasonal and long-term variations

The 1991 eruption of Avacha volcano resulted in a lava plug inside its crater, making high-temperature fumaroles available for sampling. At present, there are two high-temperature fumarolic fields: the Eastern (up to 665 °C) and the Western (up to 840 °C), both associated with a fissure in the lava plug caused by a weak 2001 explosion. The paper presents chemical and isotopic compositions (H-O-C-S) of the directly sampled fumaroles over the period 2013–2023, mainly from the Eastern field. We revealed seasonal variations of water isotopic composition and concentrations of some components of the gas. High-temperature gases from Avacha volcano are characterized by chemical and isotopic compositions typical for volcanoes in subduction zones, but with a slightly increased content of H₂O, a reduced content of HCl. A relatively high concentration of methane is noted in the gases of low-temperature field. Methane in high-temperature gas with δ¹³C(CH₄) = −16.8 ‰ has abiogenic origin. For high-temperature gases, their redox state (H₂/H₂O and CO/CO₂) is controlled mainly by the sulfur gas buffer (H₂S/SO₂); methane is not chemically equilibrated. The molar ratio C/S ∼ 1 is typical for volcanoes in the Kuril-Kamchatka Arc. The measured fumarolic temperatures at the Eastern field are descending over time from 626 °C in 2013 to 410 °C in 2023. The apparent equilibrium temperatures calculated for reactions that include CO, CO₂, H₂, H₂O, H₂S and SO₂ are generally higher than the measured temperatures and do not show the descending trend. However, calculated equilibrium temperatures for the H₂O-CO-CO₂-CH₄ system are very close to the measured temperatures. Two periods of the increased seismic activity which occurred from 2013 to 2023, in November 2014–January 2015 and October–December 2019, correlated with changes in the morphology and gas flow rates at the Western fumarolic field.