Kalacheva, Elena

Many volcanoes at any tectonic settings host hydrothermal systems. Volcano-hydrothermal systems (VHS) are result of interaction of the upper part of plumbing systems of active volcanoes with crust, hydrosphere and atmosphere. They are heated by magma, fed by magmatic fluids and meteoric (sea) water, transport and re-distribute magmatic and crustal material. VHS are sensitive to the activity of a host volcano. VHS may have specific features depending on the regional and local tectonic, geologic and geographic settings. The studies reported in this volume help to illustrate the diversity of the approaches and investigations that are being conducting at different volcano-hydrothermal systems over theworld and the results ofwhichwill be of important value in furthering our understanding of the complex array of the processes accompanying hydrothermal activity of volcanoes. About 60 papers were submitted to a special session of “Volcano-Hydrothermal Systems” at the 2015 fall meeting of the American Geophysical Union. The papers in this special issue of the Journal of Volcanology and Geothermal Research were originally presented at that session.

Golovnin caldera on the southernmost Kuril Island arc Kunashir Island is characterized by intense hydrothermal activity and thermal manifestations of different types inside and outside the caldera. In this paper we report our results of the 2015 field campaign together with already published data and discuss unusual geochemical features of the whole system. Acid chloride sulfate waters discharging inside the caldera are different from hot sulfate chloride waters discharging along the coast of the Sea of Okhotsk. The difference is in the ratios of themain conservative components (Cl, B, Na) and a high fraction of a Ca-SO4 enriched component in the coastal springs. Another unusual feature of the system is the existence of boiling Na-Cl springs outside the caldera, between the caldera thermal fields with Cl-SO4 and SO4 acid waters and SO4-Cl acid-to-neutral springs along the coast. Fumarolic and bubbling gases fromthe caldera are characterized by low3He/4He values (~3.5Ra), isotopically heavy CO2 (δ13C N−2.6‰) and isotopically lightmethane (δ13C≤−40‰). This is a rare case when “chemical” (C-H-O) temperatures are higher than the “isotopic” (CO2-CH4) equilibriumtemperatures. Trace element hydrochemistry shows preferential congruent rock dissolution in ultra-acid steam-heated SO4 waters inside the caldera andmore complicated water-rock interaction for other types of waters. The REE patterns for chloride-sulfate and sulfatechloride waters normalized by average rock show depletion in LREE caused, most probably, by co-precipitation of LREE with mineral assemblages characteristic for argillic and advanced argillic alteration. The only source of chloride in the drainage fromthe Golovnin caldera is the Kipyaschee Lake (Cl-SO4 hot springs on the lake bottom and at its shore). Solute output fromthe Golovnin caldera is lower than that from the other studied volcano-hydrothermal systems of Kuril Islands (5.7 t/d of Cl and 7.3 t/d of SO4). Natural heat output by hot water and steam discharges is estimated as 63 ± 20 MW.

This paper reports a detailed geochemical study of thermal occurrences as observed in the edifice and on the flanks of Mendeleev Volcano, Kunashir Island in August and September 2015. We showed that three main types of thermal water are discharged there (neutral chloride sodium, acid chloride sulfate, and acid sulfate types); these waters exhibit a zonality that is typical of volcano-hydrothermal island arc systems. Spontaneous and solfataric gases have relatively low 3Не/4Не ratios, ranging between 5.4Ra and 5.6Ra, and δ13С-СО2 between –4.8‰ and –3.1‰, and contain a light isotope of carbon in methane (δ13С ≈ –40‰). Gas and isotope geothermometers yield relatively low temperatures around 200оС. The isotope compositions in all types of water are similar to that of local meteoric water. The distribution of microcomponents varies among different types. The isotope composition of dissolved Sr varies considerably, from 0.7034 as observed in Kunashir rocks on an average to 0.7052 in coastal springs, which may have resulted from admixtures of seawater. The total hydrothermal transport rates of magmatic Сl and SO4, as observed for Mendeleev Volcano, are 7.8 t/d and 11.6 t/d, respectively. The natural outward transport of heat by the volcano’s hydrothermal system is estimated as 21 MW.

Karymsky Volcanic Centre (KVC) at the middle of the frontal volcanic chain of the Kamchatka arc consists of two joined calderas (Akademii Nauk and Karymsky volcano) and hosts two hydrothermal systems: Akademii Nauk (AN) and Karymsky (K). The AN is a typical boiling system, with Na-Cl waters (TDS ~ 1 g/l), low gas content (CO2-N2), with deep calculated temperatures of ~200 °C. In contrast, springs of the K systemhave lower temperatures (up to 42 °C), strong gas bubbling, TDS ~2.5 g/l, and are enriched in HCO3 − and SO4 2−, with Mg2+ as the main cation. There are two intriguing characteristics of the K field: (i) their CO2-rich gas (N97 mol%) has the highest 3He/4He ratios ever measured for hydrothermal systems in Kamchatka of ~8 Ra (where Ra = 1.4 × 10−6) and (ii) their thermal waters have an unusual cation composition (Mg N Na N Ca). After the 1996 sublimnic eruption within AN caldera, new hot springs appeared close to the eruption site. In this paper we synthesize all published and new geochemical data sets. The Karymsky Lake and post-1996 new thermal springs demonstrate exponential decreases in their main dissolved species, with a characteristic time of 5 to 8 years. The chemistry of AN and K springs did not change after the eruption. However, the concentration of chloride in the lake water approached ~35 mg/l, compared with a background of 8–11 mg/l revealing a possible new source of hot water within the Karymsky Lake. All thermal fields of the KVC are drained by the Karymsky River with an outflow rate at the source of ~2 m3/s (flowing out from Karymsky Lake) and at the exit from the Karymsky caldera of ~4.5 m3/s. Using the measured solute fluxes at the source (AN springs) and at the exit (AN+K springs) the natural heat flux fromthe two systems can be estimated as ~67MWand ~120MW, respectively, and ≥20 t/d for the chloride output from both systems.

The CO2 flux provided by the hydrothermal activity within the Karymsky Volcanic Centre, Kamchatka, was measured, and the CO2 balance of the Karymsky caldera lake was estimated in the framework of a Deep Carbon Observatory (DCO) project. The Karymsky Volcanic Centre located in the SE of the Kamchatka Peninsula, in the middle of the modern volcanic front, consists of two calderas, hosts a caldera lake and is characterized by hydrothermal activity that is manifested at several thermal fields. Within the Akademii Nauk (AN) caldera which is filled by a caldera lake, the Akademii Nauk springs discharge boiling water into the lake. The lake is drained by the Karymsky River that then crosses the caldera of the Karymsky volcano (Karymsky caldera) and drains the thermal field of CO2-rich Karymsky springs. The lake after the 1996 sublimnic eruption is in a steady-state condition with the total dynamic CO2 budget of about 4 t/day, and has a total amount of CO2 stored inside of the lake of around 8000 t. The thermal springs of the Karymsky caldera drained by the Karymsky River enrich the river in dissolved carbon species. A total CO2 output of 320 t/day from both Karymsky Centre calderas was estimated, carrying around 130 t/day carbon species (expressed as CO2) as dissolved species (HCO3 and CO2(aq)), and emitting to the atmosphere around 190 t/day of CO2 as the diffusion flux fromthe soil and bubbling emanations from the springs.

The Kuril Island arc extending for about 1,200 km from Kamchatka Peninsula to Hokkaido Island is a typical active subduction zone with 40 historically active subaerial volcanoes, some of which are persistently degassing. Seven Kurilian volcanoes (Ebeko, Sinarka, Kuntomintar, Chirinkotan, Pallas, Berg, and Kudryavy) on six islands (Paramushir, Shiashkotan, Chirinkotan, Ketoy, Urup, and Iturup) emit into the atmosphere>90% of the total fumarolic gas of the arc. During the field campaigns in 2015–2017 direct sampling of fumaroles, MultiGas measurements of the fumarolic plumes and DOAS remote determinations of the SO2 flux were conducted on these volcanoes. Maximal temperatures of the fumaroles in 2015–2016 were 5108C (Ebeko), 4408C (Sinarka), 2608C (Kuntomintar), 7208C (Pallas), and 8208C (Kudryavy). The total SO2 flux (in metric tons per day) from fumarolic fields of the studied volcanoes was measured as 1,8006300 t/d, and the CO2 flux is estimated as 1,2506400 t/d. Geochemical characteristics of the sampled gases include dD and d18O of fumarolic condensates, d13C of CO2, d34S of the total sulfur, ratios 3He/4He and 40Ar/36Ar, concentrations of the major gas species, and trace elements in the volcanic gas condensates. The mole ratios C/S are generally <1. All volcanoes of the arc, except the southernmost Mendeleev and Golovnin volcanoes on Kunashir Island, emit gases with 3He/4He values of >7RA (where RA is the atmospheric 3He/4He). The highest 3He/4He ratios of 8.3RA were measured in fumaroles of the Pallas volcano (Ketoy Island) in the middle of the arc.