Seawater hydrothermal system in the middle of the Kuril Arc: Yankich Island, Ushishir Archipelago
Introduction
The Kuril Island arc located between Kamchatka Peninsula and Hokkaido Island in the NW Pacific (Fig. 1a), is an ∼1200 km long chain that consists of about 30 islands, 17 of them possess one or more active volcanoes. Several islands host volcano-hydrothermal systems, most of them are acidic Cl-SO4 type (Taran and Kalacheva, 2019, Taran and Kalacheva, 2020). In the middle of the arc there is the Ushishir archipelago that consists of two islands, Yankich and Ryponkich. The Yankich Island is the upper 400 m of a volcano named Ushishir rising from ∼2000 m depth. The island is a small, closed bay with a maximal depth of 60 m, which is separated from the ocean by a shallow strait (Fig. 1, Fig. 2a) and is known to host an unusual hydrothermal system, more similar to seafloor magmatic-seawater hydrothermal systems. The system has been preliminary geochemically studied in the 1980s (Maltseva, 1984; Taran et al., 1993). Details about the bathymetry, temperature distribution and ecology of the bay can be found in several papers (e.g., Tarasov et al., 2008, and references therein). Here we report new data obtained during 2016, which include some trace elements, chemical and isotopic composition of fumarolic gases, (including 3He/4He) and isotopic composition of dissolved SO4 and Sr. Since the chemical composition of the Ushishir springs is similar to the fluid compositions of seafloor hydrothermal systems, we conduct a detailed comparison of these objects to show how and why a coastal seawater hydrothermal system discharging at the surface differs from the deep seafloor systems in terms of the water-rock interaction and conditions of the discharge.
Section snippets
General settings
General information about the volcanism of the Kuril Arc can be found in Gorshkov (1970) and Avdeiko et al. (1991).
The Ushishir crater bay is 1 km wide and is separated from the ocean by a strait, which is <1 m deep at low tide (Fig. 1, Fig. 2a). The maximum elevation of the crater rim is about 350 m asl. The exposed summit of the volcano is composed of andesitic rocks, and three small dacitic domes occur inside the crater (Fig. 1, Fig. 2a). The hydrothermal activity is concentrated at the SE
Methods
Steam vent and bubbling gas samples were collected using Titanium or plastic funnels, Giggenbach bottles and 10 ml vacutainers with septa stopcocks. Gas samples from Giggenbach bottles were analyzed following the methodology described by Giggenbach and Goguel (1989). Headspace gases from Giggenbach bottles and dry gas from vacutainers were analyzed by gas chromatograph techniques. The analytical error was ∼5%. Dry gas was collected for the analysis of carbon isotopes in CO2. He isotopes and
Results and discussion
Our data are shown in Table 1, Table 2. Both data sets – gas and water – are compared with representative analyses of samples collected in 1986 (Taran et al., 1993). The new data include isotopic composition of C, S, He, Sr, concentrations of Al and Fe and minor elements (Li, Rb, Cs, Ba and Sr).
Summary
The Ushishir geothermal system in the middle of the Kuril Island Arc brings to the surface water and a steam-gas mixture with all the geochemical and isotopic features of a high-temperature geothermal system, with a fluid formation temperature not lower than 300 °C. The fluid is the partially diluted altered seawater, which makes this system similar to the seafloor systems formed in the zones of ocean ridges or backarc spreading. A comparison of the Ushishir system and seafloor systems showed
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors thank E. Voloshina, T. Kotenko and the crew of the RV Afina for their assistance during the fieldworks. Andrea Rizzo (INGV-Palermo) is thanked for the He isotope analyses. We thank two anonymous reviewers for constructive comments. William Bandy is thanked for the polishing English. This work was supported by a grant from Russian Science Foundation # 20-17-00016.
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