Seawater hydrothermal system in the middle of the Kuril Arc: Yankich Island, Ushishir Archipelago

Abstract

Ushishir volcano is located in the middle of the Kuril Arc. The Ushishir crater, a closed bay connected with the ocean by a narrow and shallow strait, is characterized by a strong hydrothermal activity. Boiling springs, hot pools, fumaroles and shallow submarine vents are manifestations of a magmatic-seawater hydrothermal system that discharges a boiling solution that is generally similar in chemical and isotopic composition to the seafloor hydrothermal fluids at different tectonic settings. The main features of the Ushishir fluids are: (1) water has close to zero δD and a large oxygen isotopic shift (6 to 9 ‰); (2) 3He/4He = 7.3Ra, very low CH4 and N2/Ar > 200; (3) high boron concentration (∼70 ppm); (4) a significant uptake of Ca and Sr from the host rock and Ca/Sr ratio higher than that for seawater with 87Sr/86Sr 0.7037–0.7039, a bit higher than that of the rock value (0.7031). The measured onshore discharge of boiling water is ∼5 kg/s; however, a large plume of the discolored seawater is released from the outer submarine slope of the volcano indicating a much higher total mass and heat output. We compare the seafloor hydrothermal fluids with boiling water of the coastal Ushishir springs and estimate the effective water/rock ratio using our data on oxygen and Sr isotopes and some trace elements. We also discuss the correctness of the application of solute and gas geothermometers for both seafloor and coastal seawater hydrothermal systems.