Geochemical characteristics of hyper-acid water at the 2018 crater of Iwoyama, Kirishima volcanoes

Abstract

Hydrothermal discharge is still continuing at the April 2018 crater at Ioyama volcano of the Kirishima Volcanic Group (Kyushu, Japan). Hyper-acidic Cl-type hydrothermal fluids with negative pH and a dissolved Cl concentration exceeding 100,000 mg/L were observed after May 2023. Such hydrothermal fluids in the craters have a flat slope of about 2.3 on the dD-d¹⁸O diagram, and some of them show heavier dD and d¹⁸O values than those of magmatic vapors. Therefore, these hyper-acidic hydrothermal fluids are considered to be enriched in D, ¹⁸O, or dissolved H⁺ and Cl^- by evaporation at the crater site.