Sulfur isotopic systematics during the October 2017 eruption of the Shinmoe-dake volcano, Japan

Abstract

Shinmoe-dake, a volcano, in Japan, began to erupt on October 11, 2017, after a 6-year period of quiescence. The pyroclastic materials ejected during October 11–14, 2017, contain sulfur-bearing minerals. Time-series sulfur isotopic compositions of sulfur species in the samples ejected during the 2017 eruption were measured in order to reveal the geochemical characteristics of the volcanic hydrothermal system that developed after the 2011 eruption and its evolution during the 2017 eruption. δ34S values for water-leached sulfate (+16.09‰ to +17.51‰), acetone-leached native sulfur (−1.23‰ to +0.42‰), fine-grained pyrite (−1.81‰ to −0.90‰), and coarse-grained pyrite (−3.48‰ to −2.57‰) remained relatively constant with time. The hydrochloric-acid-leached sulfates of the pyroclastic materials ejected from October 11 to October 12 exhibit a relatively limited range of δ34S (+9.87‰ to +13.17‰). Then, on October 14, the δ34S value suddenly increased to +16.85‰, which is similar to that of the water-leached sulfate from the same day (+16.09‰). δ34S values for all the sulfur species were probably controlled by the disproportionation of magmatic SO2 in the presence of water in the hot (about 400 °C) deep and cooler (about 300 °C) shallow acidic hydrothermal alteration zones below the summit crater. These zones existed separately before and during the first few days of eruption, but were ruptured in succession as the eruption progressed. The sulfur isotopic similarity between the hydrochloric-acid-leached sulfate and the water-leached sulfate of the pyroclastic material on October 14 implies that the mixing, dissolution, and re-equilibration of the recycled sulfate from both alteration zones occurred at about 300 °C after the second day of the eruption.