Temporal changes in the chemistry of lake water within Yugama Crater, Kusatsu-Shirane Volcano, Japan: Implications for the evolution of the magmatic hydrothermal system

• Published in: Journal of volcanology and geothermal research 2008-12, Vol.178 (2), p.131-144
• Main Authors: Ohba, Takeshi, Hirabayashi, Jun-ichi, Nogami, Kenji
• Format: Article
• Language: English
• Subjects: crater lake; Crystalline rocks; Earth sciences; Earth, ocean, space; Exact sciences and technology; Freshwater; hydrothermal system; Igneous and metamorphic rocks petrology, volcanic processes, magmas; rock–water interaction; sealing zone
• ISSN: 0377-0273; 1872-6097
• DOI: 10.1016/j.jvolgeores.2008.06.015

The chemical composition and temperature of lake water within Yugama Crater, Kusatsu-Shirane Volcano, Japan, have fluctuated markedly over the period 1966 to 2005. From 1966 to 1981, water temperature was relatively low, and Cl − and SO 4 2− concentrations gradually decreased. These trends are explained by the growth of a sealing zone that formed between the region of hot rock surrounding the degassing magma and cool crust saturated with groundwater. The sealing zone acted to restrict the emission of magmatic volatiles. In 1982 and 1983, several steam-driven explosive eruptions occurred within Yugama Crater. Following the eruptions, lake temperature increased and SO 4 2− and cation concentrations increased markedly, although no increase in Cl − was recorded. These changes are attributed to a breach of the sealing zone. The increased interaction of groundwater with the hot rock region surrounding the magma chamber, including the sealing zone, produced a cation- and sulfate-rich fluid that was transported to Yugama Lake. A large number of earthquakes occurred in the region of the volcano in 1990. At this time, Cl − concentrations in lake water increased rapidly, while water pH dropped. These changes are explained by the invasion of groundwater through the breached sealing zone. The invading water interacted with the inner region of hot rock close to the molten magma, and Cl stored in the hot rock was leached to the water as HCl. The HCl-enriched water was then transported to Yugama Lake, resulting in the observed increase in Cl − concentration and decrease in pH. The sequence of hot rock–water interaction followed by the extraction of HCl from hot rock recurred in 1996 to 2005, although the degree of the interaction was much smaller than the former sequence in 1982 to 1995, resulting in no occurrence of significant eruptions.