Chemical Geothermometry Studies on a Geothermal System in Manza Hot Springs Near Kusatsu-Shirane Volcano, Japan

Changes in temperatures in shallow geothermal systems beneath the Manza area in the Kusatsu-Shirane volcano region, Japan with time, were reconstructed by using chemical geothermometers derived from long-term data on the water chemistry of three typical hot springs in the area from almost the last fifty years. The calculated equilibrium temperatures for quartz, anhydrite and alunite were helpful in understanding the state of the hydrothermal systems in the area. Although the measured water temperatures of the hot springs at their vents have decreased gradually since the 1970s, the temperatures of the geothermal systems in the area do not seem to be in a downward trend based on the geothermometry. The temperatures of the geothermal systems have probably fluctuated in response to changes in the volcanic activity of the Kusatsu-Shirane volcano. The changes in the estimated equilibrium temperatures for appropriate minerals give us helpful information on the volcanic activity that we can-not obtain directly, by monitoring of water chemistry of hot springs in the volcanic area.