[A-HW23] 同位体水文学2021

[AHW23-08] Origin and hydro-chemical evolution of geothermal waters from Nozawa-Onsen hydrothermal springs in the northern Nagano Prefecture, central Japan

*赤澤 舜介¹、Satish-Kumar Madhusoodhan²、渡部 直喜³、Ould-Elemine Cheibany⁴、柚原 雅樹⁵ (1.新潟大学大学院自然科学研究科、2.新潟大学理学部地質科学科、3.新潟大学災害・復興科学研究所、4.Ensero Solutions、5.福岡大学理学部地球圏科学科)

キーワード：野沢温泉、地熱水、同位体組成、水文地球化学、断層

Nozawa-Onsen hydrothermal springs are located between the northern segment in the active fault zone along the western margin of the Nagano basin and the Quaternary Kenashi Volcano that was active during the period from 1.7 to 1.0 Ma at approximately 40 km northeast of Nagano City, central Japan. Geothermal waters have temperatures ranging from 85.4 to 26.4 degree centigrade, pH values from 8.1 to 9.2 and electric conductivity from 47.6 to 149.0 mS/m, which is correlated to the total ion concentration. They are categorized as sodium-sulfate type, even though respective waters have different temperatures and concentrations. In this study we have carried out hydro-chemical and isotopic analyses to better understand the origin and evolution of geothermal waters. The isotopic results of delta O-18 and delta D values reveals that these waters are meteoric in origin. In addition, delta S-34 values of sulfate indicate that volcanic fluids did not contribute to the formation of geothermal waters. Relationship between chloride concentration and water temperature suggest that several waters were diluted with low-temperature groundwater. Variations of sulfate and hydrogen sulfide concentration in these waters can be explained by the precipitation of native sulfur. It is likely that the calcite was removed as precipitates from some geothermal waters during transportation through faults on the basis of the relationship between Sr-87/Sr-86 and Rb-87/Sr-86 ratios. In conclusion, the evolution process of geothermal waters from Nozawa-Onsen hydrothermal springs are closely related to (1) the recharge of meteoric waters to deep zone, (2) dissolution of soluble minerals such as plagioclase, gypsum and pyrite in Neogene sediments, (3) removal of calcite from these waters during transportation through faults, (4) dilution with low-temperature groundwaters around shallow aquifers and (5) precipitation of the small amount of native sulfur near the earth surface.