[A-HW24] 同位体水文学2024

[AHW24-04] Geochemical Constraints and Evolution of Geothermal Fluids in the Kiejo Mbaka Volcanic-Graben Geothermal Prospect, Southwest Tanzania

*NYORA DONALD KOBARE^1,2、Koki Kashiwaya¹、Katsuaki Koike¹ (1.Department of Urban Management, Graduate School of Engineering, Kyoto University, Katsura、2.Department of Geosciences, SoMG, University of Dar es Salaam, Tanzania)

キーワード：Fluid evolution, Geothermometry, Recharge area, Circulation depth, Kiejo-Mbaka prospect

The Kiejo-Mbaka Geothermal Prospect (KGS) is situated within the East African Rift System (EARS) at the triple junction of half-graben basins (Usangu, Karonga, and Songwe) in southwestern Tanzania. This study aims to glean insights into the evolution and constraints of the KGS through an analysis of the geochemistry of geothermal fluids. A total of 21 thermal spring samples and 10 gas vent samples were collected from the KGS. The hydrochemical type of geothermal water is identified as alkaline Na-K-HCO3, resulting from the interaction between meteoric waters and basement rocks (primarily gneisses), sustained by the conversion of CO2 to HCO3. The major cations observed are Na (1,180 ± 27 mg/L) and K (64 ± 2 mg/L), while the Ca reservoir concentration is 2.1 mg/L, assuming calcite saturation and the Mg reservoir content is 2.3 mg/L, based on the hypothesis of equality between silica and K-Mg temperature. The geothermal reservoir temperature within the fractured reservoir rock is estimated to be between 113–137 oC, in acceptable agreement with the saturation indices vs. temperature plot. Chalcedony and Quartz exhibit equilibrations and spontaneously respond to temperature variations. In the ascending process, contrasted Na-K geothermometers indicate that the thermal springs re-equilibrate slowly upon cooling at approximately 250-260 oC. The computed geoindicators of the KGS manifestations reveal two distinct upflow zones, suggesting outflows towards the SW and SE directions. Additionally, δ13C values of CO2, ranging from -5.5 to -6.0 ‰, reveal a significant supply of CO2 from deep sources. The stable isotope δ2H and δ18O composition of geothermal water imply infiltration of meteoric water from the surrounding mountains at an average elevation of 1,957 ± 23 masl, circulating to an average depth of 2400 b.s.l.