Combining Lithium Isotope Systematics, Stable Isotopes, and Major Elements as a Powerful Tool for Detection of the Slab-Dehydrated Fluids in Southwestern Japan

Abstract

This study aims to evaluate whether an integrated geochemical approach using lithium isotope systematics (δ⁷Li), Li and Cl concentrations, Cl/Li ratios, stable isotopes (δD and δ¹⁸O), and major-element compositions can provide useful constraints on the contribution of thermal water to the hydrothermal system. The study is based entirely on previously published geochemical data. Lithium isotope (δ⁷Li) data for four Arima samples reported by Umam et al. (2022) were re-evaluated together with Li and Cl concentration data reported by Umam et al. (2022) and Kusuda et al. (2014). Major-element concentrations and stable isotope (δD and δ¹⁸O) data for Arima hot spring waters reported by Kusuda et al. (2014) were also integrated into the analysis. The four Arima samples with available δ⁷Li data have δ⁷Li values < 5‰ and Cl/Li ratios < 1000 and are interpreted as representing thermal water. Although δ⁷Li data are not available for the Arima samples reported by Kusuda et al. (2014), 11 of these samples satisfy the proposed thermal-water criteria of Li concentrations > 0.1 mg/L, Cl concentrations > 1000 mg/L, and Cl/Li ratios < 1000. One sample is interpreted as representing mixing water and has a Cl concentration of 435 mg/L. These results suggest that integrating lithium isotope systematics with Li and Cl concentrations, Cl/Li ratios, stable isotopes, and major-element compositions may provide a useful geochemical framework for evaluating the characteristics and possible contribution of slab-dehydrated fluids to the hydrothermal system.