Lithospheric water contribution in geothermal systems of northern Sumatra: Insights from OLWC-based isotope reconstruction and Cl–B–Li geochemistry

The origin and evolution of fluids in subduction zones are still poorly understood, largely because of the complex interactions between deep lithospheric fluids and meteoric water in the shallow geothermal systems. Therefore, this study aimed to estimate the contribution of lithospheric fluids to geothermal systems in northern Sumatra, Indonesia, using the Ocean-Origin Lithospheric Water Curve (OLWC). The wide range of stable isotope values (δ²H and δ¹⁸O), alongside the systematic deviation from the meteoric domain towards the OLWC and supported by hydrochemical tracers (Cl, B and Li), showed significant compositional variability. The observed nonlinear trend suggests that the lithospheric-fluid component was a product of progressive water–rock interaction, rather than mixing with seawater or magmatic fluids. Furthermore, OLWC-based reconstruction enabled the estimation of potential lithospheric endmembers and the determination of semi-quantitative lithospheric water fractions ( f LW ). The results showed no systematic increase in calculated f LW was with elevating chloride concentration. Furthermore, some high-chloride waters had only a low to moderate f LW . Increased salinity was not directly associated with greater contribution of lithospheric water and cannot be considered a reliable proxy for deep fluid input. Consistent with this interpretation, B and Li geochemistry showed that fluid compositions were strongly influenced by post-source processes, including water–rock interaction, vapor separation, and near-surface modification, leading to systematic decoupling between isotopic and chemical signatures. These results suggest that geothermal fluids are dynamically evolving fluid systems rather than simple mixture of fixed endmembers. OLWC-based isotope reconstruction, integrated with Cl–B–Li geochemistry, provides an effective method for evaluating lithospheric-fluid evolution and post-source modification in subduction-related geothermal systems.