Isotopic Fractionation of Lithium and Boron in Geothermal Fluids of the East African Rift System: Tracing Mantle-Crust Interaction and Reservoir Temperatures

The East African Rift System (EARS) represents an active continental rift offering an unprecedented natural laboratory to investigate mantle-crust dynamics, fluid-rock interactions, and high-enthalpy geothermal reservoir processes. In this study, we present high-precision coupled lithium (δ⁷Li) and boron (δ¹¹B) isotopic analyses, combined with major and trace element geochemistry, for thermal fluids collected across major geothermal fields in the Main Ethiopian Rift (Aluto-Langano) and the Kenya Rift (Olkaria, Menengai, and Eburru). Measured δ⁷Li values in deep geothermal liquids range from +1.2‰ to +8.5‰, while δ¹¹B values vary between −6.4‰ and +2.1‰. The isotopically lightest signatures (δ⁷Li ≈ +1.2‰ to +3.0‰; δ¹¹B ≈ −6.4‰ to −2.5‰) occur in the hottest reservoir fluids (>300°C), closely matching the isotopic compositions of subcontinental lithospheric mantle and rift-related bimodal volcanic host rocks. Systematic enrichment of heavier isotopes (δ⁷Li and δ¹¹B) in lower-temperature peripheral springs reflects temperature-dependent fractionation during fluid ascent, driven by the preferential incorporation of ⁶Li and ¹⁰B into secondary clay minerals (e.g., smectite, illite, and chlorite). Quantitative modeling of coupled Li-B isotopic fractionation enables the calibration of a empirical dual-isotope geothermometer that yields subsurface reservoir temperature estimates (265–340°C) concordant with deep downhole measurements, outperforming conventional chemical geothermometers susceptible to re-equilibration. This work highlights the power of light stable isotopes in deciphering deep magmatic-hydrothermal inputs and refining reservoir thermal models in active rift settings.