Boron isotope evolution during burial diagenesis: integrating a Rayleigh model with hot spring observations

Hot spring waters are valuable geoscientific samples that provide insights into subsurface water circulation. Among the various geochemical tracers, boron isotopes are particularly effective for identifying deep-seated fluids that mix with hot spring waters, owing to their high mobility and large isotopic fractionation. However, the processes controlling variations in boron isotopic composition remain poorly understood. In this study, we investigate the evolution of boron concentrations and isotope ratios (δ11B) during early burial diagenesis, using fossil seawater-derived hot spring waters from the Niigata region of Japan. The samples have a wide range of δ11B values (+9.4‰ to +41.6‰), with a general trend of decreasing δ11B values with increasing boron concentrations. To explore the controlling mechanisms, we constructed a numerical model incorporating boron release from organic matter and minerals, and isotopic exchange via adsorption and desorption on mineral surfaces. The simulation results successfully reproduced the observed δ11B–1/B trends in hot spring waters and were consistent with published data from subseafloor porewaters and mud volcano fluids. In contrast, simpler Rayleigh-type models without surface exchange failed to reproduce the observed patterns, highlighting the importance of adsorption–desorption processes in shaping boron isotope evolution during early diagenesis.