H–O–B–Li isotope systematics for assessing slab-derived fluids: Importance of postdehydration processes

In this study, the isotopic alteration of boron (B) and lithium (Li) in slab-derived fluids after they undergo dehydration processes is investigated using deep groundwater samples from the eastern Kii Peninsula of Japan. Isotopic and elemental analyses of hydrogen (H), oxygen (O), B, and Li reveal complex subsurface fluid mixing and geochemical interactions that exceed simple meteoric recharge. Three distinct groundwater groups were identified on the basis of δ 2 H–δ 18 O–Cl relationships: Group I has strong slab-derived water signatures, particularly along the Median Tectonic Line and near 34°50′N, with up to 60 % slab-derived contributions; Group II reflects mixing between meteoric water and ocean water; and Group III is predominantly meteoric. Spatial variations in δ 11 B and δ 7 Li isotopes suggest that local geochemical processes such as clay adsorption, silicate weathering, and igneous rock contributions significantly alter the original slab-derived isotopic signals. Rayleigh-type fractionation models explain the increased isotopic values in some wells, whereas low δ 11 B and δ 7 Li values elsewhere indicate localized sources. Notably, high Li values with low δ 7 Li values in the southwest indicate a Li supply from Miocene igneous intrusions beneath the Mesozoic accretionary complex. These findings highlight the need for an integrated multi-isotope approach to accurately trace fluid origins and reconstruct the geochemical pathways of slab-derived fluids in forearc groundwater systems. • H–O–B–Li isotopes trace slab-derived fluids in a forearc groundwater system. • Mixing analysis reveals up to ∼60 % slab-derived water along major fault zones. • B and Li isotopes record post-dehydration modification by rocks and clays. • Multi-isotope integration separates primary slab signals from secondary overprints.