Ikuya Adachi, Rofiqul Umam, Tsutomu Yamanaka
Received 8 October, 2025
Accepted 17 December, 2025
Published online 18 April, 2026
Ikuya Adachi1), Rofiqul Umam2), Tsutomu Yamanaka2)
1) Degree Programs in Life and Earth Sciences, University of Tsukuba, Japan
2) Institute of Life and Environmental Sciences, University of Tsukuba, Japan
Hot spring water in central Japan contains lithospheric water components such as fossil seawater and slab-derived fluids. We measured boron isotopic compositions (δ11B) and concentrations for 35 springs and combined these results with a Rayleigh-type model of δ11B evolution during subduction that includes serpentinization and serpentinite breakdown, and secondary alteration. The boron concentrations for 19 springs in the mountainous region of central Japan ranged from 14.3–291 mg/L (26.6–557 mg/L for reconstructed lithospheric material), and δ11B ranged widely from –1.5 to +34.1‰. Lithospheric endmembers show a weak negative δ11B–δ18O trend and a positive relationship between B/Cl and δ18O. Modeling indicates that slab-fluid δ11B generally decreases with increasing horizontal distance from the trench. Serpentinization enriched 11B in the fluid, and serpentinite breakdown slightly reduced δ11B, with weaker effects on the Philippine Sea Plate and stronger effects on the Pacific Plate. In most cases, interpretations based on δ11B are consistent with δ2H–δ18O classifications and confirm previous studies. In several springs, interpretations based on δ11B differ from those based on δ2H–δ18O. δ11B, which was used in a complementary multi-tracer framework, confirms previous studies and provides additional perspectives. Interpretations should explicitly consider potential slab heterogeneity and secondary alteration.
Copyright (c) 2025 The Author(s) CC-BY 4.0
