ABSTRACT

Spring waters from 16 sites distributed across the Kii Peninsula and surrounding islands in SW Japan were studied, to understand the origins of the metamorphic fluid; they were first classified into five types, based on a Piper diagram. Spring waters enriched in Na-K-HCO₃ (referred to as type IV) exhibited rare earth element patterns with strong positive Eu/Eu* anomalies, evidencing significant influence of plagioclase decomposition. Type IV samples were generally from inland sites, with relatively low Cl concentrations, while their total ion concentrations tended to increase as their Li/B ratios increased. Most of samples enriched in Na-K-Cl, distributed on the western side of the peninsula, were explained by the mixing of seawater with type IV groundwater. Some non-meteoric, δD-δ¹⁸O signatures detected in these samples may have been produced by clay mineral dehydration in the pelitic schist of the Sambagawa metamorphic zone, and not necessarily by slab-derived fluid. We also found one exceptional sample, which was enriched in Ca-Mg-Cl, and had a high Cl proportion without seawater mixing, which suggested the contribution of dehydrated fluid from Cl-enriched serpentinite.