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Geochemical Journal
Geochemical Journal An open access journal for geochemistry
Published for geochemistry community from Geochemical Society of Japan.

Combined tracers in hot spring waters across the Kii Peninsula, Japan: Implications for the origins of metamorphic fluids of the SW Japan forearc

Koki Momose, Makoto Shiogama, Masako Hori, Kazuhiko Kandori, Yoshiro Nishio
Geochemical Journal, Vol. 55, No. 5, P. 289-300, 2021

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.

KEYWORDS

hot spring, oxygen isotopes, REE pattern, piper diagram, Cl-B-Li diagram

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