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

Copyright © 2021 by The Geochemical Society of Japan. slab-derived fluid. However, non-meteoric δD-δ18O signatures in spring waters are not always coupled with volcanic-like He isotopic signatures. According to observations from across the Kii Peninsula, non-meteoric δDδ18O signatures are specifically distributed on the western side of the median tectonic line (MTL), despite the wide distribution of groundwater with high 3He/4He ratios (Umeda et al., 2006; Morikawa et al., 2016). Spatial variations of spring waters may be attributed to compositional variations of the original deep fluid (Iwamori et al., 2020). Moreover, non-meteoric δD-δ18O signatures can also be produced by clay mineral dehydration (Dählman and Lange, 2003). Because the δD-δ18O signatures of dehydrated water from clay minerals overlap the isotopic range of magmatic water, the signature could not be used for distinguishing the origins. In this study, we used Cl-B-Li systematics and a mixing model to distinguish the origins of the water from 16 hot springs distributed across the Kii Peninsula, Japan. Li and B are light elements that are characterized by high mobility and are similar to Cl, but with different degrees of incompatibility, depending on the associated mineral phases and temperatures. Therefore, the proportion of these elements has been used for understanding diagenetic and metamorphic processes (Trompetter et al., 1999; Marschall et al., 2009; Reyes and Trompetter, 2012). Reyes and Trompetter (2012) showed the different Cl-BCombined tracers in hot spring waters across the Kii Peninsula, Japan: Implications for the origins of metamorphic fluids of the SW Japan forearc