Strontium Isotopes and Rare Earth Elements as Tracers of Water–Rock Interactions in Taiwan Hot Springs

This study investigates water–rock interactions of Taiwan hot springs by analyzing rare earth elements (REEs) concentrations and strontium (Sr) isotopes. REEs were separated from samples using RE resin, and their concentrations were measured by HR-ICPMS. Strontium was isolated using SrSPEC resin, and the strontium isotopic ratio was determined by MC-ICPMS. The ΣREE in the hot springs ranges from 3.17 ng/L to 29.7 µg/L, with the highest levels found in the Tatun Volcano Group, followed by springs from sedimentary and metamorphic regions. The primary factors controlling REE compositions are lithology and pH. REE patterns of hot springs can be categorized into five types, indicating that the hot springs were affected by various mechanisms. The most distinct hot spring samples are from Tatun Volcano, Ginshan, and Kuantzuling. The 87Sr/86Sr ratios range from 0.70468 to 0.71730, with the most radiogenic samples originating from metamorphic regions, reflecting the nature of the parent rock interacting with the hot spring water. Seawater intrusion and preferential weathering of carbonate also have minor effects on Sr isotope composition. The findings indicate that the types of surrounding rocks and the pH values of the hot springs significantly influence REE patterns and Sr isotope compositions in Taiwan’s hot springs.