Application of Rare-Earth Elements in Spring Waters to Indicate Surficial Water-Rock Interaction Process in the Wayang Windu Geothermal Field, Indonesia

Abstract

Although water-rock interaction (WRI) process is important to characterize geothermal fluid compositions, this process is complex to be clarified by traditional hydro-geochemical analyses. To approach this problem, we applied a rare earth element (REE) analysis as WRI indicator. In total twenty samples of surface waters were collected from the Wayang Windu geothermal field, West Java, Indonesia. The samples were separated to hot and spring waters by the temperature (16–63 °C), pH (2.8–8.4), and chemical composition. The hot spring waters were classified into three types: steam-heating product (SO42-), sulfate-bicarbonate, and bicarbonate water (HCO3-), meanwhile all the cold spring waters were sodium-bicarbonate type. REE concentrations range from 1.2×10-3 to approximately 6.9×10-3 times the chondrite reference. A REE speciation model indicates that Ln3+ occupies 60% of the total dissolved REE concentrations resulting from enrichment in sulfate (LnSO4+) and bicarbonate sulfate (LnCO3+) in both the hot and cold spring waters. The results revealed the LnSO4+ and LnCO3+ complexes associated with REE mineralization and oxidation of sulfides and carbonates during weathering process. Additionally, atypical HREE-enriched pattern against LREE-enriched patterns is probably caused by the preference of LREEs absorbance to the surfaces of rock particles, whereas HREEs are preferentially retained in the solutions. Consequently, weathering and absorption are the most plausible influencing factors of the REE behaviors in geothermal waters, and the REE concentration can be used to estimate degree of advance in WRI process.