APPLICATION OF RARE-EARTH ELEMENTS IN SPRING WATERS TO INDICATE SURFICIAL WATER-ROCK INTERACTION PROCESS IN THE WAYANG WINDU GEOTHERMAL FIELD, INDONESIA
Although water-rock interaction (WRI) process is important to characterize the nature of fluid composition, this process is very complex to be clarified by customary water geochemistry analyses. To approach this problem, examining behavior of rare earth elements (REEs) application as WRI indicator is worth to examine. Total eight samples of surface waters located within the conductive zones were collected from Wayang Windu geothermal field, West Java, Indonesia. The samples were separated to hot and spring waters by the temperature (16–63 °C), pH (1.3–8.4), and chemical composition. The hot spring waters were classified into two types: steam-heating product (SO42-) and sulfate-bicarbonate, and all the cold spring waters were bicarbonate types. REEs concentrations range from 6.3×10-7 to approximately 2 times chondrite. A REEs speciation model indicates that LnSO4+occupies up to 70% of the total dissolved REEs concentrations resulting enrichment in sulfate in hot springs and bicarbonate LnCO3+ (up to 80%) including lanthanides free ions Ln3+ (up to 70%) in cold spring waters. Additionally, atypical HREE-enriched pattern compared to LREE-enriched patterns is caused by the preference of LREEs absorbance to the surfaces of particles, whereas HREEs are preferentially retained in solution. Consequently, LnSO4+ and LnCO3+ are proven as REEs mineralization and oxidation of sulfides and carbonates complexes during weathering process in surficial features.