Geothermal systems on the island of Java, Indonesia

• Volcano-hosted and fault-hosted geothermal systems on the Java island, Indonesia
• HCO3 concentrations and Mg/Na ratios indicate tectonic setting
• Stable isotopes separate volcano-hosted from fault-hosted geothermal systems
• Magma heating of the fault-hosted geothermal systems
• Fault-hosted geothermal systems have a potential for geothermal energy

This paper presents an overview of all known geothermal systems on the island of Java by presenting physicochemical data for associated hot springs, cold springs and acid crater lakes. A total of 69 locations were sampled and classified based on their position in either a volcanic complex (volcano-hosted) or a fault zone (fault-hosted). In particular the potential of a magmatic heat source for fault-hosted geothermal systems was investigated. Volcano-hosted geothermal systems had higher HCO3− concentrations and higher Mg/Na ratios than fault-hosted geothermal systems. This geochemical difference is likely due to degassing and subsequent CO2-water reaction in the volcano-hosted systems, which is absent in the fault-hosted geothermal systems. The HCO3 vs. Cl and Mg/Na vs. SO4/Cl systematics indicated that fault-hosted geothermal systems located in the active Quaternary volcanic belt received shallow magmatic fluids, hence should be classified as volcano-hosted geothermal systems. The heat source of fault-hosted geothermal systems located in the old (Tertiary) volcanic belt was investigated by a combination of Li enrichment and calculated reservoir temperatures. There a shallow magmatic heat source was indicated only for the Cilayu and Cisolok geothermal systems. Thus, a deep seated magma was considered to be the heat source for the fault-hosted geothermal systems of Cikundul, Pakenjeng, Parangtritis and Pacitan.In ten of the volcano-hosted geothermal systems, 2H and 18O isotope enrichments were found, but not in any of the fault-hosted geothermal systems. Stable isotope enrichment due to evaporation was recognized in the Kawah Candradimuka and Kawah Sileri, Kawah Hujan and Candi Gedong Songo geothermal systems. A combination of intensive evaporation and magmatic gases input produced very heavy stable isotopes in the hot acid crater lakes of the Kawah Kamojang, Kawah Sikidang and Kawah Putih geothermal systems. The addition of substantial amounts of andesitic water to the geothermal fluid was observed in the Candi Songgoriti, Banyuasin and Pablengan geothermal systems.Contrary to established belief fault-hosted geothermal systems on Java could be considered a potential source for geothermal energy.