The boron isotopic composition of geothermal waters and crater lakes from Java, Indonesia

Tuesday, 16 December 2014
Budi Joko Purnomo, University of Bremen, Bremen, Germany, Thomas Pichler, Universität Bremen, Bremen, Germany and Chen-Feng You, NCKU National Cheng Kung University, Tainan, Taiwan
The presences of two different types of geothermal systems, volcanic-hosted and fault-hosted, and two different types of acid crater lakes, acid sulfate and acid chloride, on Java, Indonesia presented the opportunity to investigate boron isotope systematic in such contrasting systems. Fault-hosted hot springs and the acid chloride crater lakes had light δ11B (-2.4 to +0.49 ‰), similar to geothermal brines. Meanwhile, the δ11B values of volcanic-hosted hot springs ranged from -0.7 to +12.8 ‰ and acid-sulfate crater lakes ranged from +5.5 to +34.8 ‰, which were heavier than the geothermal vapor phase (δ11B= +3.8 ‰). The absence of magmatic gases and the fast ascent of the fault-hosted thermal waters inhibited B isotope fractionation, thus their original light δ11B signature from the reservoir was maintained. In contrast, in volcanic-hosted geothermal systems magmatic degassing produced a more reactive geothermal water, which combined with the relatively longer ascent produced heavy δ11B signatures. The light δ11B signature of acid chloride crater lakes was produced by rock dissolution, also indicated by its B/Cl ratios, which were similar to those of andesitic rocks. The continuous H2S, SO2 and HCl gases supply favors rock dissolution in the acid chloride crater lakes and thus little to no B isotope fractionation was observed. In contrast, condensation of the geothermal vapor phase combined with evaporation and B adsorption onto clay minerals caused a δ11B enrichment in the acid sulfate crater lakes. The very heavy δ11B value of +34.8 ‰ in the acid sulfate crater lake of Kawah Sikidang likely involved some reaction with sedimentary rocks in the subsurface.