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

Abstract:

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.