Boron isotope variations in geothermal systems on Java, Indonesia

This paper presents δ¹¹B data for hot springs, hot acid crater lakes, geothermal brines and a steam vent from Java, Indonesia. The processes that produce a large range of the δ¹¹B values were investigated, including the possible input of seawater as well as the contrast δ¹¹B compositions of acid sulfate and acid chloride crater lakes. The δ¹¹B values of hot springs ranged from -2.4 to +28.7‰ and acid crater lakes ranged from +0.6 to +34.9‰. The δ¹¹B and Cl/B values in waters from the Parangtritis and Krakal geothermal systems confirmed seawater input. The δ¹¹B values of acid sulfate crater lakes ranged from +5.5 to +34.9‰ and were higher than the δ¹¹B of +0.6‰ of the acid chloride crater lake. The heavier δ¹¹B in the acid sulfate crater lakes was caused by a combination of vapor phase addition and further enrichment due to evaporation and B adsorption onto clay minerals. In contrast, the light δ¹¹B of the acid chloride crater lake was a result of acid water-rocks interaction. The correlations of δ¹¹B composition with δ¹⁸O and δ²H indicated that the B isotope corresponded to their groundwater mixing sources, but not for J21 (Segaran) and J48 (Cikundul) that underwent ¹¹B isotope enrichment by B adsorption into minerals.