Gas Geochemistry and Carbon-13 Systematics of Ungaran Geothermal Field, Central Java, Indonesia

Abstract

Ungaran geothermal field has numerous thermal manifestations, e.g. fumaroles and hot springs with temperature range from 31°C to 86°C. Fluid samples have been taken from fumaroles at Gedong Songo (Dasamuka) and gas bubbling from various hot springs to analyse its gas composition and carbon-13 ratio. The result of gas analysis shows that Ungaran’s fluids are meteoric origin with some degree of magmatic contribution, with CO2 as the dominant gas. CO2-H2-Ar gas ratio grid shows that only samples from Gedong Songo fumaroles that plotted inside the two-phase grid with temperature about 280°C, while other samples plotted outside the grid indicates re-equilibration at lower temperatures. The range of carbon-13 ratio is about -8-32 o/oo to -5.89 o/oo (vs PDB). These values indicate that CO2 in Ungaran geothermal fluids are magmatic origin, not from soil or carbonate source.