The Geothermal System of the Arjuno-Welirang Volcano (East Java, Indonesia)

Abstract:

Arjuno-Welirang is a twin strato-volcano system located in the East of Java (Indonesia). It features two main peaks: Arjuno (3339 masl) and Welirang (3156 masl). The last recorded eruptive activity took place in August 1950 from the flanks of Kawah Plupuh and in October 1950 by the NW part of the Gunung Welirang.

This strato-volcano is characterized by a solfataric area, with high T-vent fumarole at least up to 220°C, located mainly in the Welirang crater zone where sulphur deposits are abundant. In addition, several hot springs vent from the flanks of the volcano, indicating the presence of a large hydrothermal system

During July 2015 we carried out a geochemical field campaign on the Arjuno-Welirang volcano-hydrothermal system area sampling water and dissolved gases from the thermal and cold springs located on the flanks of the volcano and from two high-T fumaroles located on the summit area of Welirang. Hydrothermal springs reveal temperatures up to 53°C and pH between 6.2 and 8.2. The hydrothermal springs show a volatile content (mainly CO₂ and He) that is several order of magnitude higher than the Air Saturated Waters values (ASW) indicating a strong gas/water interaction processes between waters of meteoric origin and deep volatiles of volcanic origin. The hydrothermal springs have dissolved helium isotopic values with clear magmatic signature (R/Ra around 7) that is remarkably close to the helium isotope values from the fumaroles (R/Ra= 7.30).