Characterising Geothermal Fluids of the Askja and Kverkfjöll Volcanic Systems, Iceland

Abstract

Askja and Kverkfjöll are neighbouring and active volcanic systems in the Northern Rift Zone (NRZ) in central Iceland. Both host relatively unexplored volcanic geothermal systems. Here, we present data on geothermal water and vapour composition from the two geothermal areas with the objective of characterising the geothermal reservoir fluids, volatile element sources and the effects of shallow secondary processes on geothermal activity at the surface. Askja is a productive and bimodal (basaltic and rhyolitic) rift volcano, centered on the axis of the NRZ. Its geothermal activity is currently mainly constrained to the shores of the Öskjuvatn lake (1052 m.a.s.l.) that fills a caldera associated with a VEI 5 eruption in 1875. At the bottom of the phreatic explosion crater Víti, a small acid lake (pH = 2.4, T = 20°C) with a diameter of 150 m is heated by hot springs seeping from the lake floor. Several, mostly diffusive, fumaroles are active along the SE inner walls of Víti. A circumneutral hot spring (pH = 6.4, T = 52°C) is found on the southern side of Víti, on the shore of the Öskjuvatn lake. Kverkfjöll sits on the flank of the NRZ and has a small volcanic output compared to the rift volcanoes. Only basaltic eruptions are known, but none from the last millennium. A vigorous geothermal system is manifested on the surface as a large geothermal area, located along a fault on the NW outer rim of the ice-filled NE caldera at a high altitude (1550-1700 m.a.s.l.). The area hosts dozens of individual fumaroles, hot springs and mud pools as well as two ice-dammed geothermal lakes. Two geothermally heated rivers Volga (pH = 7.8, T = 20°C) and Hveragil (pH = 8.5, T = 44°C) are found at lower altitudes. The vapour-dominated acid geothermal area on the top of Kverkfjöll and the mildly alkaline hot rivers probably reflect large fractions of the vapour and liquid phases, respectively, formed by boiling of a parental one-phase fluid at shallow depths.