TRACKING VOLATILE RATIOS THROUGH THE MAGMATIC AND HYDROTHERMAL SYSTEM OF KAWAH IJEN VOLCANO, INDONESIA

Kawah Ijen is an active stratovolcano in eastern Java, where magmatism is the result of melting of a depleted MORB-like mantle with fluids fluxed from the subducting Indo-Australian plate. Kawah Ijen is unusual because it hosts the world’s largest hyper-acid crater lake and an actively growing magmatic dome that emits gases of magmatic origin (T range from 350 to >600°C). The dome emits an important amount of native S and is one of the world’s largest active sulfur mines. Kawah Ijen sits above 20 km of accretionary wedge material and as a result, mafic, volatile-rich magmas can erupt at the surface with relatively little crustal contamination or crystal fractionation.

Analyses of olivine-hosted melt inclusions reveal that mafic magma (Mg#=78-80) rich in volatiles (up to 6 wt.% H₂O, 1600 ppm CO₂, 2200 ppm S, 1400 ppm Cl and 1600 ppm F) supplies the magmatic plumbing system. Analyses of the fumarole gases on the active dome confirm that a deep-seated, mafic magma is currently degassing beneath the crater floor, although the composition of the dome itself is unrecognizable due to pervasive pyritic alteration in the near surface.

SO₂ flux measurements reveal a modest SO₂ budget of ~100 tonnes/day, equivalent to 300 tonnes of S_total/day. This is, however, a significant underestimate due to degassing beneath the lake resulting in S deposition and dissolution into the lake water. Portable multi-gas analysis of the plume rising off the dome confirms extensive SO₂ loss from the system likely due to the formation and deposition of native S. Compared to other arc volcanoes worldwide, Kawah Ijen has comparable H₂O/CO₂ and H₂O/S_total ratios in both the dissolved and exsolved gas phase but deposits more native S than other volcanoes of its size. This may in part reflect optimal fO₂/T° conditions during ascent of the gases.

H₂O/CO₂ ratios in the fumaroles are slightly lower than in the primitive melt inclusions. This agrees with partial degassing of CO₂ prior to melt inclusion formation and the same effect is seen in the CO₂/S_total ratios. The H₂O/S_total ratios in the fumaroles agree well with the ratio found in the melt inclusions. These results suggest that the major gas species released at the dome are not substantially affected by the hydrothermal system that lies beneath the lake.