Geochemistry of the magmatic-hydrothermal system of Kawah Ijen volcano, East Java, Indonesia

Samples from Kawah Ijen crater lake, spring and fumarole discharges were co llected between 1990 and 1996 for chemical and isotopic analysis. An extrem ely low pH (<0.3) lake contains SO4-Cl waters produced during absorption of magmatic volatiles into shallow ground water. The acidic waters dissolve t he rock isochemically to produce "immature" solutions. The strong D and O-1 8 enrichment of the lake is mainly due to enhanced evaporation at elevated temperature, but involvement of a magmatic component with heavy isotopic ra tios also modifies the lake D and O-18 content. The large Delta(SO4-S0) (23 .8-26.4 parts per thousand) measured in the lake suggest that dissolved SO4 forms during disproportionation of magmatic SO2 in the hydrothermal condui t at temperatures of 250 similar to 280 degrees C. The lake delta(18)O(SO4) and delta(18)O(H2O) values may reflect equilibration during subsurface cir culation of the water at temperatures near 150 degrees C. Significant varia tions in the lake's bulk composition from 1990 to 1996 were not detected. H owever, we interpret a change in the distribution and concentration of poly thionate species in 1996 as a result of increased SO2-rich gas input to the lake system. Thermal springs at Kawah Ijen consist of acidic SO4-Cl waters on the lakesh ore and neutral pH HCO3-SO4-Cl-Na waters in Blawan village, 17 km from the crater. The cation contents of these discharges are diluted compared to the crater lake but still do not represent equilibrium with the rock. The SO4/ Cl ratios and water and sulfur isotopic compositions support the idea that these springs are mixtures of summit acidic SO4-Cl water and ground water. The lakeshore fumarole discharges (T = 170 similar to 245 degrees C) have b oth a magmatic and a hydrothermal component and are supersaturated with res pect to elemental sulfur. The apparent equilibrium temperature of the gas i s similar to 260 degrees C. The proportions of the oxidized, SO2-dominated magmatic Vapor and of the reduced, H2S-dominated hydrothermal vapor in the fumaroles varied between 1979 and 1996. This may be the result of interacti on of SO2-bearing magmatic vapors with the summit acidic hydrothermal reser voir. This idea is supported by the lower H2S/SO2 ratio deduced for the gas producing the SO4-Cl reservoir feeding the lake compared with that observe d in the subaerial gas discharges. The condensing gas may have equilibrated in a liquid-vapor zone at about 350 degrees C. Elemental sulfur occurs in the crater lake environment as banded sediments exposed on the lakeshore and as a subaqueous molten body on the crater floo r. The sediments were precipitated in the past during inorganic oxidation o f H2S in the lake water. This process was not continuous, but was interrupt ed by periods of massive silica (poorly crystallized) precipitation, simila r to the present-day lake conditions. We suggest that the factor controllin g the type of deposition is related to whether H2S- or silica-rich volcanic discharges enter the lake. This could depend on the efficiency with which the lake water circulates in the hydrothermal cell beneath the crater. Quen ched liquid sulfur products show delta(34)S values similar to those found i n the banded deposits, suggesting that the subaqueous molten body simply co nsists of melted sediments previously accumulated at the lake bottom. (C) 2 000 Elsevier Science B.V. All rights reserved.