Geochemistry of magmatic gases from Kudryavy volcano, Iturup, Kuril Islands

Abstract

Volcanic vapors were collected during 1990–1993 from the summit crater of Kudryavy, a basaltic andesite volcano on Iturup island in the Kuril arc. The highest temperature (700–940°C) fumarolic discharges are water rich (94–98 mole% H2O and have δD values of −20 to −12%o. The chemical and water isotope compositions of the vapors (temperature of thirteen samples, 940 to 130°C) show a simple trend of mixing between hot magmatic fluid and meteoric water; the magmatic parent vapor is similar in composition to altered seawater. The origin of this endmember is not known; it may be connate seawater, or possibly caused by the shallow incorporation of seawater into the magmatic-hydrothermal system. Samples of condensed vapor from 535 to 940°C fumaroles have major element trends indicating contamination by wall-rock particles. However, the enrichment factors (relative to the host rock) of many of the trace elements indicate another source; these elements likely derive from a degassing magma. The strongest temperature dependence is for Re, Mo, W, Cu, and Co; highly volatile elements such as Cl, I, F, Bi, Cd, B, and Br show little temperature dependence. The Re abundance in high-temperature condensates is 2–10 ppb, sufficient to form the pure Re sulfide recently discovered in sublimates of Kudryavy. Anomalously high I concentrations (1–12 ppm) may be caused by magma-marine sediment interaction, as Br/I ratios are similar to those in marine sediments. The high-temperature (>700°C) fumaroles have a relatively constant composition (∼2 mol% each C and S species, with SO2/H2S ratio of about 3:1, and 0.5 mol% HCl); as temperature decreases, both St and CI are depleted, most likely due to formation of native S and HCl absorption by condensed liquid, in addition to the dilution by meteoric water. Thermochemical evaluation of the high-temperature gas compositions indicates they are close to equilibrium mixtures, apart from minor loss of H2O and oxidation of CO and H2 during sampling. Calculation to an assumed equilibrium state indicates temperatures from 705 to 987°C. At high temperature (≈900°C), the redox states are close to the overlap of mineral (quartz-fayalite-magnetite and nickel-nickel oxide) and gas (H2OH2SO2H2S) buffer curves, due to heterogeneous reaction between the melt and gas species. At lower temperatures (<800°C), the trend of the redox state is similar to the gas buffer curve, probably caused by homogeneous reaction among gas species in a closed system during vapor ascent.