GEOCHEMISTRY OF MAGMATIC GASES FROM KUDRYAVY VOLCANO, ITURUP, KURIL ISLANDS

Volcanic vapors were collected during 1990-1993 from the summit crater of Kudryavy, a basaltic andesite volcano on Iturup island in the Kuri l are. The highest temperature (700-940 degrees C) fumarolic discharge s are water rich (93-98 mole% H2O) and have delta D values of -20 to - 12 parts per thousand. The chemical and water isotope compositions of the vapors (temperature of thirteen samples, 940 to 130 degrees C) sho w a simple trend of mixing between hot magmatic fluid and meteoric wat er; the magmatic parent vapor is similar in composition to altered sea water. The origin of this endmember is not known; it may be connate se awater, or possibly caused by the shallow incorporation of seawater in to the magmatic-hydrothermal system. Samples of condensed vapor from 5 35 to 940 degrees C fumaroles have major element trends indicating con tamination by wall-rock particles. However, the enrichment factors (re lative to the host rock) of many of the trace elements indicate anothe r source; these elements likely derive from a degassing magma. The str ongest temperature dependence is for Re, Mo, W, Cu, and Co; highly vol atile elements such as Cl, I, F, Bi, Cd, B, and Br show little tempera ture 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 degr ees C) fumaroles have a relatively constant composition (similar to 2 mol% each C and S species, with SO2/H2S ratio of about 3:1, and 0.5 mo l% HCl); as temperature decreases, both S, and Cl are depleted, most l ikely due to formation of native S and HCl absorption by condensed liq uid, in addition to the dilution by meteoric water. Thermochemical eva luation of the high-temperature gas compositions indicates they are cl ose to equilibrium mixtures, apart from minor loss of H2O and oxidatio n of CO and H-2 during sampling. Calculation to an assumed equilibrium slate indicates temperatures from 705 to 987 degrees C. At high tempe rature (approximate to 900 degrees C), the redox states are close to t he overlap of mineral (quartz-fayalite-magnetite and nickel-nickel oxi de) and gas (H2O-H-2-SO2-H2S) buffer curves, due to heterogeneous reac tion between the melt and gas species. At lower temperatures (<800 deg rees C), the trend of the redox state is similar to the gas buffer cur ve, probably caused by homogeneous reaction among gas species in a clo sed system during vapor ascent.