FLUXES AND SOURCES OF VOLATILES DISCHARGED FROM KUDRYAVY, A SUBDUCTION ZONE VOLCANO, KURILE ISLANDS

The Kudryavy volcano, a 996-m-high basaltic-andesite cone on the north eastern shore of Iturup Island in the Kuriles, erupted last in 1883 an d has since been in a persistent state of high-temperature, >900 degre es C fumarolic activity. Its flux of SO2, measured by COSPEC, is 73 +/ - 15 t/d, or 416 Mmol/a. In combination with the chemical composition of the parent gas supplying the high-temperature vents and the isotopi c compositions of He and C, it allows the evaluation of contributions from major source components, such as the mantle, the crust, and subdu cted sediments and carbonate. The He-3/He-4 ratio of 6.7 R-A correspon ds to a 84% to mantle origin and a flux of 2200 mol/a of mantle He. At a He concentration of 2200 mol/Mt, the mass of mantle material requir ed to generate this flux is 1.0 Mt/a. The same mass produces a flux of 0.025 mol/a of He-3 and of 50 Mmol/a of mantle CO2 at a CO2/He-3 rati o of 2.10(9). In conjunction with the C-isotopic composition of fumaro lic CO2 of -7.2 parts per thousand, about 12% of the CO2 are derived f rom the mantle, 67% from marine carbonate in subducted, altered oceani c crust, 21% are of subducted organic sedimentary origin. The Aux of 2 80 Mmol/a of carbonate-derived CO2 requires 0.41 Mt/a of oceanic crust with a CO2 content of 3 wt%, and 0.35 Mt/a of sedimentary material to supply the organic CO2 flux of 86 Mmol/a. Nitrogen from the mantle co ntributes at most 2% to the total N-2 flux of 5.4 Mmol/a. Assuming N t o be derived from the subducted sediments, its concentration there is 460 mg/kg. The total volume of mantle and subducted material required to maintain the flux of volatiles over the 100 a period of high-temper ature fumarolic activity of Kudryavy is 0.07 km(3). Steady-state relea se of volatiles from the depth of are magma generation to the fumarole s and continuously high heat flow from the mantle are proposed as the main process supporting the long-term high-temperature degassing at Ku dryavy. In this steady-state system, the calculated volatile fluxes ar e balanced over time by volatiles originating from subducted sediments , hydrothermally altered oceanic crust below the Kudryavy volcano and the mantle wedge. This has significant implications for volatile cycli ng from the Earth's crust and mantle to the atmosphere. (C) 1998 Elsev ier Science B.V. All rights reserved.