In: Earth and Planetary Science Letters, vol. 160, p. 81-96 (1998)
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Abstract
Kudryavy volcano, a 996 m high basaltic-andesite
cone on the northeastern shore of Iturup island in the Kuriles, erupted
last in 1883 and has since been in a persistent state of high-temperature,
>900°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 isotopic
compositions of He and C, it allows the evaluation of contributions from
major source components, such as the mantle, the crust, and subducted sediments
and carbonate. The 3He/4He ratio of 6.7RA corresponds to a 84% 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 required to generate this flux is 1.0 Mt/a.
The same mass produces a flux of 0.025 mol/a of 3He and of 50 Mmol/a of
mantle CO2 at a CO2/3He ratio of 2*109. In conjunction with the C-isotopic
composition of fumarolic CO2 of -7.2â, about 12% of the CO2 are derived
from the mantle, 67% from marine carbonate in subducted, altered oceanic
crust, 21% are of subducted organic sedimentary origin. The flux of 280
Mmol/a of carbonate-derived CO2 requires 0.41 Mt/a of oceanic crust with
a CO2 content of 3 weight-%, and 0.35 Mt/a of sedimentary material to supply
the organic CO2 flux of 86 Mmol/a. Nitrogen from the mantle contributes
at most 2% to the total N2 flux of 5.4 Mmol/a. Assuming N to 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-temperature fumarolic activity
of Kudryavy is 0.07 km3. Steady state release of volatiles from the
depth of arc magma generation to the fumaroles and continuously high heat
flow from the mantle are proposed as the main process supporting the long-term
high temperature degassing at Kudryavy. In this steady state system,
the calculated volatile fluxes are balanced over time by volatiles originating
from subducted sediments, hydrothermally altered oceanic crust below Kudryavy
Volcano and the mantle wedge. This has significant implications for
volatile cycling from the Earth's crust and mantle to the atmosphere.
Key words: subduction zone, helium, carbon, nitrogen,
water, isotopes, volatile flux, Kudryavy, volcano, arc magmas, volcanic
gases, volatiles.
= deceased as of November 1997