A delta(34)S value of +6.3 +/- 1.5% was estimated for the rhyodacitic
degassing magma present underneath the hydrothermal system of Nisyros,
based on the S isotope ratios of H2S in fumarolic vapors. This value was
estimated by modeling the irreversible water-rock mass transfers
occurring during the generation of the hydrothermal liquid which
separates these fumarolic vapors. The S isotope ratio of the rhyodacitic
degassing magma of Nisyros is consistent with fractional crystallization
of a parent basaltic magma with an initial delta(34)S value of +4%
(+/-at least 1.5%). This positive value could be explained by mantle
contamination due to by either transference of fluids derived from
subducted materials or involvement of altered oceanic crust, whereas
contribution of biogenic sulfides from sediments seems to be negligible
or nil. This conclusion agrees with the lack of N-2 and CO2 from thermal
decomposition of organic matter contained in subducted sediments, which
is a characteristic of the whole Aegean arc system. Since hydrothermal S
at Milos and Santorini has isotope ratios similar to those determined at
Nisyros, it seems likely that common controlling processes are active
throughout the Aegean island arc. (C) 2002 Elsevier, Science B.V. All
rights reserved.