Abstract
We used thermochemical equilibrium calculations to constrain the oxygen
fugacity (f(O2)) of volcanic gases on Io, Three types of calculations
were done: (1) Upper limits for f(O2) from Voyager IRIS upper limits
for the SO3/SO2 ratio and the O-3 abundance in the Loki volcanic
plume; (2) lower limits for f(O2) from the observed SO/SO2 ratio
in Io's atmosphere;(3) oxygen fugacities as a function of temperature,
total pressure, and O/S ratio for volcanic gases. We find that hot
SO2 (e.g. Loki volcanic gases) has oxygen fugacities between the
Ni-NiO and hematite-magnetite oxygen fugacity buffers. Pele-type
volcanic gases (i.e. So(2-)S(2) mixtures) have f(O2) values ranging
from Ni-NiO to a few log f(O2), units lower. These f(O2) values are
similar to those for most terrestrial volcanic gases and magmas,
This coincidence indirectly indicates the predominantly silicate
character of volcanism on Io. The oxidized nature of volcanic gases
and their probable source magmas indicates that Io is differentiated
and that metallic iron and free carbon are not present (or at least
not abundant) in bulls silicate Io. This deduction agrees with the
earlier inference from Galileo data of an iron core in Io. The inferred
oxidation state of bulk silicate Io is plausibly due to loss of most
of its initial water inventory via hydrogen escape and consequent
oxidation of Fe and Fe2+ -bearing minerals to magnetite and other
Fe3+-bearing phases. Geochemical analyses of Io's surface and volcanic
plumes, while difficult, are possible and can test our predictions.
(C) 1999 Academic Press.
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