Competition between Na_2SO_4 and Na sulfide in the upper crust of Io

Abstract

The Na atmosphere of Io requires a Na-S-O phase in the outer surface layers. Considering the various mechanisms for extraction of Na to the surface, the possible primary phases are Na_2O, Na_2S_x and Na_2SO_4. However, regardless of the primary phases brought to the surface, the shallow crustal recycling of material implied by the ongoing volcanism will tend to produce thermochemical equilibrium and cause all Na to end up as Na_2SO_4 or Na_2S_x. This hypothesis is investigated by relatively model-independent thermodynamic calculations. The major assumption is that material is statistically circulated to sufficiently high temperatures by burial that thermochemical equilibrium can be attained. For a wide range of assumed crustal (PT) conditions, Na_2O will be converted to Na_2SO_4. During residence in the shallowest crustal regions dominated by liquid SO_2, e.g., SO_2 geysers or fumaroles, or for any crustal regimes where SO_2 and S are in comparable abundances, Na-sulfides will be converted to Na_2SO_4. However, in high-temperature, low-pressure regimes with a low relative abundance of SO_2 relative to S (e.g., due to outgasing of SO_2), Na_2SO_4 is converted to Na sulfides. Such regimes could be relatively common on Io, e.g., associated with flows, lava lakes, or shallow intrusions. Consequently, because of thermochemical equilibration in different crustal environments, both Na_2SO_4 and Na sulfides will coexist on the Io surface.

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