Ocean world plumes at Enceladus, Triton, and possibly Europa are astrobiologically significant. These active processes may transport fresh material from potentially habitable subsurface environments to the surface and atmosphere/exosphere, where they can be accessed by spacecraft and telescopic observations. However, it is currently unclear if chemical fractionation or other modification processes might occur during subsurface transport and eruption and potentially lead to changes in concentrations of habitability indicators relative to the source reservoir. To explore this phenomenon in a natural setting, we investigated the cold CO2 geysers in Green River, Utah, which have eruptions driven by volatile exsolution. We collected samples from two geysers with different vent diameters and discharge volumes and compared the chemical composition of the erupted effluent and mineralogy of evaporite deposits with their respective pre-erupted waters; we also performed geochemical modeling to reconstruct the original chemical speciation of the source waters. Observed increases in electrical conductivity for both the erupted effluents may be due to an influx of warm fluids enriched in CO2-charged brine entering the aquifer and initiating eruption via CO2 exsolution and buoyant acceleration. Modeling results indicate source waters extremely rich in dissolved CO2 with pH values significantly lower than those of erupted waters. The outgassing of CO2 and significant levels of sulfate, Na/K ratio, and acidic pH suggest that the effluent from this geysering system may serve as a natural analog for putative plume deposits on Europa. The larger geyser (Crystal) had evaporites that were carbonate-rich, while the smaller geyser (Champagne) produced evaporites dominated by sulfate minerals. Furthermore, in a sample of erupted Champagne waters cooled rapidly in vacuum to replicate a frozen plume deposit, vitreous MgSO4 was the primary constituent; this was not the main component in solution or identified in the evaporite or surrounding tufa. Overall, our observations suggest that geyser discharge volume, eruptive energy, and/or proximity to the host reservoir may all play a role in the composition of plume ejecta and surface deposits, and care should be taken in integrating both in situ and remote sensing observations to fully characterize plume deposits and make robust inferences of ocean composition. Key Words: Enceladus—Europa—Reflectance spectroscopy—Raman spectroscopy—Habitability indicator—Plume. Astrobiology 26, 79–98.
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