Analyzing Fe/Mg-Silicate Chemical Gardens as Analogs to Silicate-Rich Hydrothermal Chimneys on Early Earth and Mars

Hydrothermal systems have been proposed as environments for prebiotic chemistry on early Earth. Ancient Mars was an ocean world and could also have had hydrothermal vents supporting biological or prebiotic processes. The Strytan Hydrothermal Field (SHF) in Iceland is a basalt-hosted alkaline vent that forms massive hydrothermal Mg-saponite chimneys and is a potential analog to basalt-hosted alkaline vents that may have existed at the Eridania basin on Mars, where saponite deposits are thought to have formed from ancient hydrothermal activity. Chemical garden experiments have previously been used to simulate aspects of hydrothermal chimney growth for other types of vent systems; however, they have not been much used in this context of a silica-rich hydrothermal system. Here, we studied the formation of Fe/Mg-silicate injection chemical gardens simulating hydrothermal chimneys that represent analogs of precipitates that could have formed in SHF-like hydrothermal vents on the early Earth and/or early Mars. We found that the Fe/Mg ratio of the exterior (ocean simulant) solutions influenced the simulated chimney chemistry under anoxic conditions, and that the precipitates were enriched in Fe compared to the surrounding solution. Simulated chimney compositions as analyzed by Raman, Scanning Electron Microscope-Energy Dispersive X-ray Spectroscopy, X-Ray Diffraction, and Visible Near Infrared Reflectance spectra were also affected by whether the chimneys were dried and/or heated post-formation. Our data were suggestive of the presence of Mg-clay like minerals such as saponite or sepiolite in the simulated chimneys, along with amorphous/nanocrystalline Fe phases and Fe-hydroxides, hydrated silica, hematite, halite, and gypsum. Although smectites were not able to be definitively confirmed with ethylene glycol treatment and XRD, this could be due to the precipitates being aged for too short a time to allow for crystallization of an ordered smectite. Though we observed evidence for Mg-silicate and clay-like minerals in the chemical gardens, we did not observe any evidence for Fe-silicate or Fe-containing clay-like minerals; only amorphous silica and Fe hydroxides/oxides, similar to what has been observed in previous studies. This suggests that in SHF-like chimneys on early Earth and/or Mars, Mg would have been present as Mg-hydroxides and Mg-silicates (and, in the presence of additional geological components such as Al, likely phyllosilicates or clays), whereas Fe would be present as Fe or Fe:Mg hydroxides. Such chimneys, containing both reactive Fe-hydroxides as well as Mg-clay-like minerals, would have increased potential for mineral-driven prebiotic chemical reactions.