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 had surface water and could also have had hydrothermal vents supporting biological or prebiotic processes. The Strýtan 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 Fe/Mg-phyllosilicate deposits (e.g., saponite, talc, sepiolite, and serpentine) 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 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 with Fe compared to the surrounding solution. Simulated chimney compositions as analyzed by Raman spectroscopy, scanning electron microscope-energy dispersive X-ray spectroscopy, X-ray diffraction, and visible-near-infrared reflectance spectroscopy were also affected by whether the chimneys were dried and/or heated post formation. Our data were suggestive of the presence of poorly ordered Mg-clay-like phases (e.g., sepiolite) in the simulated chimneys, along with amorphous/nanocrystalline Fe phases and Fe oxides/hydroxides, hydrated silica, hematite, halite, and gypsum. Saponite was not produced in our experiments because of an absence of Al in solution. Though we observed evidence for Mg-silicate and clay-like minerals in the chemical gardens, we only observed weak 12.5 Å peaks for Fe-silicate or Fe-containing clay-like minerals; however, amorphous silica and Fe oxides/hydroxides were confirmed, similar to what has been observed in previous chemical garden 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 saponite or other aluminous clay minerals), whereas Fe would be present as Fe or Fe:Mg-hydroxides. Such chimneys, containing both reactive Fe hydroxides as well as Mg-clay-like phases, would have increased potential for mineral-driven prebiotic chemical reactions.