Simulated Fe/Mg-silicate chimneys as analogs to saponite-rich hydrothermal systems on early Earth and Mars

Hydrothermal systems have been proposed as environments for organic synthesis and prebiotic chemistry on early Earth, and also could have supported biological or prebiotic processes on ancient Mars. The Strytan Hydrothermal Field (SHF) in Iceland is a basalt-hosted alkaline vent that forms massive hydrothermal Mg-saponite deposits; the SHF is an analog to basalt-hosted alkaline vents that may have existed on early Earth, and to the Eridania basin on Mars where saponite deposits are thought to have formed from ancient hydrothermal activity. In this study we simulated the growth of Fe- and/or Mg-silicate 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 seawater influenced the chimney mineralogy under anoxic conditions, and that chimneys in Fe-containing systems were enriched in Fe compared to the surrounding seawater. The chimney chemical properties as analyzed by Raman, SEM-EDX, XRD, and VNIR were also affected by how the chimneys were dried and/or aged post-formation (e.g. air-dried and/or heated at low-grade hydrothermal temperatures). Observed mineral assemblages in the chimneys included Mg-montmorillonite (when only Mg was present) or a possible Fe-saponite and amorphous Fe phases (when Fe was present), along with hydrated silica, hematite, magnetite, and other crystalline phases. Overall our data were suggestive of montmorillonite, saponite, and/or nontronite clays though this was not able to be confirmed with ethylene glycol treatment and XRD; this could be due to the precipitates being aged for only a short time. The Fe/Mg minerals in our laboratory analog chimneys are interesting for prebiotic chemistry, and are also known to protect/preserve organics; therefore, the similar Fe/Mg-clay-rich regions on Mars might be good candidates for detection of past prebiotic systems.