Author name: Nicholas J. Tosca

Over the last decade, high-resolution petrographic examinations of the sedimentary record revealed that greenalite was deposited across several continental margins and throughout many Archean successions. What physical and chemical processes could be responsible for this distribution? Combined sedimentological observations and geochemical results identify and strongly constrain greenalite’s origins in Precambrian sediments, specifically for iron formation deposits. Although greenalite often formed as a pore water or bottom water precipitate, the Precambrian greenalite factory may have resided at the interface between subseafloor hydrothermal vent fluids and anoxic seawater. Once formed, however, greenalite’s stratigraphic distribution was ultimately controlled by its susceptibility to oxidation, a property first recognised by geologists over 120 years ago.

The unusual structural properties of the Fe-Mg serpentine minerals permit significant chemical variability, but the mechanisms and extent of elemental substitution have only recently come to light. New results show that greenalite forms solid solutions with the Fe(III) end-member hisingerite, cronstedtite, and Mg-serpentines, with the composition depending on formation conditions. Leveraging this new mineralogical context enables quantitative estimation of H₂ production on Earth and Mars. Together, these advances indicate that greenalite solid solutions in ancient rocks produced and released H₂ and thus contributed to planetary habitability. Examination of Martian rocks and analogous Earth materials shows greenalite-hisingerite minerals were responsible for H₂ fluxes to the ancient Martian atmosphere and could be important contributors to planetary habitability throughout the Solar System.