Fe-rich olivine weathering under a CO₂ atmosphere: Implications for iron oxidation and phyllosilicate formation on early Mars

Secondary minerals on Mars, particularly iron oxides, record the oxidative evolution of the planet. However, the origin and timing of large-scale oxidation of the Martian surface remain poorly constrained. Here we investigate the aqueous alteration of fayalite-forsterite (Fa₁₀₀-Fa₀) olivine solid solutions under a 1-bar CO₂ atmosphere at 45 ℃. We show that Fe-rich olivine exhibits enhanced yet non-linear dissolution behavior relative to Mg-rich forsterite, and that weathering proceeds through two stages: an initial stage (0-3 days) characterized by rapid dissolution and cation release, followed by a prolonged stage (4-180 days) dominated by Fe(III) precipitation. Alteration products are primarily Fe-enriched, Mg-depleted amorphous silicate gels, consistent with precursors to phyllosilicate formation on Mars. Crystalline Fe(III) oxides (hematite and goethite) form exclusively in the most Fe-rich systems (Fa₇₁ and Fa₁₀₀), indicating that Fe(II) oxidation can occur through water-mediated redox pathways effectively even under anoxic CO₂ atmosphere. However, the abundance of Fe oxides produced in our experiments is substantially lower than the 6-20 wt.% ferric phases observed on the Martian surface, suggesting that Fe-rich olivine weathering under CO₂ alone cannot account for the global ferric inventory. Despite their limited abundance, these early-formed Fe(III) oxides may have acted as catalytic nuclei, facilitating subsequent oxidation processes involving other redox-active species. Our results suggest that the earliest ferric oxides on Mars could have formed locally within Fe-rich crustal terrains under CO₂-dominated conditions, initiating spatially heterogeneous but progressively amplified oxidative evolution on early Mars.