<b>The olivine alteration on Earth and Mars: Lessons from terrestrial analogue and Martian meteorite studies</b>

Olivine, regardless of its composition (whether magnesium-rich or iron-rich), undergoes a transformation into iddingsite, a process observed on both Earth and Mars. The formation of iddingsite is primarily controlled by fractures and is a result of limited, aqueous alteration under a low water-rock (W/R) ratio. The mineralogical assemblages include phyllosilicate (saponite), hematite, and goethite, indicative of low-temperature aqueous alteration and thereby suggested for a comparable physicochemical condition in Earth and Mars. Similar conditions of aqueous alteration are also comprehended based on comparable elemental enrichment and cation mobility. The serrated microtextural features are consistent with in situ dissolution processes occurring concurrently with precipitation. The formation of iddingsite leads to a depletion of Fe and Mg, alongside a progressive enrichment of Si under acidic conditions. The relatively high Chemical Index of Alteration (CIA) observed in terrestrial olivine alteration suggests a relatively higher water-to-rock ratio as compared to the Martian counterpart. The associated mineralogy attributes oxidative alteration, and the Al-enrichment in secondary minerals further argues for a hydrothermal alteration process. Geochemical modelling of terrestrial picritic basalts further aligns for a transient hydrothermal fluid circulation, preferably in a low water-to-rock ratio (1-10), resulting in a gradual transition to Mg saponite as the pH increases.