The Role of Chlorate‐Driven Oxidative Weathering in Shaping Clay Deposits on Mars

Oxidized clays and the widespread Al/Si‐over‐Fe/Mg stratigraphy on Mars imply prolonged water‐rock interactions and efficient oxidative weathering despite the largely anoxic conditions of Early Mars when most deposits formed during the climate optimum. Oxychlorine species are now known to be common on Mars, and laboratory kinetics show that chlorate oxidizes Fe(II) orders of magnitude faster than at low temperatures, motivating a chlorate‐centered oxidative weathering framework. We develop a 1‐D reactive transport model that explicitly includes Fe(II) oxidation by chlorate to evaluate whether low‐temperature alteration of basalt can produce oxidized Fe(III)‐rich smectites (nontronite) and associated weathering‐derived minerals commonly observed on Mars. Model simulations span oxidant supply (chlorate –; oxygen partial pressure (, –0.21 bar), climate (0–; 30–), and parent basalt mineralogy. Across this space, only saponite, nontronite, kaolinite, and amorphous silica consistently formed. Dissolved chlorate on the order of – reproduces the canonical Al/Si cap over Fe/Mg‐smectites within ∼200 kyr and builds clay‐bearing intervals approaching 100 m. Clay‐bearing unit accumulation is nonlinear, with most deposition occurring in the early stages of oxidative weathering, highlighting limitations of constant net‐rate assumptions for long‐term thickness. An intermediate climate window (–; 50–) preserves thick sections with an Al‐rich cap. Certain clay phases are sensitive to parent basalt mineralogy (plagioclase favors kaolinite; olivine/pyroxene favor saponite), whereas nontronite reflects oxidant capacity and amorphous silica marks cool–dry conditions. These findings provide a testable chlorate‐driven framework that reconciles oxidized clays under anoxic conditions and links stratigraphy to redox, climate, and bedrock composition.