Oxidative Alteration of Ferrous Smectites and Implications for the Redox Evolution of Early Mars

Abstract

Surface conditions on early Mars were likely anoxic, similar to early Earth, but the timing of the evolution to oxic conditions characteristic of contemporary Mars is unresolved. Ferrous trioctahedral smectites are the thermodynamically predicted products of anoxic basalt weathering, but orbital analyses of Noachian‐aged terrains find primarily Fe3+‐bearing clay minerals. Rover‐based detection of Fe2+‐bearing trioctahedral smectites at Gale Crater suggests that ferrous smectites are the unoxidized progenitors of orbitally detected ferric smectites. To assess this pathway, we conducted ambient‐temperature oxidative alteration experiments on four synthetic ferrous smectites having molar Fe/(Mg + Fe) from 1.00 to 0.33. Smectite suspension in air‐saturated solutions produced incomplete oxidation (24–38% Fe3+/ΣFe). Additional smectite oxidation occurred upon reexposure to air‐saturated solutions after anoxic hydrothermal recrystallization, which accelerated cation and charge redistribution in the octahedral sheet. Oxidation was accompanied by contraction of the octahedral sheet (d(060) decreased from 1.53–1.56 Å to 1.52 Å), consistent with a shift toward dioctahedral structure. Ferrous smectite oxidation by aqueous hydrogen peroxide solutions resulted in nearly complete Fe2+ oxidation but also led to partial Fe3+ ejection from the structure, producing nanoparticulate hematite. Reflectance spectra of oxidized smectites were characterized by (Fe3+,Mg)2‐OH bands at 2.28–2.30 μm, consistent with oxidative formation of dioctahedral nontronite. Accordingly, ferrous smectites are plausible precursors to observed ferric smectites on Mars, and their presence in late‐Noachian sedimentary units suggests that anoxic conditions may have persisted on Mars beyond the Noachian.

Mineralogy of Vera Rubin Ridge From the Mars Science Laboratory CheMin Instrument

“…Conformable units below VRR and the Blunts Point member contain abundant smectite (Bristow et al, 2018), and units that are stratigraphically equivalent to Jura in the Glen Torridon locale to the south of VRR contain the most smectite observed to date (Bristow et al, 2019). Fe‐bearing smectite alteration on VRR could result in a relatively low abundance of phyllosilicate on VRR, in addition to precipitation of Fe oxides and oxyhydroxides from the Fe present in the smectite structure and the precipitation of opaline silica from leaching of silica from clay mineral tetrahedral layers (e.g., Chemtob et al, 2017; Sherman et al, 1962).…”

Experimental hydrothermal alteration of basaltic glass with relevance to Mars

“…Studies by Catalano () and Chemtob et al. () suggest that the low abundance of ferrous phyllosilicates might be a result of postformational oxidation with formation of ferric smectites, rather than original formation in an oxidizing environment. Moreover, Catalano () suggested that the scarce hydrous Al‐bearing silicate identifications on Mars might indicate that subsurface alteration at elevated temperature is an unlikely scenario.…”

“…Strongly reducing conditions in terrestrial hydrothermal systems has been postulated as a source for hydrogen production observed in the Yellowstone National Park and in the Columbia River Basalt McKinley 1995, 2000;Spear et al 2005). Studies by Catalano (2013) and Chemtob et al (2017) suggest that the low abundance of ferrous phyllosilicates might be a result of postformational oxidation with formation of ferric smectites, rather than original formation in an oxidizing environment. Moreover, Catalano (2013) suggested that the scarce hydrous Al-bearing silicate identifications on Mars might indicate that subsurface alteration at elevated temperature is an unlikely scenario.…”

Evidence for Multiple Diagenetic Episodes in Ancient Fluvial‐Lacustrine Sedimentary Rocks in Gale Crater, Mars

“…Alternatively, the observed smectites could be detrital and represent chemical weathering products from a source region supplying sediment to the lake. In this scenario, the variable di:tri ratios could reflect changes in weathering signatures from the source or along the fluid pathway, or different mixtures of detrital and authigenic clays (e.g., Chemtob et al, 2015Chemtob et al, , 2017Mangold et al, 2018).…”