Oxidative Alteration of Ferrous Smectites and Implications for the Redox Evolution of Early Mars
| Autor: | Richard V. Morris, S. M. Chemtob, Jeffrey G. Catalano, R. D. Nickerson, David G. Agresti |
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| Rok vydání: | 2017 |
| Předmět: |
Recrystallization (geology)
010504 meteorology & atmospheric sciences Chemistry Inorganic chemistry Nontronite Hematite 010502 geochemistry & geophysics 01 natural sciences Anoxic waters Redox Article Ferrous Geophysics Space and Planetary Science Geochemistry and Petrology visual_art Earth and Planetary Sciences (miscellaneous) medicine visual_art.visual_art_medium Ferric Clay minerals 0105 earth and related environmental sciences medicine.drug |
| Zdroj: | J Geophys Res Planets |
| ISSN: | 2169-9100 2169-9097 |
| Popis: | 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 Fe(3+)-bearing clay minerals. Rover-based detection of Fe(2+)-bearing trioctahedral smectites at Gale Crater suggest 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% Fe(3+)/ΣFe). Additional smectite oxidation occurred upon re-exposure 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 towards dioctahedral structure. Ferrous smectite oxidation by aqueous hydrogen peroxide solutions resulted in nearly complete Fe(2+) oxidation but also led to partial Fe(3+) ejection from the structure, producing nanoparticulate hematite. Reflectance spectra of oxidized smectites were characterized by (Fe(3+),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. |
| Databáze: | OpenAIRE |
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