Other
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p31d1722b&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P31D-1722
Other
[1039] Geochemistry / Alteration And Weathering Processes, [5470] Planetary Sciences: Solid Surface Planets / Surface Materials And Properties, [6225] Planetary Sciences: Solar System Objects / Mars
Scientific paper
Data collected by the OMEGA spectrometer and the CRISM instrument indicate the presence of iron-bearing phyllosilicates on Mars' surface. Identified species include chlorite, saponite (Mg-rich smectite), and nontronite (Fe(III)-bearing smectite). The observed phyllosilicates occur in units that were deposited during the Noachian, which is thought to have had chemically reducing and alkaline conditions. Phyllosilicates are expected aqueous weathering products of basaltic minerals; the aqueous activity may have occurred episodically and hydrothermally, or as prolonged, low temperature alteration. Aqueous alkaline and reducing conditions favor the initial formation of ferrous iron-bearing phyllosilicates; subsequent surface alteration events are required to have oxidized these units into ferric smectites. Understanding the formation and oxidation of ferrous phyllosilicates can offer insight into the early Martian environment by allowing us to determine by what mechanism the oxidation occurred. We have investigated chemical and structural changes that occur upon oxidation of a synthetic ferrous saponite to determine the conditions under which such a process can produce nontronite or other ferric smectites. Both H2O2 and NO3- were used as oxidants. Hydrogen peroxide is likely the dominant oxidant currently present on Mars and nitrate is a plausible oxidant produced through photochemical processes. Deposition of photochemical nitrate is observed in the Antarctic dry valleys where it co-occurs with perchlorate, which was recently identified in Martian soil by the Phoenix lander. The initial ferrous saponite contains Fe(II) in the octahedral sheet. X-ray absorption spectroscopy (XAS) indicates that in the presence of a 1m nitrate solution under hydrothermally conditions the ferrous saponite undergoes oxidation to an Fe(III)-bearing phyllosilicate. Similar oxidation is not observed at 22°C, but this appears to be a kinetic phenomenon as oxidation is thermodynamically favorable. In contrast, exposure to hydrogen peroxide causes significant oxidation regardless of temperature. However, oxidation at 22°C produces a poorly crystalline material lacking substantial structural Fe. Recrystallization upon aging under mild hydrothermal conditions (150°C) produces an Fe(III)-bearing phyllosilicate. X-ray diffraction indicates that this oxidized phyllosilicate is a 2:1 clay with lattice parameters similar to nontronite. XAS demonstrates that this oxidized clay contains Fe in similar local coordiation environments as two nontronite standards, NAu-2 and SWa-1. In both systems the need for hydrothermal aging to produce a nontronite-like clay likely reflects the slow rate of crystallization of smectites at room temperature on laboratory timescales rather than changes in their favorabtility of formation. Oxidation of ferrous smectites formed through weathering of basalt under reducing conditions is thus a viable formation pathway for the observed Martian nontronites.
Beehr A. R.
Catalano Jeffrey G.
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