Other
Scientific paper
Dec 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agufm.p23b..05m&link_type=abstract
American Geophysical Union, Fall Meeting 2004, abstract #P23B-05
Other
5415 Erosion And Weathering, 5470 Surface Materials And Properties, 5480 Volcanism (8450), 6225 Mars, 3929 Nmr, Mossbauer Spectroscopy, And Other Magnetic Techniques
Scientific paper
The Moessbauer spectrometers on the MER rovers have measured the relative abundances of iron with respect to both oxidation state and iron-bearing phase at Gusev Crater (Spirit rover) and Meridiani Planum (Opportunity rover). The assemblage of phases indicates aqueous alteration processes at both landing sites. Although the rock and soil of the Gusev Crater plains are dominated by Fe(2+) in olivine-bearing basalt (~Fo60), a Fe(3+)-rich component (nanophase ferric oxide, np-Ox) has significant abundance in surface soils (13-28% of total Fe) and in the surface coatings (rinds) of certain rocks (39%) but not in rock interiors exposed by grinding (5-6%). The mode of occurrence of np-Ox implies that it is the product of oxidative alteration of Fe(2+) silicate and oxide phases in the presence of H2O. The ubiquitous presence of sulfur in soil and in rock coatings, as determined by the MER-A APXS instrument, suggests that the alteration occurred under acid-sulfate conditions, so that both hydrolytic and sulfatic reactions are viable. A possible source for the weathering agents is volcanic emanations rich in H2O and SO2. Generally, rocks in the Columbia Hills are significantly more altered than those in the Gusev plains, with a higher proportion of Fe(3+) oxide phases compared to Fe(2+) silicate phases. This mineralogical dichotomy implies a difference in the timing, rate, duration, and/or mechanism of alteration for basaltic material in the Gusev plains compared to basaltic material in the Columbia Hills. It is possible, for example, that the basaltic material in the Columbia Hills underwent aqueous alteration in a paleoclimate that favored nearly complete alteration and that the basaltic material of the Gusev plains will not achieve the degree of alteration exhibited by the Columbia Hills under current martian surface conditions. Because its structure contains the hydroxide anion, the Moessbauer detection of the hydroxide sulfate jarosite (K,Na)Fe3(SO4)2(OH)6 in outcrops (~28% of total Fe) at Meridiani Planum is direct mineralogical evidence for oxidative aqueous alteration of basaltic precursors. We calculate that average outcrop has the equivalent of ~2 wt. % H2O contained in the jarosite. By analogy with terrestrial jarosite occurrences, acid-sulfate conditions at low pH are required. Acid-sulfate conditions could arise through different pathways, including oxidation of sulfide minerals by aqueous fluids and volcanic emanations with a high SO2/H2O ratio. Hematite is pervasive and is found within the outcrop matrix (~35% of total Fe) and in spherules (interpreted as concretions) dispersed throughout the outcrop. Whole and fragmented spherules occur on outcrop surfaces and are concentrated in the crests of ripples (lag deposits). The process for spherule formation is incompletely understood, but presumably involved aqueous transport processes. As at Gusev Crater, basaltic surface soils at Meridiani Planum have significant concentrations of nanophase ferric oxide (12-37%). In particular, a high-albedo aeolian deposit (np-Ox ~ 29%) on the rim of Eagle Crater suggests that martian dust is basaltic in bulk composition and is a product of oxidative, aqueous alteration and subsequent disintegration of coarser basaltic materials.
Klingelhoefer G.
Morris Richard V.
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