Mathematics – Logic
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.p11c1349t&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #P11C-1349
Mathematics
Logic
[1094] Geochemistry / Instruments And Techniques, [5494] Planetary Sciences: Solid Surface Planets / Instruments And Techniques, [6225] Planetary Sciences: Solar System Objects / Mars
Scientific paper
On Earth, sulfates and sulfides are found in various geologic settings including volcanic, hydrothermal, evaporitic, and low-temperature chemical-weathering environments. The key to understanding the sulfur history on Mars is to identify and determine the sulfate/sulfide compositions and then to draw from geologic clues about their environments of formation. Theoretical, chemical, and spectroscopic evidence suggest that abundant sulfate and sulfide minerals occur on Mars; however, only a few specific types of sulfate minerals have been identified on the surface. As demonstrated here, ChemCam will have the ability to detect and quantify sulfur on the martian surface, and provide information on its mineralogy. A suite of 12 samples was selected, including minerals representative of Ca sulfates that are widespread on Mars, as well as some Mg and Fe sulfates found more locally. Pure sulfur and some common rock-forming sulfides were also included, as well as sulfides found in meteorites. Because of the vastly different compositions, mineralogies, and hydration states, standard reporting of geochemical analyses was not sufficient. Baseline compositions were converted to atomic fractions using a number of assumptions regarding mineral stoichiometry. Standoff LIBS analysis was performed at 7 m using procedures described in Tucker et al [1]. S II lines were detected most prominently in the regions of 540-550 nm and 559-567 nm. Partial least squares regression analysis was used to measure S contents of rocks in this dataset. Quantification was most successful for the naturally-occurring sulfate bearing rocks when only the regions containing the S lines were considered. Mineralogical identification/classification was successfully achieved by plotting the fitted areas of three spectral lines on a ternary diagram: S (564.7 nm), H (656.3 nm), and O (778 nm). Elemental sulfur plots at 100% S, the sulfides plot along the S-O axis, and the sulfates plot along the O-H axis. Among the sulfates, the O/H line strength ratio is indicative of the hydration state of the mineral, as the hydrated sulfates plot closer to H compared to the hydroxylated and anhydrous sulfates. The presence of other cationic elemental lines in the spectra (e.g. Fe, Ca, Zn) can further constrain mineralogies but with the caveat that distinguishing cations in the sulfur-bearing phase from the substrate rock is non-trivial in these spectra. We note also that classification using conventional principal component analysis (PCA) could not be used because the S and H lines important to mineralogical classification are too weak relative to the strong spectral lines from dominant cationic species such as Ca, Na, and Fe, which dominate the spectra. [1] Tucker, J.M. et al. (2010) Chem. Geol., in press.
Clegg Samuel M.
Darby Dyar M.
Humphries Stephen
Lane Melissa D.
Tucker Jordan
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