Other
Scientific paper
Dec 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufm.p21c..02m&link_type=abstract
American Geophysical Union, Fall Meeting 2007, abstract #P21C-02
Other
5220 Hydrothermal Systems And Weathering On Other Planets, 5415 Erosion And Weathering, 5480 Volcanism (6063, 8148, 8450), 6225 Mars, 8424 Hydrothermal Systems (0450, 1034, 3017, 3616, 4832, 8135)
Scientific paper
The Alpha Particle X-ray Spectrometer (APXS) instrument on the Mars Exploration Rover (MER) Spirit measured three targets on or adjacent to Home Plate in Gusev Crater that have unusually high SiO2 concentrations (68 to 91%), unusually low FeO concentrations (1 to 7%, with total Fe as FeO), and unusually high TiO2/FeO ratios (0.2 to 1.2 by weight). Two targets are high albedo soil (Gertrude Weise) exposed by the rover wheels (Kenosha Comets and Lefty Ganote), and one target is a rock (Fuzzy Smith). Kenosha Comets has the highest SiO2 concentration, lowest FeO concentration, and highest TiO2/FeO ratio. Mineralogical evidence from the MER Miniature Thermal Emission Spectrometer (Mini-TES) suggests that high proportions of amorphous (non- crystalline) SiO2 account for the high SiO2 concentration of Gertrude Weise. Mini-TES data were not acquired for Fuzzy Smith. The spectral evidence for amorphous SiO2 includes deep emissivity minima near 9 and 21 microns. Amorphous SiO2 is typically characterized by a shoulder near 8 microns, but the Gertrude Weise spectra are instead characterized by a well-defined emissivity minimum. The difference is attributed to scattering and/or geometric effects. The chemistry of Gertrude Weise and Fuzzy Smith is very similar to that for a tholeiitic basaltic rock altered under acid-sulfate conditions in a fumarolic (hydrothermal) environment on Kilauea Volcano (Hawaii). The terrestrial acid sulfate alteration resulted in compositions having about 62 to 91% SiO2, 1 to 8% FeO, and 0.3 to 5 TiO2/FeO. The SiO2 and TiO2 are passively enriched while all other major elements are removed by leaching. XRD analysis shows that SiO2 and TiO2 are present as amorphous SiO2 (opal-A) and anatase, respectively. Alunite is also present. Thermal emission spectra of the Kilauea rock obtained from its exterior surface and from 500-1000 micron and <150 micron powders derived by grinding and sieving are all characterized by the emissivity features at 8, 9, and 21 micron for opal-A. The features at 9 and 21 microns are emissivity minima and, like Gertrude Weise, the feature near 8 microns is also an emissivity minimum. The <150 micron powder has additional spectral features in the transparency region between about 9.5 and 12 microns. The correspondence of available chemical and thermal emission data for the terrestrial analog and martian samples implicates a common formation pathway, i.e., acid sulfate alteration in a fumarolic environment. Although other formation pathways merit investigation for the martian samples (e.g., SiO2 sinters), acid-sulfate conditions in a fumarolic environment are consistent with the proposed volcanic origin for Home Plate and with the presence of SO3-rich (22 to 35 %) soils and clods in the vicinity of Home Plate. These ferric sulfate rich materials also show excess SiO2 by mass balance. By analogy with VNIR spectra for the Kilauea sample, a broad M-OH spectral feature at 2.25 microns detected from martian orbit by CRISM or OMEGA can be interpreted as evidence for high-SiO2 deposits on the surface.
Arvidson Ray E.
Clark Ben C.
Gellert Ralf
Graff Trevor G.
Ming Douglas W.
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