Other
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.p43d..06m&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #P43D-06
Other
3665 Mineral Occurrences And Deposits, 3672 Planetary Mineralogy And Petrology (5410), 5410 Composition (1060, 3672), 5464 Remote Sensing
Scientific paper
Mawrth Vallis contains the largest exposure of phyllosilicates on Mars, as observed by OMEGA and CRISM. OMEGA identified nontronite and montmorillonite as the most common phyllosilicates in the Mawrth Vallis region, and CRISM has confirmed and refined this observation with the further identification of kaolinite and hydrated silica [1-3]. In addition, saponite has been identified in one image within a crater. Two main phyllosilicate units occur in the Mawrth Vallis region: the lowermost unit is nontronite-bearing and is unconformably overlain by an Al-phyllosilicate-bearing unit composed of montmorillonite plus hydrated silica, with a thin layer of kaolinite plus hydrated silica at the top. These two units are capped by a spectrally- unremarkable unit [4, 5]. Spectra retrieved near the boundary between the nontronite and Al-phyllosilicate units exhibit a strong positive slope from 1-2 μm, likely from a ferrous component within the rock. This component could be ferrous olivine, troilite/phyrrhotite, or glauconite, for example, and indicates either rapid deposition or reducing conditions, sometimes supplied on Earth by microorganisms. Each of the phyllosilicate minerals identified form only under hydrolytic conditions, indicating a regional wet period in the Noachian. It is likely the two units formed by separate events; however, the processes are similar so we address them together. The large areal extent, stratigraphy, mineralogy, and observed layering of the two primary units suggest that they may be formed in a manner similar to bentonite: volcanic ash of different composition deposited into a body of water and then altered. This suggests the presence of a stable ground water table and extensive volcanism near Mawrth Vallis in order to produce the volume of phyllosilicates observed here. Other potential formational processes include: i) sediment deposition into a marine or lacustrine basin, ii) pedogenesis, or iii) hydrothermal alteration due to an overlying hot impact melt sheet (Al-phyllosilicate unit only). Clay deposits in marine basins typically reflect nearby continental conditions, and the species and volumes of phyllosilicates identified indicate a warm-wet climate. Pedogenesis in a warm-wet environment will produce laterites with alteration horizons of kaolinite and other Al-phyllosilicates and nontronite in the lowermost horizons. Impacts can mobilize frozen water as well as liquid water and the mineral assemblage formed is the same for a given temperature and water to rock ratio, so the climate cannot be determined if the Al-phyllosilicates were formed by an impact melt sheet hydrothermal system. It seems likely that these phyllosilicates formed in a warm-wet climate and are another indicator of a more hospitable environment in Mars' past. [1]Bibring, J.-P., et al. (2006) Science, 312, 400-404. [2]Poulet, F., et al. (2005) Nature, 438, 632-627. [3]Bishop, J. L., et al. (2008) Science, 321, 830-833. [4]Michalski, J. R., et al. (2007) Geology, 35, 951-954. [5]Wray, J. J., et al. (2008) GRL, 35, L12202.
Bibring J.
Bishop Janice L.
Ehlmann Bethany L.
McKeown Nancy K.
Murchie Scott L.
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