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Journal of Geophysical Research Planets 117 (2012) E00J05
Extensive surface pedogenic alteration of the Martian Noachian crust suggested by plateau phyllosilicates around Valles Marineris
Laetitia Le Deit ( ) 1, Jessica Flahaut 2, Cathy Quantin 2, Ernst Hauber 3, Daniel Mège 4, Olivier Bourgeois 4, Joanna Gurgurewicz, M. Masse 5, R. Jaumann 1

Thousands of phyllosilicate-rich outcrops, mainly iron or magnesium-rich are exposed on Noachian terrains in theMartian southern highlands. We analyzed 90 CRISMobservations and more than a hundred HiRISE images located on the plateaus surrounding Valles Marineris. We mapped an extensive Al-and Fe/Mg-phyllosilicate-rich formation covering at least similar to 197,000 km(2), for which we introduce the name "Plateau Phyllosilicates." Tens of meters in thickness, this light-toned formation crops out at various elevations on top of the Noachian units Npl(1) and Npl(2), as flat exposures on plateaus and along scarps such as valley walls, chasma walls, pit walls and impact crater rims. The Fe/Mg-phyllosilicate-rich lower member of the formation is composed of Fe/Mg-smectites (nontronite, saponite) and vermiculite. The Al-phyllosilicate-rich upper member of the formation contains Al-smectites (montmorillonite, beidellite) and locally kaolinite and/or halloysite. We suggest that the Plateau Phyllosilicates were mainly formed by pedogenesis related to the weathering of the Noachian bedrock by percolation of meteoric water or melted snow under a temperate and subarid climate during the Noachian Epoch in an alkaline to neutral environment. Kaolinite and/or halloysite may have formed in areas of more intense drainage at the surface under slightly acidic environments during the Noachian and Hesperian Epochs. Fluvial activity and deuteric alteration may have locally contributed to the genesis of phyllosilicates. This study suggests that the alteration of the Noachian basement of the plateaus surrounding Valles Marineris was widespread during the Noachian Epoch, and was still active during the Hesperian Epoch even though the water availability was limited.
1:  Institute of Planetary Research
German Aerospace Center (DLR)
2:  Laboratoire de Géologie de Lyon - Terre, Planètes, Environnement (LGL-TPE)
CNRS : UMR5276 – INSU – Université Claude Bernard - Lyon I (UCBL) – École Normale Supérieure (ENS) - Lyon
3:  Institute of Planetary Research
Deutsches Zentrum für Luft- und Raumfahrt
4:  Laboratoire de Planétologie et Géodynamique de Nantes (LPGN)
CNRS : UMR6112 – INSU – Université de Nantes
Sciences of the Universe/Earth Sciences