Post by Anne Gaudin, University Nantes, CNRS, Laboratoire LPGN, France
On Earth, weathering profiles that have developed in ultramafic rocks under tropical climate show a mineralogical transition between a Fe, Mg-rich smectite zone and an Al-rich kaolinite-bearing zone (e.g. Colin et al., 1990; Gaudin et al., 2005; Yongue-Fouateu et al., 2009). This evolution is due to an intense leaching of Mg2+ cations during the weathering process. The Murrin Murrin (MM) site is an example of such a profile located in the Archean Eastern Yilgarn Craton, in Western Australia. The MM profile is developed in serpentinized peridotite massifs over a 40 m thick sequence (Image 1) and shows three zones: serpentinized peridotites at the bottom, immediately overlain by Fe/Mg-bearing smectites and then Al-bearing phyllosilicates (kaolinite) mixed with iron hydroxides.
This mineralogical sequence is similar to that spectrally detected on several regions on Mars (e. g. Ehlmann, 2009; Loizeau et al., 2007; Noe Dobrea et al., 2010). Data from the the Nili Fossae region illustrates this (Image 2). The Nili Fossae bedrock, detected as excavated material in crater fill, indicates an olivine-rich composition suggesting an ultramafic nature. As in Murrin Murrin, the altered materials observed
The origin of phyllosilicates detected on Mars is still under discussion and leading hypotheses include diagenesis, hydrothermalism and weathering processes. By analogy, we propose that the mineral assemblage locally detected on Mars is the result of long-term weathering under a climate conditions that were significantly warmer than present.
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