The Mawrth Vallis Phyllosilicates Within a Regional Context: Extent, stratigraphy, and mineralogy of Phyllosilicates around Mawrth Vallis and Western Arabia Terra

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The Mawrth Vallis Phyllosilicates Within a
Regional ContextExtent, stratigraphy, and
mineralogy of Phyllosilicates around Mawrth
Vallis and Western Arabia Terra
E. Z. Noe Dobrea, J.L. Bishop, N.K. McKeown, R.
Fu, C. Rossi, G. Swayze, J.R. Michalski, F.
Poulet, J.-P. Bibring, J.F. Mustard R. Arvidson,
R.V. Morris, S. Murchie, A.S. McEwen, E. Malaret,
C. Hash, and the CRISM Team.
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Outline

• Introduction, Purpose
• Spectroscopic/Morphological evidence for
  regional-scale aqueous activity
• Formation hypotheses

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• The two largest phyllosilicate exposures
• plains around Mawrth Vallis
• region around Nili Fossae
• Smaller exposures also identified throughout the
  highlands
• Numerous hypothesis have been suggested regarding
  their formation
• Hydrothermal, lacustrine, diagenetic
• one thread of observation that appears to be
  common
• all appear to have been exhumed or excavated from
  under dark, practically unaltered (possibly
  mafic) material.
• coupled to the global distribution of the
  observed phyllosilicates, this suggests that
  aqueous alteration was a globally-active process
  early in Martian history.
• Point of talk is to
• place the detailed observations made at Mawrth
  Vallis into a large regional context
• Evaluate the possible formation hypotheses from
  this regional perspective

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OMEGA Mineralogy
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Important questions

• Extent of layered hydrated units (is this the tip
  of the iceberg)?
• Geologic origin of light-toned and dark units?
• Impact
• Volcaniclastic
• Sedimentary
• Aeolian
• Lacustrine / marine deposition
• Origin of hydrated material?
• Aqueous deposition in a lacustrine system
• Diagenesis
• Hydrothermal

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Unit 1 (bottom)
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Unit 2 (middle)
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Unit 2Mineralogical mixtures
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FRT00008438
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FRT00008438
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Mawrth Vallis Al-phyllosilicates
Yellow - Kaolinite, Magenta - Montmorillonite
Kaolinite
Montmorillonite
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Summary of observations

• Extent
• A) Hydrated phyllosilicates are identified
  everywhere that deeper layers have been exhumed
• extending up to 1000 km x 1000 km
• B) Mineralogical stratigraphy is same
  throughout region
• Fe/Mg phyllo. underlies Al-phyllo and/or
  hydrated silica, which underlies cap unit,
  regardless of elevation (2000 m. vertical
  change).
• ? regional process
• Hydrates occur under a dark, boulder-shedding,
  highly cratered, and unaltered cap unit, which is
  uncomformably overlain on layered hydrated units
• ? transition from altered to less-altered units
• 3. Presence of kaolinite and other leaching
  products suggests leaching
• ? significant amounts of water in a warm
  climate
• 4. We see geomorphic evidence for fluvial
  activity preserved in the geologic record in the
  form of channels.
• ? Extensive fluvial activity during and/or after
  the deposition of now hydrated units, but before
  the emplacement of cap unit.

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Origin of hydrated units

• Al/Si-OH
• Observations
• Finely layered
• Stronger (cliff-former)
• 20-40 m thick
• Contains Al-smectites mixed w/ kaolinite,
  hydrated silica, and possibly other products
• Hypotheses
• Deposition in a lacustrine system
• Diagenetic alteration of volcanic ash
• Pedogenesis

• Fe/Mg - OH
• Observations
• Finely layered
• Weak (slope-former)
• gt200 m thick
• Contains Fe/Mg smectites mixed w/ Chlorites
• Hypotheses
• Deposition in a lacustrine system metamorphism
• Diagenetic alteration of volcanic ash
  metamorphism
• Alteration of impact ejecta

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Geological Scenarios
Deposition of mafic primaries
Deposition of sequence of primaries (mafic ?
felsic)
Deposition of sequence of hydrates (lacustrine)
Aqueous alteration ? Al/Si-OH from felsic ?Fe/Mg
smectites from mafic
Alteration to Fe/Mg smectites
Low-T metamorphism at depth ? chlorites
Leaching of upper layers ? Al/Si-OH
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1.42
2.19
1.37
2.28
Madejova et al., 2008
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Conclusions

• Mineralogical relationships observed near Mawrth
  Vallis are repeated everywhere that we have
  observed capping unit to be eroded away (up to
  1000 km from Mawrth Vallis), suggesting a
  regional process in their formation.
• Mineralogic sequence is the same over a wide
  range in elevations and locations
• ? The clays of Mawrth Vallis are common, and
  representative
• ? mineralogical transitions are representative
  of a regional, if not global, process
• The most likely alternatives for the formation
  of hydrated materials include diagenesis of
  primary minerals to form hydrated
  phyllosilicates, possibly followed by
  pedogenesis, and low-grade metamorphism.
• Morphological evidence suggests that water flowed
  on the surface of Mars during and/or after the
  deposition of the clay-bearing units, but before
  the deposition of the cap unit.
• Cap unit is relatively unaltered, and it overlies
  altered bedrock
• ? possibly representative of a transition period