The Mawrth Vallis Phyllosilicates Within a

Regional Context:

Extent, 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.

OutlineOutline

Introduction, PurposeIntroduction, Purpose Spectroscopic/Morphological Spectroscopic/Morphological

evidence for regional-scale aqueous evidence for regional-scale aqueous activityactivity

Formation hypothesesFormation hypotheses

The two largest phyllosilicate exposuresThe two largest phyllosilicate exposures1.1. plains around Mawrth Vallis plains around Mawrth Vallis 2.2. region around Nili Fossaeregion around Nili Fossae

Smaller exposures also identified throughout the highlands Smaller exposures also identified throughout the highlands

Numerous hypothesis have been suggested regarding their Numerous hypothesis have been suggested regarding their formationformation Hydrothermal, lacustrine, diageneticHydrothermal, lacustrine, diagenetic

one thread of observation that appears to be common:one thread of observation that appears to be common: all appear to have been exhumed or excavated from under dark, all appear to have been exhumed or excavated from under dark,

practically unaltered (possibly mafic) material. practically unaltered (possibly mafic) material.

coupled to the global distribution of the observed phyllosilicates, coupled to the global distribution of the observed phyllosilicates, this suggests that this suggests that aqueous alteration was a globally-active aqueous alteration was a globally-active processprocess early in Martian history. early in Martian history.

Point of talk is to:Point of talk is to: place the detailed observations made at Mawrth Vallis into a large place the detailed observations made at Mawrth Vallis into a large

regional contextregional context Evaluate the possible formation hypotheses from this regional Evaluate the possible formation hypotheses from this regional

perspectiveperspective

OMEGA MineralogyOMEGA Mineralogy

50 m

layers

Important questionsImportant questions

Extent of layered hydrated units (is this the Extent of layered hydrated units (is this the tip of the iceberg)?tip of the iceberg)?

Geologic origin of light-toned and dark units?Geologic origin of light-toned and dark units? ImpactImpact VolcaniclasticVolcaniclastic SedimentarySedimentary

AeolianAeolian Lacustrine / marine deposition Lacustrine / marine deposition

Origin of hydrated material?Origin of hydrated material? Aqueous deposition in a lacustrine systemAqueous deposition in a lacustrine system DiagenesisDiagenesis HydrothermalHydrothermal

Unit 1 (bottom)

Chlorite + Nontronite

CRISM

Unit 2 (middle)

Unit 2Mineralogical mixtures

FRT00008438

FRT00008438

Mawrth Vallis Al-phyllosilicates

Kaolinite Montmorillonite

Yellow - Kaolinite, Magenta - Montmorillonite

1.1. ExtentExtentA) Hydrated phyllosilicates are A) Hydrated phyllosilicates are identified everywhereidentified everywhere

that deeper layers have been exhumedthat deeper layers have been exhumed:: extending up to ~1000 km x ~1000 kmextending up to ~1000 km x ~1000 km

B) B) Mineralogical stratigraphy is same throughout Mineralogical stratigraphy is same throughout region:region:

Fe/Mg phyllo. underlies Al-phyllo and/or hydrated silica, which Fe/Mg phyllo. underlies Al-phyllo and/or hydrated silica, which underlies cap unit, underlies cap unit, regardless of elevation (~2000 m. vertical regardless of elevation (~2000 m. vertical change).change). regional processregional process

2.2. Hydrates occur under a Hydrates occur under a dark, boulder-shedding, highly dark, boulder-shedding, highly cratered, and unaltered cap unit, which is cratered, and unaltered cap unit, which is uncomformably overlainuncomformably overlain on layered hydrated units on layered hydrated units

transition from altered to less-altered unitstransition from altered to less-altered units3.3. Presence of Presence of kaolinitekaolinite and other leaching products suggests and other leaching products suggests

leaching leaching significant amounts of water in a warm climatesignificant amounts of water in a warm climate

4.4. We see geomorphic evidence for fluvial activity preserved in We see geomorphic evidence for fluvial activity preserved in the geologic record in the form of channels.the geologic record in the form of channels.

Extensive fluvial activity during and/or after the Extensive fluvial activity during and/or after the deposition of now hydrated units, but before the deposition of now hydrated units, but before the

emplacement of cap unit.emplacement of cap unit.

Summary of observationsSummary of observations

Al/Si-OHAl/Si-OH

Observations:Observations:1.1. Finely layeredFinely layered2.2. Stronger (cliff-former)Stronger (cliff-former)3.3. ~20-40 m thick~20-40 m thick4.4. Contains Al-smectites Contains Al-smectites

mixed w/ kaolinite, mixed w/ kaolinite, hydrated silica, and hydrated silica, and possibly other productspossibly other products

Hypotheses:Hypotheses:1.1. Deposition in a Deposition in a

lacustrine systemlacustrine system2.2. Diagenetic alteration of Diagenetic alteration of

volcanic ashvolcanic ash3.3. PedogenesisPedogenesis

Origin of hydrated unitsOrigin of hydrated units

Fe/Mg - OHFe/Mg - OH

Observations:Observations:1.1. Finely layeredFinely layered2.2. Weak (slope-former)Weak (slope-former)3.3. >200 m thick>200 m thick4.4. Contains Fe/Mg smectites Contains Fe/Mg smectites

mixed w/ Chloritesmixed w/ Chlorites

Hypotheses:Hypotheses:1.1. Deposition in a lacustrine Deposition in a lacustrine

system + metamorphismsystem + metamorphism2.2. Diagenetic alteration of Diagenetic alteration of

volcanic ash + volcanic ash + metamorphismmetamorphism

3.3. Alteration of impact Alteration of impact ejecta ejecta

Geological ScenariosGeological Scenarios

Deposition of sequenceof hydrates (lacustrine)

Deposition of sequenceof primaries

(mafic felsic)

Deposition of mafic primaries

Aqueous alteration Al/Si-OH from felsicFe/Mg smectites from mafic

Alteration to Fe/Mg smectites

Low-T metamorphismat depth chlorites

Leaching of upper layers Al/Si-OH

2.19

2.28

1.37

1.42

Madejova’ et al., 2008

Conclusions:Conclusions: Mineralogical relationships observed near Mawrth Vallis are Mineralogical relationships observed near Mawrth Vallis are

repeated everywhere that we have observed capping unit to be repeated everywhere that we have observed capping unit to be eroded away (up to ~1000 km from Mawrth Vallis), suggesting a eroded away (up to ~1000 km from Mawrth Vallis), suggesting a regional process in their formation. regional process in their formation.

Mineralogic sequence is the same over a wide range in Mineralogic sequence is the same over a wide range in elevations and locationselevations and locations The clays of Mawrth Vallis are common, and representativeThe clays of Mawrth Vallis are common, and representative mineralogical transitions are representative of a regional, if mineralogical transitions are representative of a regional, if not global, processnot global, process

The most likely alternatives for the formation of hydrated The most likely alternatives for the formation of hydrated materials include diagenesis of primary minerals to form materials include diagenesis of primary minerals to form hydrated phyllosilicates, possibly followed by pedogenesis, and hydrated phyllosilicates, possibly followed by pedogenesis, and low-grade metamorphism.low-grade metamorphism.

Morphological evidence suggests that water flowed on the Morphological evidence suggests that water flowed on the surface of Mars during and/or after the deposition of the clay-surface of Mars during and/or after the deposition of the clay-bearing units, but before the deposition of the cap unit.bearing units, but before the deposition of the cap unit.

Cap unit is relatively unaltered, and it overlies altered bedrock Cap unit is relatively unaltered, and it overlies altered bedrock possibly representative of a transition periodpossibly representative of a transition period