Transcript Document

Which Clays are Really Present on Mars?
or
Are you sure about those squiggly lines?
Ralph Milliken (JPL/Caltech)
50 m Clays in Shalbatana Vallis (HiRISE)
VIS-NIR reflectance spectra can be used
to distinguish between major
phyllosilicate groups:
- kaolinite, serpentine (1:1, 7Å)
- smectites, micas (2:1, 10Å)
- chlorites (2:1+1, 14Å)
However, there are some potential
sources of confusion for distinguishing
between different minerals within the
groups.
1
1
2.2 µm
0.8
0.8
0.6
0.6
Reflectance
Reflectance
Smectites
0.4
0.2
Muscovite: Al
Phlogopite: Mg
Biotite: Mg, Fe(II)
Celadonite: Al, Fe(III), Mg, Fe(II)
Illite: Al
Micas
0.4
0.2
Montmorillonite: Al
Nontronite: Fe(III)
Hectorite: Mg
Saponite: Mg, Fe(II)
2.31 µm
0
0
0.50
1.0
1.5
Wavelength (µm)
2.0
2.5
1.0
1.5
2.0
Wavelength (µm)
2.5
Kaolinite Group
1
Using CRISM and OMEGA, we should be
able to distinguish kaolinite from dickite
(high-temp polymorph)…
Reflectance
0.8
0.6
but it may be difficult to tell the difference
between halloysite and a mixutre of
kaolinite + a hydrated mineral (e.g. zeolite).
0.4
0.2
Dickite: Al
Kaolinite: Al
Halloysite: Al
0
0.50
1.0
1.5
2.0
2.5
Wavelength (µm)
1
Fe/Mg-bearing Smectites
Nontronite
Nontronite w / Mg
Nontronite w / Mg
Saponite
Hisingerite: Fe(III)
We can also tell the difference between
Al-rich smectites and Mg/Fe-rich
smectites…
Reflectance
0.8
0.6
but it may be difficult to tell the difference
between the various types of Mg/Fe
smectites due to cation substitutions.
0.4
0.2
0
0.50
1.0
1.5
Wavelength (µm)
2.0
2.5
Not all ‘smectite’ spectra look similar.
Are the ‘smectite’ deposits actually smectites,
or could they be mixed-layer
smectite/chlorite?
Smectites vs. S/C
1
Nontronite
1.4
Mawrth
Gale Crater
0.8
1.36
0.6
1.32
1.28
0.4
Mixed-Layer Smectite/Chlorite
1.24
Nili
0.2
1.2
0
2.1
2.2
2.3
2.4
Wavelength, µm
2.5
2.6
CRISM I/F Ratio, Nili
CRISM I/F Ratio, Mawrth & Gale
Nili Fossae
Burial Diagenesis of Clays
On Earth, burial can (and often
does) lead to transitions in clay
structures and compositions.
Observations of smectite
changing to illite or chlorite with
depth on Mars can inform us
about temperature and fluid
chemistry.
Smectite, mixed-layer S/C, &
chlorite have been observed in
CRISM data.
1.3
0.6
Chlorite (Clinochlore)
1.29
0.5
0.4
1.27
1.26
0.3
1.25
0.2
CRISM
1.24
0.1
1.23
1.22
0
2
2.1
2.2
2.3
Wavelength (um)
2.4
2.5
Reflectance
CRISM I/F Ratio
1.28
What about serpentine?
CRISM Image
West of Juventae
Chlorite has been detected in the walls of V. Marineris, Nili,
and throughout the southern highlands (e.g., Mustard et al., 2008).
However, could some of these chlorite detections be confused
with Fe-serpentines (e.g. greenalite)?
We need to improve our spectral libraries for Fe-rich clays!
Fe2+ Alteration Products?
1.2
1.02
1
1
CRISM
0.98
0.8
0.96
0.6
0.94
Greenalite? (serpentine)
Clinochlore (chlorite)
0.92
Chamosite (chlorite)
0.4
0.9
1
1.2
1.4
1.6
1.8
2
Wavelength (um)
2.2
2.4
2.6
Reflectance
CRISM I/F Ratio
opaline silica
Both chlorite and greenalite have broad features centered at
wavelengths longer than 2.3 µm; presence of Al can cause
additional features.
Smectite to Illite transition: Geothermometer for Martian Crust?
Montmorillonite detections may have significant amounts of interlayered illite.
To date, there have been very few detections of illite/muscovite on Mars.
- chlorite is more dominant, likely related to low abundance of K+ on Mars
However, we need to search for possible smectiteillite transitions because
illitization can be used as a geothermometer to constrain crustal heat flow.
1.4
1
Central mound of crater in southern highlands
Smectite
(montmorillonite)
1.35
1.3
Illite/Smectite
(60/40)
Illite/Smectite
(70/30)
0.5
CRISM I/F Ratio
Reflectance (offset)
1.25
1.2
Illite
1.15
0
1.1
CRISM
1.05
1
-0.5
1
1.5
2
Wavelength (um)
2.5
VIS-NIR spectra can be used to distinguish between major phyllo groups:
- smectite, mica/illite, chlorite, kaolinite, serpentine
Potential sources of confusion:
- saponite & nontronite with Fe+2,+3-Mg substitutions (trioctahedral vs. dioctahedral)
- illite & muscovite (but other micas are spectrally distinct)
- physical mixtures of clays versus mixed-layered clays? (TBD)
- sepiolite versus Mg-bearing smectites such as saponite
Different clays have implications for environment and fluid chemistry, so we must be
careful when assigning names and inferred chemistry to orbital clay detections.
1
1.5
Mica (Celadonite)
Saponite
0.8
Chlorite (Clinochlore)
1
Illite-Smectite (Rectorite)
Illite
Smectite (Montmorillonite)
0.5
Reflectance
Reflectance, offset
Sepiolite
0.6
0.4
Kaolin (Kaolinite)
0.2
0
0
1.0
1.5
2.0
Wavelength (µm)
2.5
1.0
1.5
2.0
Wavelength (µm)
2.5
Uzboi-Ladon-Margaritifer System
Margaritifer
Ladon
Ritchey
Holden
Uzboi
Argyre
CRISM: Mg-Rich Clays
1.15
0.6
Fe/Mg clay
0.4
0.2
1.05
0
1
-0.2
CRISM
Sepiolite (Mg clay)
Nontronite (Fe clay)
-0.4
Smectite-Chlorite (Fe/Mg clay)
Saponite (Mg clay)
0.95
0.5
1
1.5
Wavelength (µm)
2
2.5
Reflectance, offset
olivine basalt
CRISM I/F Ratio
1.1
clays
(more H2O)
clays
(less H2O)
V.E . = 2
Clay signatures are strongest at the bottom of the
stratigraphic section & spectrally similar to clays in the
crater rim: source to sink
V.E.=2
Ladon Basin: Stratigraphic variations in clay signatures
CRISM FRT8076
1
CRISM
CRISM
1.1
0.8
0.6
1.06
0.4
1.04
0.2
Nontronite
Nontronite w / Mg
Saponite
Hisingerite
1.02
0
1
1.5
2
Wavelength (µm)
2.5
Reflectance
CRISM I/F Ratio
1.08
Physical versus Chemical Weathering
Chlorite is concentrated
at high latitudes where
physical, not chemical,
weathering is dominant
(chlorite present in
source rocks).
Eslinger & Pevear (1988)
Data from the Mars landing
sites indicates that there is
minimal chemical
segregation….evidence
that physical weathering is
dominant?
Provided by Joel Hurowitz, JPL
Clays Formed in the Hesperian?
Clays are present in Hesperian-age deposits…..but are they authigenic or detrital?
- We are looking at alteration products, not primary minerals.
- Just because clays are found in Noachian aged units doesn’t mean that they
formed in the Noachian.
- Noachian crust is heavily cratered, fractured, and materials likely have high surface
area to volume ratios, this will favor alteration, especially with low water-to-rock ratios.
HiRISE color
So what do we know?
OMEGA & CRISM have ‘definitively’ detected:
nontronite (reducing conditions)
montmorillonite
chlorite (w/ Al)
illite/muscovite
kaolin mineral (kaolinite or ‘halloysite’)
OMEGA & CRISM have also detected:
Mg-clay: saponite, sepiolite, something else?
Mixed-layer clays: smectite/chlorite? smectite/illite?
Analcime (or some other zeolite?)
Clays are most commonly found in the ancient cratered terrains (Noachian).
Some clays have been transported by fluids and deposited as sedimentary rocks.
Some clays are associated with Hesperian sulfates (V. Marineris, Gale): not acidic!
Majority of clays are the Mg/Fe varieties, but many show evidence of Al substitution.
‘Chlorite’ much more common than illite: related to availability of K, Na, etc.?
How do we distinguish between formation and depositional environments?
On Earth, the vast majority of clays in sediments are detrital.
- plate tectonics, crustal recycling
- average crustal composition is granitic
- large fraction of clays are derived from pre-existing sedimentary rocks
- contribution from soils and the role of organics
- clays on Earth are primarily a story of erosion, transport, and (re)deposition.
The same is likely true for Mars, with some important differences:
- no plate tectonics or crustal recycling
- average crustal composition is basaltic
- hydrated minerals are commonly Fe/Mg varieties
This is consistent with low water-to-rock ratios, but does it require this?
- minimal leaching or continuous fluid flow (kaolinite, gibbsite not dominant)
- chlorites at the surface suggest physical, not chemical, weathering has been
dominant since their exposure (>3 Ga?)
How do we reconcile the paucity of end-stage weathering products with the abundant
geomorphic evidence for extensive water flow over the surface?
Role of Excess Cations
Formation of smectite requires a lot of silica.
Clay deposits on Mars are often not
associated with other hydrated minerals or
alteration products (at least not that we see
from orbit).
However, dissolution of basalt and
precipitation of smectites would result in a
significant excess in cations….where is the
complementary salt?
Possibilities are OH-, Cl-, SO3, SO4, CO3
Determining this phase is the key to
understanding the atmospheric chemistry,
oxidation state, and fluid chemistry on early
Mars.
[Figure provided by Joel Hurowitz, JPL/Caltech]
1
Particle Size
Montmorillonite: Particle Size Effects
- does not have noticeable effect on position
of specific bands
Reflectance
0.8
- spectra of large particles may lose weaker
bands at long wavelengths (illite vs mont.)
0.6
- band strength is not necessarily directly
comparable to clay abundance
0.4
0-45 µm
45-75 µm
75-125 µm
125-250 µm
250-500 µm
>500 µm
0.2
Loss of H2O and/or OH
0
0.50
1.0
1.5
2.0
2.5
Wavelength (µm)
- 1.9 µm H2O band can disappear; reversible
- H2O bands can shift during dehydration
- can lose metal-OH bands; irreversible
- heating can change structure
1
1
Heated Fe-Sm ectite
0.8
0.8
0.6
0.6
Reflectance
Reflectance
Nontronite: Particle Size Effects
0.4
0.4
0.2
0.2
0-25 µm
25-45 µm
45-75 µm
75-125 µm
125-250 µm
Nontronite, heated to 500°C
Nontronite, unheated
Hisingerite
0
0
0.50
1.0
1.5
Wavelength (µm)
2.0
2.5
0.50
1.0
1.5
Wavelength (µm)
2.0
2.5
Clays in the Noachian Crust
CRISM spectra exhibit Al-OH, Mg/Fe-OH, and H2O absorption features,most consistent
with smectites (montmorillonite, nontronite, saponite).
Mapping the band depths of these features suggests they generally occur in separate
locations, but a closer inspection of the spectra suggests some regions are mixtures of
these clays.
These deposits are mineralogically similar to those in Mawrth Vallis.
Clays South of Eos Chasma (CRISM HRL67F1)
1.07
Al-smectite
1
Fe/Mg-smectite
0.9
Mg/Fe smectite
Al smectite
1.06
0.7
1.05
0.6
1.04
0.5
0.4
1.03
Montmorillonite
Nontronite
0.3
1.02
0.2
1
1.5
2
Wavelength (µm)
2.5
Reflectance
CRISM I/F Ratio
0.8