Which Clays are Really Present on Mars?

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Which Clays are Really Present on Mars? or Are
you sure about those squiggly lines? Ralph
Milliken (JPL/Caltech)
Clays in Shalbatana Vallis (HiRISE)
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VIS-NIR reflectance spectra can be used to
distinguish between major phyllosilicate
groups - kaolinite, serpentine (11, 7Å) -
smectites, micas (21, 10Å) - chlorites (211,
14Å) However, there are some potential sources
of confusion for distinguishing between different
minerals within the groups.
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Using CRISM and OMEGA, we should be able to
distinguish kaolinite from dickite (high-temp
polymorph) but it may be difficult to tell the
difference between halloysite and a mixutre of
kaolinite a hydrated mineral (e.g. zeolite).
We can also tell the difference between Al-rich
smectites and Mg/Fe-rich smectites but it may
be difficult to tell the difference between the
various types of Mg/Fe smectites due to cation
substitutions.
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Nili Fossae
Not all smectite spectra look similar. Are the
smectite deposits actually smectites, or could
they be mixed-layer smectite/chlorite?
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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.
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What about serpentine?
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)? Both chlorite and greenalite have
broad features centered at wavelengths longer
than 2.3 µm presence of Al can cause additional
features. We need to improve our spectral
libraries for Fe-rich clays!
CRISM Image West of Juventae
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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.
Central mound of crater in southern highlands
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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 Fe2,3-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.
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Uzboi-Ladon-Margaritifer System
Margaritifer
Ladon
Ritchey
Holden
Uzboi
Argyre
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olivine basalt
Fe/Mg clay
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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
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Ladon Basin Stratigraphic variations in clay
signatures
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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
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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 doesnt 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
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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.?
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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 (gt3 Ga?) How do we reconcile the
paucity of end-stage weathering products with the
abundant geomorphic evidence for extensive water
flow over the surface?
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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
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Particle Size - does not have noticeable effect
on position of specific bands - spectra of
large particles may lose weaker bands at long
wavelengths (illite vs mont.) - band strength
is not necessarily directly comparable to
clay abundance Loss of H2O and/or OH - 1.9 µm
H2O band can disappear reversible - H2O bands
can shift during dehydration - can lose metal-OH
bands irreversible - heating can change
structure
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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.
Al-smectite
Fe/Mg-smectite