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CLAY MINERALOGY AS A GUIDE TO ALTERATION
ENVIRONMENTS ON MARS
David Bish and David Vaniman
Indiana UniversityLos Alamos National Laboratory
What can clay minerals tell us?
• That liquid water was present!• That alteration took place at (relatively)
low temperatures• Water compositions-open vs closed
hydrologic systems• Whether any post-formation alteration has
taken place (diagenesis, metamorphism)
Cheto bentonite
Tundra
mic
a,
chlo
rite
mix
ed-
laye
r I/S
kaol
inite
hem
atite
, gi
bbsi
te
Prairie
Intensity of W
eathering
Laterite
Mineralogy vs. intensity of weathering(modified from Velde, 1985).
Stability diagram for minerals in the Al2 O3 -SiO2 -H2 O system at 25ºC and 1 atm. The “p” in axis labels refers to –log[].
From Kittrick (1969)
Solutions are supersaturated with the respective phase to the left of or above the solid line.
Smectites vs. Zeolites• Smectites or zeolites can form from
volcanic ash, depending on conditions– smectite in near- or below-neutral pH
conditions– zeolites under alkaline conditions– cannot assume that basaltic ash will always
alter to phyllosilicates– both smectites and zeolites would indicate a
more persistent and evolved hydrogeologic system
Alteration of Volcanic Glass to Clinoptilolite
St. Cloud Mining, Buckhorn, NM
Smectites vs. Kaolin Minerals• Kaolin minerals form on Earth most commonly in tropical
climates, usually under more-acidic conditions and with high water:rock ratios (well drained).
• Can form hydrothermally, accompanied by amorphous silica and TiO2 minerals such as anatase.
• On Mars, a Ti-Si association has been considered to support acid-vapor alteration (Yen et al. 2007)—not unique.
• Detection of 10Å hydrated halloysite, a more hydrated kaolin mineral, on Mars would imply that the mineral had never experienced dehydration after formation, as hydrated halloysite irreversibly dehydrates to a 7.2Å phase under low-RH conditions.
Stability relations of phases in the K2 O-Al2 O3 -SiO2 -H2 O system at 25ºC and 1 atm. Solid circles represent of analyses of waters from
arkosic sediments.From Garrels and Christ (1965)
• Kaolinite stable at low pH, low K, and a(SiO2 ) > qtz
• Mica stable at low a(SiO2 ), med pH, and med- high K
Short-Term Clay Mineral Stability and Sample Return
• ANY change in RH (or T) will change smectites (and other hydrous minerals)– Reversibility of rxn is important– sealed samples that are heated will change
due to change in RH!– as T goes up, RH goes down, etc.
• Large changes in RH or T will have little or no effect on kaolin or chlorite minerals.
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10
11
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16
0 10 20 30 40 50 60 70 80 90 100
Increasing %RH
Decreasing %RH
Relative Humidity (%)
d(00
1) re
flect
ion,
Å
3 4 5 6 7 8 9 10 11 122-Theta (CuK )a
NG-1 Nontronite(Na-Exchanged)
RH Effects
Long-Term Clay Mineral Stability from Mars
• Poorly ordered clay minerals (e.g., smectites and illite/smectites) do not occur in old rocks on Earth--it is often assumed that they gradually transform to more stable phases such as illite, micas, and chlorites.
• The discovery of smectites in Noachian terrains has important implications for the long-term stability of clay minerals and suggests an alternative hypothesis, namely that tectonic activity on Earth eventually results in the progressive alteration of low-temperature minerals to higher-temperature assemblages.
• Smectites on Mars in rocks >3 Gya would rewrite our understanding of clay mineral stability and suggest that, in the absence of (plate) tectonic activity, “metastable” clay minerals may be “stable” for times on the order of the age of our planet.
0
50
100
150
200
250
0 50 100 150 200 250
Time (My)
T (º
C)
Time-temperature limits on clay minerals (modified from Velde, 1992).This figure implies that mixed-layer illite/smectites are not stable over
long times even at low temperatures.
smectite andmixed-layer illite/smectite
illite ± chlorite
Relationship between T and extent of smectite-to-illite reaction(Hower and Altaner, 1983)
What is Lost on Return?
• Hydration states of minerals (not just smectites)
• So-called H2 O of hydration can be reversibly driven off—isotope info lost
• Potential for reaction between minerals– hydrous sulfates can react together– Ca-smectite + epsomite⇒Mg-smectite +
gypsum!
What is Gained by Return?
• Ability to perform analyses not easily done remotely– Clay mineral analyses are best done in the
laboratory– Discrimination between different smectites,
smectite and illite/smectite, stacking order– Isotope analyses (e.g., Ar-Ar)
XRD patterns for air-dry and glycolated smectite (SWy-1)
Lin (Cps)
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100
200
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3100
3200
2-Theta - Scale3 10 20 30 40 50 60
9.6Å
16.9Å
Discrimination of Clay MineralsTreatments used in terrestrial
laboratories will not be accessible on Mars (e.g., saturation with
ethylene glycol vapor).
• The occurrence of clay minerals ⇒ aqueous alteration has occurred.
• Specific clay minerals can put limits on the conditions of mineral formation, e.g.,
–kaolin ⇒ high water:rock, low pH–zeolites ⇒ low water:rock, closed system, high pH–smectites ⇒ open system, med pH, I/S ⇒T
• ID of “old” smectites on Mars can rewrite our understanding of clay stability.
• Clay mineralogy can clarify alteration mechanisms-- the entire mineral assemblage can greatly constrain processes responsible for today’s martian mineralogy.
• Unlike other hydrous minerals, there is no significant loss on sample return.
Summary
Cheto bentonite