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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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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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