1. GEOCHEMICAL MODELS OF LOW-TEMPERATURE ALTERATION OF MARTIAN ROCKS. Annika
Wallendahl1
and Allan H. Treiman2
. 1
Dept. of Earth Sciences, Montana State Univ, Bozeman, MT 59717 (anni-
kaw@montana.edu). 2
Lunar and Planetary Institute, 3600 Bay Area Boulevard, Houston, TX 77058.
Alteration of martian basalt, T<150°C, is modeled
with a chemical equilibrium computer code. Extensive
alteration at high rock/water will yield: clay + carbon-
ate + qz and near neutral pH water if CO2 = 6mbar;
and clay + serpentine + diopside, gas rich in H2, and
very alkaline water (to pH~12) if CO2 is not externally
buffered.
Introduction. Liquid water may have been stable
on (or near) the surface of ancient Mars, and may now
be stable at depth. We focus on chemical reactions at
low temperature between this water and martian basalt
for their possible effects on storage and generation of
volatiles. We consider temperatures between 0° and
150°C, corresponding to ambient weathering and dis-
tal alteration around hydrothermal centers. Earlier
studies have focused on the higher temperatures asso-
ciated with hydrothermal centers and impact sites [1].
Method. Alteration was modeled with the com-
puter program Geochemist’s Workbench® (GWB)
[2], which uses the LLNL thermochemical database
(like the similar program EQ3/6 [3]). GWB does not
allow for solid solutions.
The composition of the rock reactant was the
Shergotty meteorite [4], which is similar to the mar-
tian dust [5]. Reactions are modeled independent of
kinetics and original mineralogy – in effect, the reac-
tant is glass and transforms completely to product
phases. Alteration temperatures were 0, 25, 60, 100
and 150°C; water:rock mass ratios were 1000:1 to
1:10. Initial ambient gas was taken as the current
martian atmosphere: p(CO2)=0.006 bars; f(O2) =
9x10-6
bar. The reactant water was essentially pure at
pH=7.
Geological Scenarios: Two scenarios were mod-
eled. In the open system scenario, f(CO2) and f(O2)
were fixed to represent reactions open to the martian
atmosphere. In effect, this model assumes that gas
transport to an aquifer is faster than reactions in it.
In the closed system scenario, rock reacts with
water that had equilibrated with martian atmosphere,
but is sealed against further chemical contact with the
atmosphere. This scenario could be realized in an aq-
uifer beneath a permafrost layer [6].
Results: In the open system scenario (fixed gas fu-
gacities), the calculated alteration assemblage is
dominated by clay and carbonate minerals, ~60% and
20% respectively below 100°C (Fig. 1). The exact
minerals vary with temperature and water-rock ratio.
Clay minerals include kaolinite, saponites and nontro-
nites. The latter contain all the reactant’s original iron
(now all ferric); iron oxide / hydroxide minerals were
absent. The solution pH is near neutral (as buffered by
CO2), permitting formation of carbonates. Carbonate
species vary with T: dolomite + magnesite at 0°C;
dolomite alone at 25 and 60°C; calcite at 100°C; and
no carbonate at 150°C; (Fig. 1). At 150°C, the altera-
tion minerals change radically to include those typical
of greenschist facies metamorphism: albite, epidote,
and andradite.
In the closed system scenario, calculated alteration
assemblages are dominated by clay (Reykjanes smec-
tite) and ‘serpentine’ (minnesotaite). Alteration min-
erals and relative abundances do not depend on T
(Fig. 2). The water becomes strongly alkaline (to
pH=11.9) through hydration of silicates, and reducing
through oxidation of ferrous iron. The original CO2 is
converted to methane, up to 0.2 bars at low wa-
ter:rock. Hydrogen gas forms abundantly from reduc-
tion of water, as also occurs on Earth [7] to greater
than 350 bars at 100°C and low water:rock.
Zeolites only appear in the calculated alteration as-
semblages when crystalline silica (quartz, tridymite,
cristobalite, chalcedony) are suppressed. This condi-
tion is not unreasonable in nature, as these phases are
often difficult to nucleate [2]. In the open system sce-
nario, the alteration assemblages contain 17% (mass)
mordenite at 0°C, 16% clinoptilolite at 25°C, and no
zeolites at higher T. In the closed system scenario,
mordenite or clinoptilolite are present from 0 through
150°C, but account for less than 3.5% of the assem-
blage. So, zeolites could potentially be important res-
ervoirs of water in the martian crust [8], but only un-
der the open system scenario at low T.
Comparison with Earlier Work: Griffith &
Shock [1] modeled alteration of a Shergotty composi-
tion at 150°C with no CO2 and low f(O2), comparable
to our closed system. Our calculated results are very
different from theirs; our results for the closed system
included clays and ‘serpentine’, but not chlorite nor
andradite [1]. GWB does not support solid solutions,
which could have been important in stabilizing chlo-
rite, and we infer that [1] did not include clay miner-
als in their model. The presence of andradite is prob-
lematic, both in [1] and in our results for the fixed
fugacity scenario, as it is not common on Earth in
low-temperature alteration. We suspect that its free
energy data are inaccurate.
Conclusions: Gratifyingly, the calculated altera-
tion assemblages are similar to those of weathered
Lunar and Planetary Science XXX 1268.pdf

2. Mars Weathering: Wallendahl A. & Treiman A.
basalt, both terrestrial and martian [9]. Calculated
mineral assemblages and proportions cannot be taken
literally because of uncertainties in the thermochemi-
cal database and GWB’s inability to model solid solu-
tions. Weathering of basalt on Mars can consume sig-
nificant quantities of volatiles: e.g., open system
weathering at 25°C yields an assemblage with ~4%
H2O and ~6% CO2. Abundant production of H2 and
CH4 gas in the closed system model may be realistic,
e.g. [7]. The high pressures of hydrogen may not be
realistic, as they assume a closed, gas-tight, system. If
this scenario is similar to that of an aquifer beneath
permafrost [6], it is possible that significant pressures
of H2 could develop. If this pressure were released
suddenly, it could produce blow-out pits or sand gey-
sers (which could mimic small volcanoes).
This work was performed during a Summer Internship to the
first author at the Lunar and Planetary Institute. Both
authors are grateful for its support. We appreciate assistance
and critical comments from S. Clifford and L. Kirkland.
Supported in part by the Lunar and Planetary Institute and
by NASA grant NAGW-5098.
[1] Griffith L. & Shock E. (1995) Nature 377, 406. Griffith
L. & Shock E. (1997) JGR 102, 9135. [2] Bethke C. (1996)
Geochemical Reaction Modeling. Oxford U. Press. Bethke
C. (1998) Geochemist’s Workbench 3.0, Users Guide. Univ.
Ill. Press. [3] Wolery T. et al. (1992) EQ3/6. LLNL. [4]
Laul J.C. (1986) GCA 50, 909. [5] Baird A.& Clark B.
(1982) Icarus 45, 113. [6] Clifford S. (1993) JGR 98,
10973. [7] Neal C. & Stangel G. (1983) EPSL 66, 315.
Stevens T. & McKinley J. (1995) Science 270, 450. [8]
Jakes P. & Rajmon D. (1998) LPS XIX, Abs. #1627. [9]
Loughnan F. (1986) Chemical Weathering of the Silicate
Minerals. Elsevier. Gooding J. (1992) Icarus 99, 28. Trei-
man A. et al. (1993) Meteoritics 28, 86.
Figure 1. Calculated alteration mineral assemblages from
Shergotty composition glass in the open-system scenario:
p(CO2) = 0.006 bars; f(O2) = 9x10-6
bar. Calcula-
tions at T = 0, 25, 60, 100, and 150°C; water:rock =
1:5. Abbreviations are: M, magnesite; Mica, includes
muscovite & paragonite; Felds includes albite & mi-
crocline; Hm, hematite; Epid, epidote; And, andra-
dite; Clays includes kaolinte and Ca-, Na-, and Mg-
saponites and nontronites. Minerals not labeled in-
clude hydroxylapatite, anhydrite, and pyrolusite.
Note change in carbonate mineralogy at low tem-
peratures.
Figure 2. Calculated alteration mineral assemblages from
Shergotty composition glass in the closed-system scenario.
Calculations at T = 25, 60, 100, and 150°C; water:rock
= 1:5. Clay is Reykjanes smectite, serpentine is actu-
ally minnesotaite. Minerals not labeled include hy-
droxylapatite, tremolite, alabandite, chromite, an-
nite, and tephroite. Note invariance of assemblage,
compared to wide changes in alteration assemblages
in Fig. 1
50
60
70
80
90
100
0 20 40 60 80 100 120 140 160
T ( o
C )
M
Dolomite
Calcite
Clays
Mica
And
Epid
Hm
Felds
Quartz
40
50
60
70
80
90
100
0 20 40 60 80 100 120 140 160
T (
o
C )
Clay
Serpentine
Diopside
Quartz
Lunar and Planetary Science XXX 1268.pdf