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Full text of "Aqueous Alteration and Hydrogen Generation on Parent Bodies of Unequilibrated Ordinary Chondrites: Thermodynamic Modeling for the Semarkona Composition"

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Lunar and Planetary Science XXXVI (2005) 


SEMARKONA COMPOSITION. M. Yu. Zolotov', M. V. Mironenko^ and E. L. Shock' ^ 'Department of Geo- 
logical Sciences, ^Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona 85287, 
^Vemadsky Institute of Geochemistry and Analytical Chem., Russian Academy of Sciences, Kosygin Str. 19, Mos- 
cow, Russia. E-mails:,, 

Introduction: Ordinary chondrites are the most abun- 
dant class of meteorites that could represent rocky parts of 
solar system bodies. However, even the most primitive un- 
equilibrated ordinary chondrites (UOC) reveal signs of mild 
alteration that affected the matrix and peripheral zones of 
chondrules. Major chemical changes include oxidation of 
kamacite, alteration of glass, removal of alkalis, Al, and Si 
from chondrules, and formation of phases enriched in halo- 
gens, alkalis, and hydrogen [1,2]. Secondary mineralogical 
changes include formation of magnetite, ferrous olivine, 
fayalite, pentlandite, awaruite, smectites, phosphates, car- 
bonates, and carbides [1,4-10]. Aqueous alteration is consis- 
tent with the oxygen isotope data for magnetite [11]. 

The presence of secondary magnetite, Ni-rich metal al- 
loys, and ferrous silicates in UOC implies that H2O was the 
oxidizing agent. However, oxidation by H2O means that H2 
is produced in each oxidative pathway. In turn, production of 
H2, and its redistribution and possible escape should have 
affected total pressure [12], as well as the oxidation state of 
gas, aqueous and mineral phases in the parent body. 

Here we use equilibrium thermodynamic modeling to 
explore water-rock reactions in UOC. The chemical compo- 
sition of gas, aqueous, and mineral phases is considered. 

Model: The composition of Semarkona (LL3.0) [13] is 
selected to exemplify UOC. Semarkona is one of the best- 
studied chondrites that reveal clear signs of aqueous altera- 
tion [2,3,5,6,8-11]. The calculations are performed with our 
new codes [14] in the 17-component system 0-H-Mg-Fe-Ca- 
Si-Al-C-P-S-Cr-Na-K-Cl-Mn-Co-Ni that also includes 117 
minerals, 21 mineral solid solutions, a non-ideal gas solution 
(7 gases) and a non-ideal aqueous solution (194 species). 
The composition of unaltered Semarkona was estimated by 
decreasing the O abundance so that Fe, Ni, Co, Cr and Mn 
were in the metal/sulfide form and P was in Fes?. 

We explored how the water/rock mass ratio (WRR), total 
pressure (P), and alteration progress affect the composition 
of the gas-aqueous-mineral system, amount of H2 produced, 
and its partitioning among phases. We also explored how P 
may change with alteration progress (also see [14]). Forma- 
tion of CH4 was suppressed because of inhibition of its for- 
mation at low temperatures and pressures, consistent with the 
presence of carbides, aromatic hydrocarbons, and carbonates 
in UOC. Initial modeling was performed for lOCC. 

Results: Chemical equilibration in the rock- water system 
leads to the formation of gas, aqueous solution and secon- 
dary minerals. An aqueous solution forms at WWR > 0.15- 
0.2. H2 is the dominant gas except at P < ~2 bar when 
1120(g) is abundant. Progress of alteration reactions causes 
water consumption through oxidation and hydration, and a 
decrease in WWR among altered parts of the rock. Formation 

of H2 and changes in volumes affect P, unless it is externally 
fixed. Generally, lower WWR and high P represent relatively 
reduced conditions. 

Mineralogy. The composition unaltered chondrite corre- 
sponds to the following normative mineralogy (in mole/kg): 
metalhc alloy (Feo.9103Nio.0652Cro.02i6Coo.0028), 3.227; troilite, 
0.5920; forsterite, 0.9531; pyroxene (Di, 0.33085; En, 
2.1669); feldspar (An, 0.05665; Ab, 0.33, Mc, 0.0226); ala- 
bandite, 0.0526; schreibersite, 0.0360; lawrencite, 3.005-10"'; 
and graphite, 0.5055. The following minerals are observed in 
the calculated assemblages: Fe-depleted metal alloys, mag- 
netite, pyrrhotite, chromite, olivine of different compositions 
(Foioo-Foo), enstatite-diopside, diopside-hedenbergite, rho- 
donite, andradite, saponite, chlorite, serpentine, whitlockite 
and halite. 

Ni-Co-rich metal is among the major secondary phases, 
especially at low WRR (< 0.1) and P > -2-30 bar (depending 
on WRR). The Fe/(Co+Ni) ratio decreases with increasing 
WWR and decreasing P. Small amounts of Co and Co-Ni 
metals form in the presence of aqueous solution at WRR > 
-0.2, and the Co/Ni ratio decreases as P increases. 

Magnetite is typically present in low-P, relatively oxidiz- 
ing conditions (low /H2) and at moderate WRR. At high 
WRR, ferrous serpentines and chlorites form instead of mag- 
netite, which only forms at P < ~5 bar. At low WRR, iron is 
mainly present as metal. However at P > -30 bar, a small 
amount of magnetite can be associated with pure fayalite that 
forms in a limited range of WRR (around 0.1). At lower 
WRR, forsteritic olivine (F093.100) is stable and ferrous hy- 
drosilicates become abundant at higher WWR. 

Secondary Ca-Mg-Fe pyroxenes are mostly represented 
by diopside-hedenbergite in which Fe/(Mg+Fe) ratio reaches 
0.34 at elevated WRR and P. In the absence of an aqueous 
solution (WRR < -0.2), much lower Fe contents are typical. 
Andradite is a minor phase that often associates with mag- 
netite at low P and high WRR conditions in the presence of a 

Saponites are present in all calculated assemblages and 
become abundant at moderate WRR (-0.02-0.2) where solu- 
tion is absent and at lower P. Typical saponites are Na-rich 
with K content of -8 mol %. Mg-rich saponites are only 
abundant at WWR < 0.05. Compared to smectites, serpen- 
tines and chlorites are more common at elevated WWR and 
P. Although Mg-rich phases are abundant at WRR ~ 0.05-0.2 
without solution and at low P, increases in WWR and P 
causes increases in the Fe/(Mg+Fe) ratio to 0.3 and 0.8 in 
serpentine and chlorite, respectively. Chlorites are about 10 
times less abundant than serpentines. 

Ni sulfide (here exemplified by NiS) forms at WRR > 
-0.2 in the presence of solution atP < -20-30 bar. Formation 

Lunar and Planetary Science XXXVI (2005) 


of Ni sulfide corresponds to a decrease in Ni/Co ratio in 
secondary metal alloys. Pyrrhotite and Mn silicate (exempli- 
fied by rhodonite) form at mildly oxidizing conditions at 
elevated WRR (> -0.1) and P < -20-30 bar. Otherwise, pri- 
mary troilite and alabandite remain stable in secondary as- 

Chromite and whitlockite are present in all calculated 
secondary associations, indicating a strong drive for oxida- 
tion of Grand P in asteroids. 

Although graphite remains stable, relatively high CO3 ' 
activity (-10 ) is observed in the solutions. This implies that 
carbonates could deposit once the solution is consumed. 
Formation of FcsC is not observed. 

Halite forms at WRR < 0.2 from concentrated solutions 
and always present in "dry" secondary assemblages. This is 
the only Cl-bearing mineral formed. 

Solution chemistry and the pH: All solutions are rich in 
Na and have alkaline pH. At P < -2 bar and/or concentrated 
{WRR - 0.15-0.2) solutions, Cl-Na-K-bearing species (CI", 
Na+, NaCl(aq), K+, CaCl2(aq), KCl) dominate at pH 8-10. At 
higher P and WRR, Na+, OH", NaOH(aq), NaHSi03(aq), HS", 
K , and KOH are the most abundant solutes at pH 1 1-12. 

H 2 production and fate. Since water is the only oxidizing 
agent, H2 is produced in all oxidation pathways: (1) Fe-rich 
metal to magnetite, ferrous silicates (olivine, hedenbergite, 
serpentine and chlorite), Ni sulfide and chromite; (2) 
schreibersite to whitlockite; (3) troilite to pyrrhotite, and (4) 
alabandite to rhodonite. Iron phosphides, Cr" and Fe° are 
unstable with respect to the oxidation that generates H2 even 
at small (> -0.05) WWR. Oxidation of Fe and Mn sulfides, 
and Ni° requires more oxidizing conditions that correspond 
to low P and/or H20-rich environments. The calculations 
show that H2 is produced until consumption of either H2O or 
the reduced mineral reactants. The maximum amount of H2 
can be produced if a bulk water-chondrite balance is close to 
the stoichiometry of the Fe° oxidation reactions that yield 
Fe * and magnetite. More H2 is produced if magnetite forms. 
However, formation of OH-bearing minerals (e.g., smectites) 
may confine the amount of H2 released. 

Although H2 is preferentially concentrated in the gas 
phase, the H2(gas)/H2(aq) ratio decreases as P increases, e.g. 
due to progress of oxidation reactions. The activity of H2(aq) 
is -lO"'' ofyH2, which is close to total P at high pressures. 
The H2(g)/H20(g) ratio also increases with P and alteration 
progress because of water condensation (mostly from 1.01 
bar to -5 bar), water consumption through oxidation and 
hydration, and H2 production. 

Increases in the volume of solids and H2 production con- 
tribute into the increase in P above that of water saturation. 
In addition, total P depends on the initial porosity. Our re- 
sults show that low initial porosity would lead to explosive 
destructions of asteroids (at -10^ bars [12]) even at low de- 
grees of alteration progress, as can be seen in Fig. la in [14]. 
Note that production of H2 and total P are limited by the 
amount of incorporated water ice. 

Alteration Scenario and Discussion: Here we summa- 
rize our results of a possible alteration scenario (see also 

[14]). Following the melting of ice, aqueous alteration in 
UOC started with interaction of water with mineral surfaces. 
This first stage was characterized by low pressure (equal to 
that of water saturation) and a dilute solution (because of 
locally high WRR) that mostly contained Na, CI, OH", HS", K 
and H2(aq). Magnetite, Fe-Mg serpentine, pyrrhotite, Ni- 
sulfide, chromite, and whitlockite formed. Further alteration 
caused consumption of water though hydration and H2 pro- 
duction, and increases in the volume of solids, /H2, and in 
total P (if gas was unable to migrate). Smectites formed at 
the expense of serpentine. An increase in total P caused con- 
densation of H2O and dilution of solution for a while; then 
the solution became saline, Cl-rich, and halite precipitates. 
Once the solution was consumed, further equilibration in a 
close system caused interaction of original (metal-rich) min- 
eral assemblages with secondary products of oxidation and 
hydration. Magnetite, serpentine, and chlorite were partially 
converted to olivine (including pure fayalite) and metal, 
while smectites remained abundant. In other words, even in 
an isothermal regime, early-formed oxidized minerals could 
be converted to more reduced secondary minerals, and early- 
formed hydrous silicates can dehydrate. These conversions 
decreased the volume of solids, /H2 and P. Note that reduc- 
tion and dehydration at high degrees of alteration progress 
were only possible at low ice/rock ratios and low porosities, 
which were typical for ordinary chondrites. 

Comparison of model results with observations [1-10] 
could be used to evaluate conditions of alteration. In brief, 
enrichments of altered solids in Na, K, and CI, the presence 
of smectites among secondary minerals in UOC, along with 
unaltered grains and cores of chondrules, are consistent with 
a low amount of incorporated ice {WWR - 10" -10" ), and 
only partial re-equilibration after consumption of solution. 
Our models shows that high-pH solutions rich in Na, OH", 
CI, HS" and H2(aq) were typical for parent bodies of UOC 
and may characterize ocean-forming fluids on large bodies. 

Acknowledgments: This work is supported by NASA 
grants from Origins, Outer Planets Research and Exobiology 

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