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Diagenesis and clay mineral formation at Gale Crater, Mars.

Bridges JC, Schwenzer SP, Leveille R, Westall F, Wiens RC, Mangold N, Bristow T, Edwards P, Berger G - J Geophys Res Planets (2015)

Bottom Line: On the basis of the observed host rock and alteration minerals, we present results of equilibrium thermochemical modeling of the Sheepbed mudstones of Yellowknife Bay in order to constrain the formation conditions of its secondary mineral assemblage.The modeling shows that the mineral assemblage formed by the reaction of a CO2-poor and oxidizing, dilute aqueous solution (Gale Portage Water) in an open system with the Fe-rich basaltic-composition sedimentary rocks at 10-50°C and water/rock ratio (mass of rock reacted with the starting fluid) of 100-1000, pH of ∽7.5-12.We therefore deduce a dissolving composition of approximately 70% amorphous material, with 20% olivine, and 10% whole rock component.

View Article: PubMed Central - PubMed

Affiliation: Space Research Centre, Department of Physics and Astronomy, University of Leicester Leicester, UK.

ABSTRACT

The Mars Science Laboratory rover Curiosity found host rocks of basaltic composition and alteration assemblages containing clay minerals at Yellowknife Bay, Gale Crater. On the basis of the observed host rock and alteration minerals, we present results of equilibrium thermochemical modeling of the Sheepbed mudstones of Yellowknife Bay in order to constrain the formation conditions of its secondary mineral assemblage. Building on conclusions from sedimentary observations by the Mars Science Laboratory team, we assume diagenetic, in situ alteration. The modeling shows that the mineral assemblage formed by the reaction of a CO2-poor and oxidizing, dilute aqueous solution (Gale Portage Water) in an open system with the Fe-rich basaltic-composition sedimentary rocks at 10-50°C and water/rock ratio (mass of rock reacted with the starting fluid) of 100-1000, pH of ∽7.5-12. Model alteration assemblages predominantly contain phyllosilicates (Fe-smectite, chlorite), the bulk composition of a mixture of which is close to that of saponite inferred from Chemistry and Mineralogy data and to that of saponite observed in the nakhlite Martian meteorites and terrestrial analogues. To match the observed clay mineral chemistry, inhomogeneous dissolution dominated by the amorphous phase and olivine is required. We therefore deduce a dissolving composition of approximately 70% amorphous material, with 20% olivine, and 10% whole rock component.

No MeSH data available.


Related in: MedlinePlus

Portage soil reacted with GPW: (a) 10°C and 10% (molar) of the Fe is Fe3+, (b) 50% Fe as Fe3+, (c) and 74.5% of Fe as Fe3+, which is equivalent to all Fe in silicates being Fe3+. Trace phases (below 3%) are not plotted and include apatite for all models. The results of these runs have led us to use the 10% Fe3+ composition in the rest of the models for the whole rock components of our runs.
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fig03: Portage soil reacted with GPW: (a) 10°C and 10% (molar) of the Fe is Fe3+, (b) 50% Fe as Fe3+, (c) and 74.5% of Fe as Fe3+, which is equivalent to all Fe in silicates being Fe3+. Trace phases (below 3%) are not plotted and include apatite for all models. The results of these runs have led us to use the 10% Fe3+ composition in the rest of the models for the whole rock components of our runs.

Mentions: The Fe2+/Fe3+ ratios in the Gale Crater host rocks are not known precisely. Previous studies of Martian water-rock interaction have taken 10% Fe as Fe3+ because of the assumed redox state in the basaltic shergottites, for details see Schwenzer and Kring [2009, 2013[ and Filiberto and Schwenzer [2013[. In order to test the validity of the 10% figure for the whole rock samples studied here, we studied the influence of varying host rock redox conditions with 10–75% Fe as Fe3+ (Figure 3). Because the soil is the most likely host rock to be influenced by other factors, such as evaporite deposition or atmosphere-soil interactions, the test was done on Portage soil. Figure 3a shows the same model as Figure 4c for the ease of comparison. Figures 3b and 3c demonstrate that with increasing Fe3+ content in the host rock, the W/R range at which nontronite is the second most abundant phase extends to much lower W/R, because nontronite is the Fe3+ phase in the precipitate. The amount of stilbite increases at low W/R with increasing Fe3+ content, which is likely to be the result of changes in the solubility of Ca and Si. The overall result of a chlorite-dominated assemblage does not change between the different Fe3+/Fetot ratio runs, and we therefore select the 10% figure for the whole rock component because it is consistent with both our results and previous work on other Martian systems. However, the bulk Fe3+/Fetot ratio in our runs (Figures 5 and 6) varies from 0 to 45% as the proportion of the components, whole rock, amorphous, olivine, whole rock, and plagioclase, is varied.


Diagenesis and clay mineral formation at Gale Crater, Mars.

Bridges JC, Schwenzer SP, Leveille R, Westall F, Wiens RC, Mangold N, Bristow T, Edwards P, Berger G - J Geophys Res Planets (2015)

Portage soil reacted with GPW: (a) 10°C and 10% (molar) of the Fe is Fe3+, (b) 50% Fe as Fe3+, (c) and 74.5% of Fe as Fe3+, which is equivalent to all Fe in silicates being Fe3+. Trace phases (below 3%) are not plotted and include apatite for all models. The results of these runs have led us to use the 10% Fe3+ composition in the rest of the models for the whole rock components of our runs.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4508961&req=5

fig03: Portage soil reacted with GPW: (a) 10°C and 10% (molar) of the Fe is Fe3+, (b) 50% Fe as Fe3+, (c) and 74.5% of Fe as Fe3+, which is equivalent to all Fe in silicates being Fe3+. Trace phases (below 3%) are not plotted and include apatite for all models. The results of these runs have led us to use the 10% Fe3+ composition in the rest of the models for the whole rock components of our runs.
Mentions: The Fe2+/Fe3+ ratios in the Gale Crater host rocks are not known precisely. Previous studies of Martian water-rock interaction have taken 10% Fe as Fe3+ because of the assumed redox state in the basaltic shergottites, for details see Schwenzer and Kring [2009, 2013[ and Filiberto and Schwenzer [2013[. In order to test the validity of the 10% figure for the whole rock samples studied here, we studied the influence of varying host rock redox conditions with 10–75% Fe as Fe3+ (Figure 3). Because the soil is the most likely host rock to be influenced by other factors, such as evaporite deposition or atmosphere-soil interactions, the test was done on Portage soil. Figure 3a shows the same model as Figure 4c for the ease of comparison. Figures 3b and 3c demonstrate that with increasing Fe3+ content in the host rock, the W/R range at which nontronite is the second most abundant phase extends to much lower W/R, because nontronite is the Fe3+ phase in the precipitate. The amount of stilbite increases at low W/R with increasing Fe3+ content, which is likely to be the result of changes in the solubility of Ca and Si. The overall result of a chlorite-dominated assemblage does not change between the different Fe3+/Fetot ratio runs, and we therefore select the 10% figure for the whole rock component because it is consistent with both our results and previous work on other Martian systems. However, the bulk Fe3+/Fetot ratio in our runs (Figures 5 and 6) varies from 0 to 45% as the proportion of the components, whole rock, amorphous, olivine, whole rock, and plagioclase, is varied.

Bottom Line: On the basis of the observed host rock and alteration minerals, we present results of equilibrium thermochemical modeling of the Sheepbed mudstones of Yellowknife Bay in order to constrain the formation conditions of its secondary mineral assemblage.The modeling shows that the mineral assemblage formed by the reaction of a CO2-poor and oxidizing, dilute aqueous solution (Gale Portage Water) in an open system with the Fe-rich basaltic-composition sedimentary rocks at 10-50°C and water/rock ratio (mass of rock reacted with the starting fluid) of 100-1000, pH of ∽7.5-12.We therefore deduce a dissolving composition of approximately 70% amorphous material, with 20% olivine, and 10% whole rock component.

View Article: PubMed Central - PubMed

Affiliation: Space Research Centre, Department of Physics and Astronomy, University of Leicester Leicester, UK.

ABSTRACT

The Mars Science Laboratory rover Curiosity found host rocks of basaltic composition and alteration assemblages containing clay minerals at Yellowknife Bay, Gale Crater. On the basis of the observed host rock and alteration minerals, we present results of equilibrium thermochemical modeling of the Sheepbed mudstones of Yellowknife Bay in order to constrain the formation conditions of its secondary mineral assemblage. Building on conclusions from sedimentary observations by the Mars Science Laboratory team, we assume diagenetic, in situ alteration. The modeling shows that the mineral assemblage formed by the reaction of a CO2-poor and oxidizing, dilute aqueous solution (Gale Portage Water) in an open system with the Fe-rich basaltic-composition sedimentary rocks at 10-50°C and water/rock ratio (mass of rock reacted with the starting fluid) of 100-1000, pH of ∽7.5-12. Model alteration assemblages predominantly contain phyllosilicates (Fe-smectite, chlorite), the bulk composition of a mixture of which is close to that of saponite inferred from Chemistry and Mineralogy data and to that of saponite observed in the nakhlite Martian meteorites and terrestrial analogues. To match the observed clay mineral chemistry, inhomogeneous dissolution dominated by the amorphous phase and olivine is required. We therefore deduce a dissolving composition of approximately 70% amorphous material, with 20% olivine, and 10% whole rock component.

No MeSH data available.


Related in: MedlinePlus