Alteration assemblages in Martian meteorites: Implications for near-surface processes

J. C. Bridges, D. C. Catling, J. M. Saxton, Timothy Swindle, I. C. Lyon, M. M. Grady

Research output: Contribution to journalArticle

204 Citations (Scopus)

Abstract

The SNC (Shergotty-Nakhla-Chassigny) meteorites have recorded interactions between Martian crustal fluids and the parent igneous rocks. The resultant secondary minerals - which comprise up to ∼1 vol.% of the meteorites - provide information about the timing and nature of hydrous activity and atmospheric processes on Mars. We suggest that the most plausible models for secondary mineral formation involve the evaporation of low temperature (25 - 150 °C) brines. This is consistent with the simple mineralogy of these assemblages - Fe-Mg-Ca carbonates, anhydrite, gypsum, halite, clays - and the chemical fractionation of Ca-to Mg-rich carbonate in ALH84001 "rosettes". Longer-lived, and higher temperature, hydrothermal systems would have caused more silicate alteration than is seen and probably more complex mineral assemblages. Experimental and phase equilibria data on carbonate compositions similar to those present in the SNCs imply low temperatures of formation with cooling taking place over a short period of time (e.g. days). The ALH84001 carbonate also probably shows the effects of partial vapourisation and dehydration related to an impact event postdating the initial precipitation. This shock event may have led to the formation of sulphide and some magnetite in the Fe-rich outer parts of the rosettes. Radiometric dating (K-Ar, Rb-Sr) of the secondary mineral assemblages in one of the nakhlites (Lafayette) suggests that they formed between 0 and 670 Myr, and certainly long after the crystallisation of the host igneous rocks. Crystallisation of ALH84001 carbonate took place 0.5 Gyr after the parent rock. These age ranges and the other research on these assemblages suggest that environmental conditions conducive to near-surface liquid water have been present on Mars periodically over the last ∼1 Gyr. This fluid activity cannot have been continuous over geological time because in that case much more silicate alteration would have taken place in the meteorite parent rocks and the soluble salts would probably not have been preserved. The secondary minerals could have been precipitated from brines with seawater-like composition, high bicarbonate contents and a weakly acidic nature. The co-existence of siderite (Fe-carbonate) and clays in the nakhlites suggests that the pCO2 level in equilibrium with the parent brine may have been 50 mbar or more. The brines could have originated as flood waters which percolated through the top few hundred meters of the crust, releasing cations from the surrounding parent rocks. The high sulphur and chlorine concentrations of the Martian soil have most likely resulted from aeolian redistribution of such aqueously-deposited salts and from reaction of the Martian surface with volcanic acid volatiles. The volume of carbonates in meteorites provides a minimum crustal abundance and is equivalent to 50-250 mbar of CO2 being trapped in the uppermost 200-1000 m of the Martian crust. Large fractionations in δ18O between igneous silicate in the meteorites and the secondary minerals (≤30‰) require formation of the latter below temperatures at which silicate-carbonate equilibration could have taken place (∼400°C) and have been taken to suggest low temperatures (e.g. ≥150°C) of precipitation from a hydrous fluid.

Original languageEnglish (US)
Pages (from-to)365-392
Number of pages28
JournalSpace Science Reviews
Volume96
Issue number1-4
DOIs
StatePublished - 2001

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SNC meteorites
Martian meteorite
carbonates
secondary mineral
carbonate
meteorite
minerals
meteorites
brines
silicate
silicates
nakhlites
igneous rocks
rocks
igneous rock
fluid
Mars
mars
clays
crystallization

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

Cite this

Alteration assemblages in Martian meteorites : Implications for near-surface processes. / Bridges, J. C.; Catling, D. C.; Saxton, J. M.; Swindle, Timothy; Lyon, I. C.; Grady, M. M.

In: Space Science Reviews, Vol. 96, No. 1-4, 2001, p. 365-392.

Research output: Contribution to journalArticle

Bridges, J. C. ; Catling, D. C. ; Saxton, J. M. ; Swindle, Timothy ; Lyon, I. C. ; Grady, M. M. / Alteration assemblages in Martian meteorites : Implications for near-surface processes. In: Space Science Reviews. 2001 ; Vol. 96, No. 1-4. pp. 365-392.
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T1 - Alteration assemblages in Martian meteorites

T2 - Implications for near-surface processes

AU - Bridges, J. C.

AU - Catling, D. C.

AU - Saxton, J. M.

AU - Swindle, Timothy

AU - Lyon, I. C.

AU - Grady, M. M.

PY - 2001

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N2 - The SNC (Shergotty-Nakhla-Chassigny) meteorites have recorded interactions between Martian crustal fluids and the parent igneous rocks. The resultant secondary minerals - which comprise up to ∼1 vol.% of the meteorites - provide information about the timing and nature of hydrous activity and atmospheric processes on Mars. We suggest that the most plausible models for secondary mineral formation involve the evaporation of low temperature (25 - 150 °C) brines. This is consistent with the simple mineralogy of these assemblages - Fe-Mg-Ca carbonates, anhydrite, gypsum, halite, clays - and the chemical fractionation of Ca-to Mg-rich carbonate in ALH84001 "rosettes". Longer-lived, and higher temperature, hydrothermal systems would have caused more silicate alteration than is seen and probably more complex mineral assemblages. Experimental and phase equilibria data on carbonate compositions similar to those present in the SNCs imply low temperatures of formation with cooling taking place over a short period of time (e.g. days). The ALH84001 carbonate also probably shows the effects of partial vapourisation and dehydration related to an impact event postdating the initial precipitation. This shock event may have led to the formation of sulphide and some magnetite in the Fe-rich outer parts of the rosettes. Radiometric dating (K-Ar, Rb-Sr) of the secondary mineral assemblages in one of the nakhlites (Lafayette) suggests that they formed between 0 and 670 Myr, and certainly long after the crystallisation of the host igneous rocks. Crystallisation of ALH84001 carbonate took place 0.5 Gyr after the parent rock. These age ranges and the other research on these assemblages suggest that environmental conditions conducive to near-surface liquid water have been present on Mars periodically over the last ∼1 Gyr. This fluid activity cannot have been continuous over geological time because in that case much more silicate alteration would have taken place in the meteorite parent rocks and the soluble salts would probably not have been preserved. The secondary minerals could have been precipitated from brines with seawater-like composition, high bicarbonate contents and a weakly acidic nature. The co-existence of siderite (Fe-carbonate) and clays in the nakhlites suggests that the pCO2 level in equilibrium with the parent brine may have been 50 mbar or more. The brines could have originated as flood waters which percolated through the top few hundred meters of the crust, releasing cations from the surrounding parent rocks. The high sulphur and chlorine concentrations of the Martian soil have most likely resulted from aeolian redistribution of such aqueously-deposited salts and from reaction of the Martian surface with volcanic acid volatiles. The volume of carbonates in meteorites provides a minimum crustal abundance and is equivalent to 50-250 mbar of CO2 being trapped in the uppermost 200-1000 m of the Martian crust. Large fractionations in δ18O between igneous silicate in the meteorites and the secondary minerals (≤30‰) require formation of the latter below temperatures at which silicate-carbonate equilibration could have taken place (∼400°C) and have been taken to suggest low temperatures (e.g. ≥150°C) of precipitation from a hydrous fluid.

AB - The SNC (Shergotty-Nakhla-Chassigny) meteorites have recorded interactions between Martian crustal fluids and the parent igneous rocks. The resultant secondary minerals - which comprise up to ∼1 vol.% of the meteorites - provide information about the timing and nature of hydrous activity and atmospheric processes on Mars. We suggest that the most plausible models for secondary mineral formation involve the evaporation of low temperature (25 - 150 °C) brines. This is consistent with the simple mineralogy of these assemblages - Fe-Mg-Ca carbonates, anhydrite, gypsum, halite, clays - and the chemical fractionation of Ca-to Mg-rich carbonate in ALH84001 "rosettes". Longer-lived, and higher temperature, hydrothermal systems would have caused more silicate alteration than is seen and probably more complex mineral assemblages. Experimental and phase equilibria data on carbonate compositions similar to those present in the SNCs imply low temperatures of formation with cooling taking place over a short period of time (e.g. days). The ALH84001 carbonate also probably shows the effects of partial vapourisation and dehydration related to an impact event postdating the initial precipitation. This shock event may have led to the formation of sulphide and some magnetite in the Fe-rich outer parts of the rosettes. Radiometric dating (K-Ar, Rb-Sr) of the secondary mineral assemblages in one of the nakhlites (Lafayette) suggests that they formed between 0 and 670 Myr, and certainly long after the crystallisation of the host igneous rocks. Crystallisation of ALH84001 carbonate took place 0.5 Gyr after the parent rock. These age ranges and the other research on these assemblages suggest that environmental conditions conducive to near-surface liquid water have been present on Mars periodically over the last ∼1 Gyr. This fluid activity cannot have been continuous over geological time because in that case much more silicate alteration would have taken place in the meteorite parent rocks and the soluble salts would probably not have been preserved. The secondary minerals could have been precipitated from brines with seawater-like composition, high bicarbonate contents and a weakly acidic nature. The co-existence of siderite (Fe-carbonate) and clays in the nakhlites suggests that the pCO2 level in equilibrium with the parent brine may have been 50 mbar or more. The brines could have originated as flood waters which percolated through the top few hundred meters of the crust, releasing cations from the surrounding parent rocks. The high sulphur and chlorine concentrations of the Martian soil have most likely resulted from aeolian redistribution of such aqueously-deposited salts and from reaction of the Martian surface with volcanic acid volatiles. The volume of carbonates in meteorites provides a minimum crustal abundance and is equivalent to 50-250 mbar of CO2 being trapped in the uppermost 200-1000 m of the Martian crust. Large fractionations in δ18O between igneous silicate in the meteorites and the secondary minerals (≤30‰) require formation of the latter below temperatures at which silicate-carbonate equilibration could have taken place (∼400°C) and have been taken to suggest low temperatures (e.g. ≥150°C) of precipitation from a hydrous fluid.

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