Untitled

Abstract

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 o 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 Ma, and certainly long after the crystallisation of the host igneous rocks. Crystallisation of ALH84001 carbonate took place 0.5 Ga 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 Ga. 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 pCO 2 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 metres of the crust, releasing cations from the surrounding parent rocks. There is no spectroscopic or photographic evidence for extensive evaporite deposits on the current martian surface but some salt precipitation through solute concentration must have occurred as floodwaters associated with many of the martian channels ponded in low-lying areas. 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. However, the SNC...

Geochemistry of Carbonates on Mars: Implications for Climate History and Nature of Aqueous Environments

“…All of these samples are igneous rocks and show some degree of weathering, frequently in the form of carbonate minerals and in low abundance (1 % or less) (Bridges et al 2001). A strong case has been made that at least some of these samples contain carbonate that formed on Mars and thus preserve the chemical signatures of aqueous environments on that planet.…”

“…The carbonates typically occur as small crystals (<100 µm) along cracks and within crushed zones of the rocks. They are sometimes associated with other aqueous alteration phases such as phyllosilicates, iron oxides, and iron sulfides (Bridges et al 2001). Bridges et al (2001).…”

WatSen: searching for clues for water (and life) on Mars

“…Carbonates are widely predicted to occur because the main constituent of Mars' atmosphere is CO 2 and this dissolves readily in water, giving a solution from which carbonate can precipitate (Equation (1)). However, apart from in minor abundances in Martian meteorites (Bridges et al 2001), and in dust (Bandfield et al 2003), carbonates have not been observed in quantity at the Martian surface. Carbonate precipitation is highly dependent on the pH of the fluid in which CO 2 is dissolved.…”

“…The rocks are all igneous, and the effects of water can be seen in some of the meteorites (Fig. 2), where two distinct types of alteration product can be found (Bridges & Grady 1999;Bridges et al 2001). One alteration product is exemplified by 'iddingsite ', a fine-grained mixture of the clay minerals smectite and illite, produced by the weathering along cracks within olivine grains.…”

“…One alteration product is exemplified by 'iddingsite ', a fine-grained mixture of the clay minerals smectite and illite, produced by the weathering along cracks within olivine grains. The second group of alteration products is of precipitated salts, including sulphates, chlorides and chemically and isotopically zoned carbonates (Bridges et al 2001). The fact that the secondary minerals are found in meteorites that were collected immediately following their observed fall implies that the meteorites were altered by weathering on Mars' surface.…”