Alteration Assemblages in Martian Meteorites: Implications for Near-Surface Processes

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 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 salt a...

Geological Processes and Evolution

“…Martian rocks, as evidenced by Martian meteorites, bear the record of a complex series of processes, both primary igneous processes and secondary processes (weathering, deposition of secondary minerals, cf. Bridges et al, 2001). The return of actual Martian samples to terrestrial laboratories is likely to be required to answer many of the outstanding questions of Martian chronology.…”

A Conspicuous Clay Ovoid in Nakhla: Evidence for Subsurface Hydrothermal Alteration on Mars with Implications for Astrobiology

“…Further evidence for a past wet history of Mars came with the discovery of evaporitic mineral assemblages in Nakhla, such as sulfates, halides, and carbonates (Chatzitheodoridis and Turner, 1990;Gooding et al, 1991;Grady, 1999, 2000;Bailey et al, 2003), along with isotopic evidence that suggests a low temperature of formation for these minerals (Grady et al, 1994, for the ALH84001 carbonates; Leshin et al, 1996;Saxton et al, 2000, for Nakhla carbonates) and the involvement of near-surface processes on Mars (Bridges et al, 2001). The formation of these evaporitic mineral assemblages in Nakhla is directly related to the formation of clays, iron oxides, and oxyhydroxides.…”

Science Priorities for Mars Sample Return

“…Martian meteorites also record a history of fluid alteration as shown by the presence of microscopic clay and carbonate phases (e.g. Gooding et al 1991, McKay et al 1996, Bridges et al 2001. Although the trace element and isotopic variability of the Martian meteorite suite far exceeds that observed in equivalent suites of basalts from Earth and Moon (Borg et al, 2003) (Schultz and Mustard, 2004), volcanic ash (Wilson and Head, 2007) , 2, 3, 4, 6, 7, 8 ( (Davis, 1998;NRC, 2002;Jones et al, 2004 (Warmflash et al, 2007 10.…”

“…Determination of CO 2 and H 2 O abundance and isotopic compositions would lead to a greater understanding of the global inventories and cycling between crust, atmosphere and poles of these compounds. For example, accurate paleotemperatures of hydrothermal systems could be determined from measurements of 18 O/ 16 O isotopic fractionation during water-mineral isotopic exchange in hydrothermal assemblages ND-SAGreport_v202.doc DRAFT-SUBJECT TO REVISION Page 19 of 70 (sampled across Mars or in meteorites) using the isotopic analyses of Martian ice as the starting water reservoir composition (Bridges et al 2001, Valley et al 1997. If a polar landing is not chosen then the regolith sample would take on additional importance as a likely source of the ice.…”