A halite‐siderite‐anhydrite‐chlorapatite assemblage in Nakhla: Mineralogical evidence for evaporites on Mars

Abstract

Abstract-We report the results of a study of a halite-siderite-anhydrite-chlorapatite assemblage in the Shergotty-Nakhla-Chassigny (SNC) Martian meteorite Nakhla. These minerals are found associated with each other in interstitial areas, with halite often being adjacent to or enclosing siderite. We suggest the halite and other minerals are Martian in origin because (1) the conditions of fall preclude significant amounts of terrestrial contamination or weathering having taken place; (2) textures indicate that the minerals within this assemblage crystallized at the same stage as some silicate and oxide minerals within the Nakhla parent; and (3) the association with siderite, which previous studies have shown has C and 0-isotopic compositions incompatible with an origin on the Earth. Siderite has the range of compositions: CaC03, 0.1-5.7; MgC03, 2.0-40.9; FeC03,23.2-87.0; MnC03,l .O-39.9 mol%. There are two compositional groupings: high-MnC03 (230 mol%) and low-MnCO3high-FeCO3, with a gap identified between the two. This may be a miscibility gap or, alternatively, the two compositional groupings may mark separate generations of carbonate. We have not found any textural evidence for the latter explanation, but acceptance of the presence of a miscibility gap would require independent work on Fe-Mn carbonates to verify its existence. Trace element abundances have been determined by ion microprobe analysis on three siderite and one anhydrite grains. Siderite has light rare earth elements (2.2-7.3 x C1) greater than heavy rare earth elements (0.32-0.79 x Cl) without Ce or Eu anomalies, and the anhydrite has a similar pattern. These abundances reflect the source composition rather than partitioning or complexing controls. They are not typical of hydrothermal signatures which generally do not have such smooth REE abundance patterns. The nature of the mineral assemblage suggests that its source rocks on Mars were evaporites. These may be common in the craters and flood plains of the Martian southern highlands. Two models are suggested in this paper to explain the incorporation of evaporitic material into the Nakhla igneous parent. It may have happened as a low-temperature process (<200 "C) by crystallisation from an aqueous fluid. An origin at low temperature is compatible with the available experimental data on siderite stability. Alternatively, we suggest evaporitic material may have been incorporated into the Nakhla parent while melt was still present and crystallized -800 "C. The latter model can more readily explain the trace element abundances and also the siderite textures that imply intergrowth with residual interstitial melt. Both high-and low-temperature models are consistent with the presence of evaporite sediments on Mars.

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“…In contrast most of the siderite in Governador Valadares and Nakhla is located in interstitial positions and has lower Ca contents. These two meteorites contain gypsum, Nakhla also containing anhydrite (Chatzitheodoridis and Turner, 1990;Bridges and Grady, 1999), halite and epsomite (Gooding et al, 1991). Figure 1b shows halite and anhydrite within a section of Nakhla.…”

“…Gooding et al (1988) suggested that the salts within the "lithology C" of EETA79001, which is largely composed of shock melt, might have been entrained from the martian surface into the parent rock during the impact melting event. Evaporite sediments might have been assimilated into the cooling Nakhla parent lava (Bridges and Grady, 1999), although subsequently a direct evaporation model, described in the previous section, was found to be more consistent with the mineralogical features in the 3 nakhlites (Bridges and Grady, 2000). Harvey and McSween (1996) argued that the carbonates in ALH84001 formed through rapid reaction between a CO 2 -rich, H 2 O-poor fluid and the parent rock at >650 • C. Subsequently, Kring et al (1998) suggested that CO 2 -charged fluids at <300 • C had been active in the ALH84001 parent rock for at least a few years.…”

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

“…During fabrication, the thin section was polished with Al 2 O 3 powder from both sides and carefully prepared by using nonpolar solvents in an effort to reduce contamination or dissolution of any soluble material of the meteorite; this is similar to the preparation procedure described by Bridges and Grady (1999). Prior to electron microscopy, the surface of the thin section was carboncoated.…”

“…Between 10 and 11 Ma, a second impact event ejected Nakhla from the martian surface (Ganapathy and Anders, 1969;Eugster et al, 2002), after which it traveled through space and fell to Earth in 1911 at a location situated in northern Egypt, where it was immediately collected Reid and Bunch, 1975). Consequently, Nakhla contains minimal terrestrial contamination (Bridges and Grady, 1999;Jull et al, 2000); therefore, almost all the observed alteration in this rock took place on Mars.…”

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

“…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.…”