Other
Scientific paper
May 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999m%26ps...34..407b&link_type=abstract
Meteoritics & Planetary Science, vol. 34, no. 3, pp. 407-415 (1999).
Other
24
Scientific paper
We report the results of a study of a halite-siderite-anhydrite-chlorapatite assemblage in the SNC (martian) meteorite Nakhla. These minerals are found associated with each other in interstitial areas, halite often being adjacent to or enclosing siderite. We suggest the halite and other minerals are martian in origin because the conditions of fall preclude significant amounts of terrestrial contamination or weathering having taken place; textures indicate that the minerals within this assemblage crystallised at the same stage as some silicate and oxide minerals within the Nakhla parent ; the association with siderite which previous studies have shown has carbon and oxygen isotopic compositions incompatible with an origin on Earth. Siderite has the range of compositions CaCO3 0.1-5.7, MgCO3 2.0-40.9, FeCO3 23.2-87.0, MnCO3 1.0-39.9 mol. %. There are two compositional groupings: high-MnCO3 (( 30 mol %) and low-MnCO3/high-FeCO3, with a gap identified between the two. This may be a miscibility gap or, alternatively, the 2 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 3 siderite and 1 anhydrite grains. Siderite has LREE (2.2 - 7.3 x C1) > HREE (0.32 - 0.79 x C1) 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 (< 200oC) by crystallisation from an aqueous fluid. An origin at low temperature is compatible with the available experimental data on siderite and halite stability. Alternatively, we suggest evaporitic material may have been incorporated into the Nakhla parent whilst melt was still present and crystallised around 800oC. The latter model can more readily explain the trace element abundances and also the siderite textures which imply intergrowth with residual interstitial melt Both high and low temperature models are consistent with the presence of evaporite sediments on Mars.
Bridges John C.
Grady Michael
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