Mathematics – Logic
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufm.p71c0470n&link_type=abstract
American Geophysical Union, Fall Meeting 2002, abstract #P71C-0470
Mathematics
Logic
1040 Isotopic Composition/Chemistry, 1060 Planetary Geochemistry (5405, 5410, 5704, 5709, 6005, 6008), 3662 Meteorites, 6215 Extraterrestrial Materials, 6225 Mars
Scientific paper
The Martian meteorite ALH84001 contains a small amount of carbonate that was deposited from aqueous fluids on the Martian surface approximately 3.9 Ga.. McKay et al. (1996) proposed evidence for the existence of life preserved within the carbonate grains. In order to determine the nature of the ancient Martian aqueous system we have combined previously collected oxygen isotopic data with new carbon isotopic measurements performed on the Cameca 6f ion microprobe at Arizona State University. Isotopic measurements were made at high mass resolution with a spot size of 10 microns. The measured carbon isotopic values range from 29.2‰ to 64.5‰ (PDB) with an average uncertainty of +/-1.6‰ (1σ ). These data agree very well with previous acid dissolution and stepped combustion experiments which range from a δ13C of +32‰ to +41‰ . As observed with the oxygen isotopic data, the carbon isotopic composition is correlated with the chemical composition of the carbonates. This allows us to establish that the earliest (Ca-rich) carbonates had the lightest carbon isotopic composition while the latest forming (Mg-rich) carbonates had the heaviest carbon isotopic composition. The large range of carbon isotopic compositions measured in this study cannot be explained by previously proposed models. Temperature change or a Rayleigh distillation process caused by progressive carbonate precipitation are insufficient to create the observed carbon isotopic compositions. Furthermore, processes such as evaporation or photosynthesis will not produce large carbon isotopic variations due to rapid isotopic equilibration with the atmosphere. We propose two possible models for the formation of the ALH84001 carbonates consistent with the isotopic data collected thus far. Carbonates could have formed from an evolving system where the carbon and oxygen isotopic composition of the carbonates reflects a mixing between magmatic hydrothermal fluids and fluids in equilibrium with an isotopically heavy atmosphere. Alternatively, carbon enrichment could have occurred as a portion of the carbon dioxide is reduced through abiotic Fischer-Tropsch synthesis or biologically mediated methanogenesis. In this scenario, cooling of the system over time would be necessary to create observed oxygen isotopic values.
Guan Yifeng
Leshin Laurie Ann
Niles Paul B.
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