Physics
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.580s&link_type=abstract
Meteoritics, vol. 30, no. 5, page 580
Physics
Isotopes, Carbon, Oxygen, Meteorites, Alh 84001, Antarctic, Soils, Weathering
Scientific paper
We report here the stable isotope composition of carbonate measured from a suite of desert soils from the Dry Valleys of Antarctica [1] to determine the 13C enrichments attributed to cryogenic freezing in terrestrial environments. These data are then used to gauge whether cryogenic freezing is a viable aqueous process that can produce extreme 13C enrichments observed in Martian carbonates (e.g., ALH 84001 [2]). Analyses of ALH 84001 have shown that the delta^(13)C of carbonate is the most-positive yet recorded for an SNC meteorite (ca. 42 per mil)[2]. The source of carbon is thought to be Martian atmospheric CO2, which has been recycled through an aqueous medium into the solid phase. The delta^(13)C of the carbonate is consistent with a precipitation temperature below ~300 degrees C [3], assuming the delta^(13)C of Martian CO2 lies somewhere between 26 and 46 per mil [4, 5]. An equilibrium temperature of formation near 0 degrees C is difficult to reconcile if the atmospheric source of carbon is <26 per mil, despite the fact that equilbrium isotope enrichments are large at this temperature (12-14 per mil) [6-8]. Low delta^(13)C for atmospheric CO2 is only compatible with high delta^(13)C for carbonate when non-equilibrium processes are the primary mechanism of isotopic fractionation. An inorganic surficial process known to enrich carbonate by >15 per mil over ambient atmospheric CO2 is cryogenic freezing [9]. Carbonate-bearing soils from Wright Valley, Antarctica were studied as a terrestrial analog to the carbonates in ALH 84001 to characterize isotopic "fingerprints" associated with cryogenic freezing. delta^(13)C and delta^(18)O carbonate values from Prospect Mesa Soil Pit range from +0.89 per mil to -20.46 per mil (PDB) within the "permanently frozen zone" (below 0.4 m), and +4.20 per mil to -11.87 per mil at the surface. The most enriched 13C and 18O tend to occur at the surface where seasonal variations in temperature or precipitation have imposed cyclical precipitation/dissolution of calcite. This observation is consistent, albeit less pronounced, with isotope enrichments in salts deposited in sediments from Lake Vanda, Antarctica [10]. Cryogenic freezing provides a possible explanation for the extreme enrichments observed in carbonate from Mars (e.g. ALH 84001, EETA 79001, Nakhla). During freezing of subsurface fluids, dissolved ions become concentrated in the residual liquid phase, to the point where dissolved inorganic carbon can no longer be held in solution due to the reduced activity of water, and CO2 exsolves. If differences in diffusivity alone control isotopic fractionation then the escaping CO2 gas should be enriched in 12C due to kinetic effects, with the resultant 13C enrichment in the fluid being passed on to any carbonate which may precipitate. Enrichments in 13C of up to 20 per mil above atmospheric CO2 are observed for carbonates from Arctic cave deposits [9] and lake beds of the Dry Valleys in Antarctica [10]. Using an enrichment of this magnitude in a fractionation model, carbonate with a d13C of 42 per mil can be generated from Martian atmospheric CO2, as long as the CO2 source has a delta^(13)C greater than about 20 per mil. This model is compatible with less extreme enrichments if the delta^(13)C of atmospheric CO2 is increased in a complimentary fashion. Cryogenic freezing is a weathering process which is shown to enrich the carbonates in 13C above that which can be obtained by isotope effect at low temperature. Those processes operating in the cold desert environment of the Dry Valleys of Antarctica may be the ideal terrestrial analog to processes acting at or near the Martian surface. References: [1] Gibson E. K. et al. (1983) Proc . LPS 13th, in JGR, 88, A912. [2] Romanek C. S. et al. (1994) Nature, 372, 655. [3] Romanek C. S. et al. (1994) GSA Abstr. with Progr., 26, A-424. [4] Carr R. H. et al. (1985) Nature, 314, 248. [5] Nier A. O. and McElroy M. B. (1977) JGR, 82, 4341. [6] Bottinga Y. (1968) J. Phys. Chem., 72, 800. [7] Chacko T. et al. (1991) GCA, 55, 2867. [8] Romanek C. S. et al. (1992) GCA, 56, 419. [9] Clark I. D. and Lauriol B. (1992) Chem. Geol., 102, 217. [10] Nakai N. et al. (1975) Geochem. J., 9, 7.
Gibson Everett K. Jr.
Romanek Christopher S.
Socki Richard A.
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