Mathematics – Logic
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.511g&link_type=abstract
Meteoritics, vol. 30, no. 5, page 511
Mathematics
Logic
4
Carbonates, Isotopes, C, Mars, Meteorites, Snc
Scientific paper
The occurrence of carbonates in martian meteorites was first established after acid dissolution and stepped combustion analyses of whole-rock Nakhla [1]. The release of CO2 after a 24 hr. reaction with 100% H3PO4 at 25 degrees C was taken to imply that the carbonate mineral present was calcite, a proposal subsequently confirmed by petrographic examination [2]. The isotopic composition of the carbon comprising the calcite was enriched in 13C (isotopically heavy) with delta^(l3)C ~ +12 per mil. An extended period of acid attack, also at 25 degrees C, released small quantities of even more 13C-enriched CO2 (delta^(13)C ~ +49 per mil), but the isotopic data were considered uncertain, and thus little significance was attached to the result, beyond the suggestion that some carbonate was perhaps dolomite or iron-bearing. Now, however, following the analysis of Fe-Mg-rich carbonates in ALH 84001 [3-5], it is apparent that previously-reported data might underestimate the abundance and delta^(13)C of carbonates in SNCs [6], and that a much higher proportion might occur as siderite or dolomite end-members. Iron- and magnesium-rich carbonates are only partially attacked at 25 degrees C, even after extended exposure to H3PO4 [7]. Given that the delta^(13)C of carbonates in SNCs has been used to deduce both environmental conditions on Mars [4, 6], and the evolution of the martian atmosphere [8], it is desirable that correct delta^(l3)c values are known. We have undertaken a reappraisal of the chemical and isotopic composition of carbonates in martian meteorites, by a programme of high resolution stepped combustion analyses and high temperature (75 degrees C) acid dissolution . Carbonates in most martian meteorites are extremely fine-grained. and therefore not easv to identify by traditional optical microscopic methods; it is not possible to determine readily the mineralogical composition of the grains. Comparison of combustion data from SNCs with that from pure materials allows the mineralogy of the carbonates to be constrained. since the peak decomposition temperature of carbonates varies with mineralogical composition. Preliminary results indicate that the carbonates in martian meteorites might indeed be more 13C-enriched than previously inferred [6]. Stepped combustion of Nakhla reveals a maximum in yield at 475 degrees C, corresponding to the release of an iron-rich carbonate, with delta^(13)C +21 per mil. Since there is an overlap of the carbonate with low temperature carbonaceous material, this value is a lower limit. Indeed, delta^(13)C from acid leaching experiments reaches a value around +50 per mil, similar to the ALH 84001 studies [1, 5]. Models of surface processes on Mars [e.g. 4, 6] have assumed that C07 from the martian atmosphere is in equilibrium with CO2 dissolved in circulating crustal fluids, resulting in the precipitation of isotopically heavy carbonates. However, if the delta^(13)C of martian carbonates is ~ +50 per mil, then the models must be re-adjusted. It is unlikely that delta^(13)C of the carbonate has increased with time, e.g. by decarbonation reactions, since there is no parallel effect in delta^(18)O, and no petrographic evidence for the reaction. The delta^(13)C of the martian atmosphere is poorly-constrained; it is possible that its isotopic composition is heavier than believed, and that delta^(13)C has decreased with time, perhaps by the addition of isotopically light magmatic carbon degassed from the planet [8]. Additional measurements of carbonates in martian meteorites will allow better comprehension of fluid-atmosphere interactions on Mars. References: [1] Carr R. H. et al. (1985) Nature, 314, 248-250. [2] Gooding J. L. et al. (1991) Meteoritics, 26, 135-143. [3] Grady M. M. et al. (1994) Meteoritics, 29, 469. [4] Romanek C. S. et al. (1994) Nature, 372, 655-657. [5] Jull A. J. T. et al. (1995) Meteoritics, 30, 311-318. [6] Wright I. P. et al. (1992) GCA, 56, 817-826. [7] Rosenbaum J. and Sheppard S. F. (1986) GCA, 50, 1147-1150. [8] Pepin R. O. (1994) Icarus, 111, 289-304.
Douglas Craig C.
Grady Michael
Pillinger Colin T.
Wright Ian P.
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