Published in Meteoritics, 29, p. 469.
Reclassification of ALH 84001 as an orthopyroxenite related to SNCs  brings the total number of martian meteorites to 10. Preliminary descriptions of ALH 84001 [1,2] and the more detailed analysis that followed  highlighted the presence of abundant Fe,Mg-carbonates distributed heterogeneously throughout the specimen. Previous studies of SNCs identified four discrete carbon-bearing components: materials that combusted at temperatures usually associated with organics (probably terrestrial contaminants), carbonates, magmatic carbon, and trapped martian atmospheric carbon dioxide. The isotopic compositions of these species are distinctive, and have been used to constrain the operation of martian surficial processes [e.g., 4]. Given the relatively high carbonate abundance in ALH 84001, detailed isotopic analyses of the specimen will undoubtedly provide further information on the formation mechanisms of these minerals. Nitrogen analysis could identify the presence of any N-bearing salts (e.g., nitrates ) and trapped atmospheric species. This abstract reports the first results from analysis of carbon in ALH 84001; measurements of N are anticipated in time for the conference. A high-resolution stepped combustion of 5.099 mg of powdered ALH 84001 was performed; the data are summarized in Table 1. The total C content (600.6 ppm) is within the range of all other SNC meteorites, but is higher than most [6,7]. The overall delta13C, of +10.6, is above any value measured for a whole-rock sample. The most outstanding feature of the analysis was the release of almost 50% of the total C (284 ppm) across a narrow temperature range from 450°-525°C, with delta13C ~ +40. This feature is interpreted as decrepitation of Fe,Mg-rich carbonates. The delta13C is far in excess of previously measured values for carbonates in most other SNCs, where delta13C lies in the range -5 to +11 . However, Carr et al.  reported delta13C up to +49 from an H3PO4-dissolution of Nakhla, but commented that the results might have been compromised by interference from organic species; in the light of data from ALH 84001, the Nakhla results may have been authentic. The high delta13C values were obtained after extended reaction times, consistent with Fe,Mg-carbonates. Above 450°C, amounts of carbon close to blank levels were liberated from ALH 84001. This material is also slightly enriched in 13C, probably due to a tailing effect from the Fe,Mg-carbonates.
|Composition||Temp. (°C)||Yield (ppm)||delta13C ()|
The enrichment of 13C in carbonates from ALH 84001 indicates beyond any doubt that these salts are truly indigenous to the meteorite (and, by implication, Mars), rather than an Antarctic weathering product. Wright et al.  defined a linear relationship between yield and C isotopic composition of carbonate in SNCs; the datum from ALH 84001 extends this association. For the carbonate to be formed by interaction of martian atmospheric CO2 (delta13C ~ +36 ) with regolith material, reaction would need to have occurred at temperatures around 100°C. Such a high temperature is unlikely on the martian surface, and therefore the carbonates more probably formed in a hydrothermal environment.
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