A carbon and nitrogen isotope study of Zagami

Samples from the two major lithologies (fine- and coarse-grained) in Zagami have been analyzed for the abundance of carbon and nitrogen and their isotopic compositions. Using the technique of stepped combustion, it was possible to evaluate the distribution of carbon and nitrogen within different species (organic, carbonate, nitrate; etc.) in the samples. Separates of shock-produced glass and pigeonite were also analyzed for carbon. The majority of the carbon and nitrogen in each sample arises from contaminants (organic materials, absorbed gases, etc.). However, carbon and nitrogen of magmatic origin is released above 600 degrees C (delta(13)C similar to -24 parts per thousand; delta(15)N similar to -5 parts per thousand). The glass separates contain a minor contribution from trapped Martian atmospheric CO2, in addition to the magmatic component. Zagami appears to contain two generations of carbonate, an isotopically light end-member with minimum delta(13)C around -23 parts per thousand, plus a more C-13-enriched carbonate. The latter is typical of Martian carbonates formed from fluid in contact with Martian atmospheric CO2, while the former might represent a more primitive source of carbon, and possibly emanate from the carbonate anion present within silicates or phosphates. The spread in delta(13)C values of carbonates in Martian meteorites clearly points to the operation of at least two processes, each imparting a characteristic carbon isotopic signature These processes are interpreted within the framework of models that describe the evolution of Zagami-type basalts on Mars.