Chassigny and the nakhlites: Carbon-bearing components and their relationship to martian environmental conditions

Abstract

The carbon and nitrogen inventories of Chassigny and the nakhlites have been investigated by low-resolution (100°C temperature increment) stepped combustion; in addition, the contents and isotopic compositions of carbonate minerals have been assessed by the use of an acid-dissolution technique. Low-temperature carbon (i.e., that which combusts below 500°C) appears to be from two different sources: one component is present in variable quantities (300–700 ppm) and has δ 13C > −26%. , consistent with identification as terrestrial organic contamination; the other is isotopically light with δ 13C ca. −36%., which is unusual for normal sources of terrestrial contamination. Curiously, the latter material is present in each sample in similar concentrations ( 45 ± 15 ppm ). Since this level of carbon is too high to be ascribed to a system blank, it is considered that this component is probably indigenous to the meteorites. The low temperature of release of the isotopically light carbonaceous material is testimony to its highly labile nature; it is evidently organic material of some description. The accompanying low-temperature nitrogen also appears to be distributed between two components (one with δ 15N of ca. −6.5 to −2.4%.; the other with δ 15N > +4%. ) but it is not yet possible to establish relationships between the nitrogen and carbon components released below 500°C. The meteorites investigated were found to contain 2.5–30 ppm carbon as carbonate, with δ 13C between −5 and +11%. and δ 18O between +23 and +29%.. Variation in δ 13C and δ 18O of the carbonates indicates either a change in conditions during formation of the carbonate minerals or that there may be two distinct carbon sources. Magmatic species, which are released on combustion at temperatures above 700°C, apparently have an isotopically light carbon isotopic composition (between −30 and −20%.), and are associated with light nitrogen ( δ 15N < −20%. ). Superimposed on the magmatic species is a release of spallogenically produced isotopically heavy nitrogen, seen most clearly in Chassigny. There is no evidence in any of the samples analysed for trapped martian atmospheric gases.