A nitrogen and argon stable isotope study of Allan Hills 84001: Implications for the evolution of the Martian atmosphere

The abundances and isotopic compositions of N and Ar have been measured by stepped combustion of the Allan Hills 84001 (ALH 84001) Martian orthopyroxenite. Material described as shocked is N-poor ([N] similar to 0.34 ppm; delta(15)N similar to +23 parts per thousand); although during stepped combustion, N-15-enriched N (delta(15)N similar to +143 parts per thousand) is released in a narrow temperature interval between 700 degrees C and 800 degrees C (along with C-13-enriched C (delta(13)C similar to +19 parts per thousand) and Ar-40). Cosmogenic species are found to be negligible at this temperature; thus, the isotopically heavy component is identified, in part, as Martian atmospheric gas trapped relatively recently in the history of ALH 84001. The N and Ar data show that ALH 84001 contains species from the Martian lithosphere, a component interpreted as ancient trapped atmosphere (in addition to the modern atmospheric species), and excess Ar-40 from K decay. Deconvolution of radiogenic Ar-40 from other Ar components, on the basis of end-member Ar-36/N-14 and Ar-40/Ar-36 ratios, has enabled calculation of a K-Ar age for ALH 84001 as 3.5-4.6 Ga, depending on assumed K abundance. If the component believed to be Martian palaeoatmosphere was introduced to ALH 84001 at the time the K-kr age was jet, then the composition of the atmosphere at this time is constrained to: delta(15)N greater than or equal to +200 parts per thousand, Ar-40/Ar-36 less than or equal to 300 and Ar-36/N-14 greater than or equal to 17 x 10(-5). In terms of the petrogenetic history of the meteorite, ALH 84001 crystallised soon after differentiation of the planet, may have been shocked and thermally metamorphosed in an early period of bombardment, and then subjected to a second event. This later process did not reset the K-Ar system but perhaps was responsible for introducing (recent) atmospheric gases into ALH 84001; and it might mark the time at which ALH 84001 suffered fluid alteration resulting in the formation of the plagioclase and carbonate mineral assemblages.