A nitrogen and argon stable isotope study of ALH 84001: Implications for the evolution of the martian atmosphere

Details A nitrogen and argon stable isotope study of ALH 84001: Implications for the evolution of the martian atmosphere A nitrogen and argon stable isotope study of ALH 84001: Implications for the evolution of the martian atmosphere

Jul 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998m%26ps...33..795g&link_type=abstract

Meteoritics & Planetary Science, vol. 33, no. 4, pp. 795-802.

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The abundances and isotopic compositions of nitrogen and argon have been measured by stepped combustion of the Allan Hills 84001 (ALH 84001) martian orthopyroxenite. Material described as shocked is nitrogen-poor ([N] ~ 0.34 ppm; (15N ~ +23e), although during stepped combustion, 15N-enriched nitrogen ((15N ~ +143e) is released in a narrow temperature interval between 700C and 800C (along with 13C-enriched carbon ((13C ~ +19e) and 40Ar). 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 nitrogen and argon data show that ALH 84001 contains species from the martian lithosphere, as well as a component interpreted as ancient trapped atmosphere, in addition to the modern atmospheric species, plus excess 40Ar from potassium decay. Deconvolution of radiogenic 40Ar from other argon components, on the basis of end-member 36Ar/14N and 40Ar/36Ar ratios, has enabled calculation of a K-Ar age for ALH 84001 as 3.5 - 4.6 Gyr, depending on assumed potassium abundance. If the component believed to be martian palaeoatmosphere was introduced to ALH 84001 at the time the K-Ar age was set, then the composition of the atmosphere at this time is constrained to: d15N ( +200e, 40Ar/36Ar ( 300 and 36Ar/14N ( 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, then subjected to a second event. This later process did not re-set the K-Ar system, but perhaps was responsible for introducing (recent) atmospheric gases into ALH 84001, and might mark the time at which ALH 84001 suffered fluid alteration resulting in the formation of the plagioclase and carbonate mineral assemblages.

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