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A nitrogen and argon stable isotope study of Allan Hills 84001: Implications for the evolution of the Martian atmosphere
Author(s) -
GRADY MONICA M.,
WRIGHT IAN P.,
PILLINGER COLIN T.
Publication year - 1998
Publication title -
meteoritics and planetary science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.09
H-Index - 100
eISSN - 1945-5100
pISSN - 1086-9379
DOI - 10.1111/j.1945-5100.1998.tb01686.x
Subject(s) - martian , radiogenic nuclide , meteorite , atmosphere of mars , meteoroid , atmosphere (unit) , isotopes of argon , mars exploration program , astrobiology , chemistry , geology , argon , geochemistry , thermodynamics , mantle (geology) , physics , organic chemistry
— 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] ∼ 0.34 ppm; δ 15 N ∼ +23%); although during stepped combustion, 15 N‐enriched N (δ 15 N ∼ +143%) is released in a narrow temperature interval between 700 °C and 800 °C (along with 13 C‐enriched C (δ 13 C ∼ +19%) and 40 Ar). Cosmogenic species are found to be negligible at this temperature; thus, the iso‐topically 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 40 Ar from K decay. Deconvolution of radiogenic 40 Ar from other Ar components, on the basis of end‐member 36 Ar/ 14 N and 40 Ar/ 36 Ar 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 palaeoatmos‐phere 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: δ 15 N ≥ +200%, 40 Ar/ 36 Ar ≤ 300 and 36 Ar/ 14 N ≥ 17 × 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.