The case for a martian origin of the shergottites: nitrogen and noble gases in EETA 79001

Abstract

Nitrogen and noble gases were measured in samples of a glass inclusion and the surrounding basaltic matrix from the antarctic shergottite EETA 79001. A nitrogen component trapped in the glass, but not present in the matrix, has a δ 15N value at least as high as +190‰. Ratios of 40Ar/ 14N and 15N/ 14N in the glass are consistent with dilution of a martian atmospheric component ( δ 15N = 620 ± 160‰, 40Ar/ 14N= 0.33 ± 0.03 ) by either terrestrial atmosphere adsorbed on the samples or by indigenous nitrogen from the minerals of the rock. Trapped noble gases in the glass reproduce, within error, the elemental and isotopic compositions measured in Mars' atmosphere by Viking, and are in general agreement with previous measurements except for much lower abundances of neutron-generated krypton and xenon isotopes. The most reasonable explanation at the present time for the noble gas pattern and the isotopically heavy nitrogen is that a sample of martian atmosphere has been trapped in the EETA 79001 glass, and that this meteorite, and thus the shergottites and probably the nakhlites and chassignites as well, originated on Mars. Nitrogen in the non-glassy matrix of EETA 79001 amounts to less than 0.5 ppm and has a spallation-corrected δ 15N value in the range 0 to −20‰; it may reflect indigenous nitrogen in the basalt or a mixture of indigenous and adsorbed terrestrial nitrogen. Spallogenic noble gases yield single-stage exposure ages between 400,000 and 900,000 years, depending on irradiation geometry. Trapped argon may have an unusually low 36Ar/ 38Ar ratio. Trapped krypton, except for a small excess at 80Kr, is smoothly mass-fractionated with respect to either terrestrial or chondritic Kr. The trapped xenon composition is consistent with addition of neutron-capture, radiogenic and fissiogenic isotopes to a base composition resembling terrestrial atmospheric Xe. The elemental 84Kr/ 132Xe ratio of 25 is close to the terrestrial value and very different from the chondritic ratio.