Predominantly Non-Solar Origin of Nitrogen in Lunar Soils

1J. Mortimer, 1A.B. Verchovsky, 1,2M. Anand
Geochimica et Cosmochimica Acta (in Press) Link to Article [doi:10.1016/j.gca.2016.08.006]
1Planetary and Space Sciences, Department of Physical Sciences, The Open University, Milton Keynes, MK7 6AA, UK
2Department of Earth Sciences, The Natural History Museum, London, SW7 5BD, UK
Copyright Elsevier

Simultaneous static-mode mass spectrometric measurements of nitrogen, carbon, helium, neon, and argon, extracted from the same aliquot of sample by high-resolution stepped combustion, have been made for a suite of five lunar soils.

Noble gas isotope ratios show that the majority of noble gases are derived from a solar wind source; for example, at peak release temperatures of 500-600 °C, 21Ne/22Ne = 0.0313 ± 0.0007 to 0.0333 ± 0.0007, and 20Ne/22Ne = 11.48 ± 0.05 to 12.43 ± 0.07, with values at the lowest temperature steps less fractionated during implantation from, and therefore even closer to, solar values (21Ne/22NeSW = 0.03361 ± 0.00018 and 20Ne/22NeSW = 14.001 ± 0.042 (Pepin et al., 2012)). Despite the co-release of nitrogen and solar wind argon, measured nitrogen isotopic signatures at each temperature step, whilst variable, are significantly more enriched in 15N compared to the measured solar wind nitrogen value from the Genesis mission. Therefore, mixing between a 15N-enriched non-solar planetary nitrogen source with solar wind nitrogen is required to explain the measured isotopic values from the stepped combustion analysis of lunar soils. Binary mixing calculations, made under different assumptions about the degree of loss of solar wind 36Ar, reveal that the majority (up to 98%) of the nitrogen released is derived from a non-solar source. The range of modelled non-solar end-member nitrogen compositions required to satisfy the measured δ15N values varies between samples and temperature steps from +5 ‰ up to +300 ‰, or between +87 ‰ and +160 ‰ for bulk samples. This range of modelled isotopic compositions for the non-solar source of nitrogen encompasses measured values for several different groups of carbonaceous chondrite, as well as IDPs.

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