86th Annual Meeting of the Meteoritical Society 2024 (LPI Contrib. No. 3036)

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Origin of nitrogen on Mars: First in situ N isotope analyses of martian meteorites
C. Deligny1,2, E. Füri1, E. Deloule1, A.H. Peslier3,4, F. Faure1, Y. Marrocchi1,1
Université de Lorraine, CNRS, CRPG, F-54000 Nancy, France, 2Department of Geosciences, Swedish Museum of
Natural History, Stockholm, Sweden (cecile.deligny@nrm.se), 3Jacobs, NASA-Johnson Space Center, Mail CodeX13, Houston, TW 77058, USA, 4Dept. of Geological Sciences, New Mexico University, Las Cruces, NM 88011,USA.
Introduction: Knowledge of the origin(s) and chronology of accretion of highly volatile elements such as nitrogenon the terrestrial planets is crucial to our understanding of planetary formation processes and the development of
habitable conditions. Due to its small size and rapid accretion, Mars is considered to be a planetary embryo [1] and ananalogue of Earth during the earliest stages of planet formation. Martian meteorites are key for assessing the isotopic
characteristics of nitrogen in different martian reservoirs (i.e., mantle, crust, and atmosphere), and, ultimately, forconstraining the source(s) of nitrogen trapped during the earliest stages of planetary accretion in the terrestrial planetforming region. Nitrogen isotopes are powerful tools to trace the origin of volatiles accreted by planetary bodies because 15N/14N ratio vary significantly across the solar system. Indeed, three major, isotopically distinct volatile reservoirs have been identified: i) the solar nebula [2], ii) chondritic meteorites (e.g., enstatite chondrites; Ivuna-like (CI)and Mighei-like (CM) carbonaceous chondrites [3],[4],[5],[6]), and iii) comets [7].
Method: In this study, we analysed, for the first time, the nitrogen content and isotopic composition of glassy melt
inclusions of Chassigny and of the mesostasis of five nakhlites (MIL 03346, Nakhla, NWA 6148, NWA 998, and Y000593) by in situ secondary ion mass spectrometry. In situ nitrogen analyses were performed with the CAMECA
1280 HR2 ion microprobe at the Centre de Recherches Pétrographiques et Géochimiques following the analytical
protocol of [8] and [9].
Results and discussions: The nitrogen content of Chassigny melt inclusions, corrected for olivine overgrowth on
the inclusion walls, varies from 4 ± 1 to 860 ± 45 ppm N, and the majority of δ15N values range from –35 ± 41 to +73± 36‰ (corrected for cosmogenic 15N). Several secondary and planetary processes (i.e., cosmic ray-induced production of 15N, post-entrapment modification, magmatic degassing) likely affected and modified the initial abundance andisotopic composition of nitrogen trapped in martian meteorites. After corrections for spallation, olivine overgrowthon the melt inclusion walls and degassing of the melt (N2 or NH3), the estimated δ15N value of the 'primitive' melt
trapped in Chassigny is 0 ± 32‰. This value implies that nitrogen trapped in the martian mantle is not inherited fromthe solar nebula, but, instead, is derived from (isotopically) chondritic material. The mesostasis of nakhlites contains
2.7 ± 0.2 to 943 ± 156 ppm N, with δ15N values from –30 ± 37 to +348 ± 43‰ (corrected for cosmogenic 15N). Whereas
degassing of N2 or NH3 can explain the lowest nitrogen isotopic ratios measured in the nakhlite mesostasis, the 15Nenriched isotopic composition (δ15N > 150‰) of four nakhlites (MIL 03346, Nakhla, NWA 6148, and Y 000593)
likely results from interaction of the mesostasis melt with the martian atmosphere during ejection. The δ15N values(+25 ± 42 and +77 ± 19‰), of two melt inclusions in Y 000593 are comparable to those of Chassigny, further confirming that these meteorites likely sample a common volatile reservoir in the martian interior. Degassing of N2 orNH3 alone cannot explain the full range of measured values. Instead, interaction of the mesostasis melt with the martian
atmosphere during ejection may explain the nitrogen isotopic variations in nakhlites. Overall, the new results indicatethat the chassignite-nakhlite reservoir did not inherit nitrogen from the solar nebula but, instead, from chondritic-likematerials. These findings further confirm that planetary bodies in the inner solar system accreted (isotopically) chondritic nitrogen during the first few million years of solar system history. Overall, our findings show that the nitrogenisotopic composition of nakhlites was more severely affected by secondary processes than that of Chassigny, although
nakhlites are the least shocked of all martian meteorites. Since the exact provenance of Chassigny and nakhlites on
Mars remains unknown, a Mars sample-return mission is crucial to collect specific, pristine samples from selectedareas of interest on Mars, and, ultimately, to improve our understanding of the origin of volatiles trapped in the interiorof Mars.
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