5:15 PM - 6:45 PM
[PPS08-P17] In-situ nitrogen chemical speciation of Martian meteorites for understanding the environmental history of Mars
Keywords:Martian meteorites, Nitrogen, In-situ speciation analysis, XAFS
Many evidences suggest that early Mars had retained the liquid water on its surface. Geochemical information of volatile elements such as H, C, N, and S, provides important clues for the long-term history of Mars. At present, nitrogen (N) makes up ~3% of the Martian atmosphere as chemically inert N2. Once N was fixed via past abiotic processes (e.g., volcanic eruptions and/or meteoritical impacts), highly reactive N-bearing compounds may have affected the planetary environment and its potential habitability. The recent rover-based investigations at Martian Gale Crater identified the presence of oxidized N (e.g., nitrate salts) in the 3.5-billion-year-old sedimentary materials (Stern et al., 2015, 2017). Moreover, the in-situ analyses of 4-billion-year-old carbonate minerals from Martian meteorite ALH 84001 revealed that those minerals had preserved organic N and carbonate-associated sulfate (CAS) provably since their formation (Koike et al., 2020, Kajitani et al., 2023). To understand the nitrogen behaviors and its evolutions on Mars, this study focuses on the N speciation of Martian meteorites. Here we report the N K-edge micro-scale X-ray absorption fine structure (µ-XAFS) analyses of three shergottites (Tissint, NWA 13367, and NWA 2975) and a single nakhlite (Yamato 000749).
Rock chips of the individual meteorites were pressed into the indium mold and polished using Al2O3 powder and ultrapure water. Their textural observation and major elements analyses were conduced by SEM-EDS and EPMA. The impact-induced melted and quenched patches (hereafter referred to as "impact glass") in the three shergottites and the aqueous alteration veins (iddingsite) in olivine crystals in the nakhlite were selected for the following N speciation analysis. The samples were then processed with FIB to reduce the adhering potential contaminants. The N K-edge X-ray absorption near edge structure (XANES) spectra of the samples were obtained at the soft X-ray beamline BL27SU of SPring-8. Reference XANES spectra of various N-bearing chemical reagents were measured along with the unknowns. The N species potentially trapped into the meteorites were estimated by comparing the obtained spectra.
According to the SEM observations, the large impact glasses with size of ~2 mm were observed both in Tissint and in NWA 2975. Many micro-sized vesicles (φ<10 µm) were also found in the Tissint’ glass. In contrast, NWA 13367 contains the smaller volumes of partially remelt and recrystallized regions (size of ~200 µm). We regard those recrystallized areas as the impact glass of NWA 13367. The narrow veins (<10 µm) of iddingsite crosscutting the olivine crystals were observed on Y-000749 nakhlite. Although the iddingsite presents sub-micron scale compositional variations, such structures were too small to distinguish by our XANES measurements.
The XANES spectra of impact glass from Tissint present absorption peaks characteristic of organic N, nitrate, and N2. The XANES spectra of NWA 13367 and NWA 2975 indicate only the organic N contributions. The presence of organic N is also suggested from the Y-000749 spectra. Previous studies pointed that the impact glasses in Tissint probably incorporated Martian subsurface fluid and organic components (Chen et al., 2015, Jaramillo et al., 2019). It is suggested that the observed nitrate and organic N may also have originated from the subsurface fluid and/or sedimentary materials on Mars. On the other hand, NWA 13367 and NWA 2975, found in Sahara hot desert, were considerably weathered. Their N species may be of terrestrial origin. Meanwhile, the apparent absence of inorganic N species (nitrate or ammonium) from the iddingsite in Y-000749 nakhlite suggests that either the Martian subsurface fluid from which iddingsite precipitated at 600-700 Ma did not contain N species or such species were lost from the samples during our experiment. Iddingsite is fragile and susceptible to be damaged by the SEM/EPMA, FIB, and µ-XAFS measurements. It is possible that primary volatile components in the iddigsite may have been decomposed due to the beam-induced damages. In contrast, the organic N observed from the iddingsite indicates that the ~600–700 Ma Martian fluid may have contained the organic components.
Rock chips of the individual meteorites were pressed into the indium mold and polished using Al2O3 powder and ultrapure water. Their textural observation and major elements analyses were conduced by SEM-EDS and EPMA. The impact-induced melted and quenched patches (hereafter referred to as "impact glass") in the three shergottites and the aqueous alteration veins (iddingsite) in olivine crystals in the nakhlite were selected for the following N speciation analysis. The samples were then processed with FIB to reduce the adhering potential contaminants. The N K-edge X-ray absorption near edge structure (XANES) spectra of the samples were obtained at the soft X-ray beamline BL27SU of SPring-8. Reference XANES spectra of various N-bearing chemical reagents were measured along with the unknowns. The N species potentially trapped into the meteorites were estimated by comparing the obtained spectra.
According to the SEM observations, the large impact glasses with size of ~2 mm were observed both in Tissint and in NWA 2975. Many micro-sized vesicles (φ<10 µm) were also found in the Tissint’ glass. In contrast, NWA 13367 contains the smaller volumes of partially remelt and recrystallized regions (size of ~200 µm). We regard those recrystallized areas as the impact glass of NWA 13367. The narrow veins (<10 µm) of iddingsite crosscutting the olivine crystals were observed on Y-000749 nakhlite. Although the iddingsite presents sub-micron scale compositional variations, such structures were too small to distinguish by our XANES measurements.
The XANES spectra of impact glass from Tissint present absorption peaks characteristic of organic N, nitrate, and N2. The XANES spectra of NWA 13367 and NWA 2975 indicate only the organic N contributions. The presence of organic N is also suggested from the Y-000749 spectra. Previous studies pointed that the impact glasses in Tissint probably incorporated Martian subsurface fluid and organic components (Chen et al., 2015, Jaramillo et al., 2019). It is suggested that the observed nitrate and organic N may also have originated from the subsurface fluid and/or sedimentary materials on Mars. On the other hand, NWA 13367 and NWA 2975, found in Sahara hot desert, were considerably weathered. Their N species may be of terrestrial origin. Meanwhile, the apparent absence of inorganic N species (nitrate or ammonium) from the iddingsite in Y-000749 nakhlite suggests that either the Martian subsurface fluid from which iddingsite precipitated at 600-700 Ma did not contain N species or such species were lost from the samples during our experiment. Iddingsite is fragile and susceptible to be damaged by the SEM/EPMA, FIB, and µ-XAFS measurements. It is possible that primary volatile components in the iddigsite may have been decomposed due to the beam-induced damages. In contrast, the organic N observed from the iddingsite indicates that the ~600–700 Ma Martian fluid may have contained the organic components.