3:00 PM - 3:15 PM
[MIS18-12] Preliminary investigation results of organic matter in the Black Beauty (NWA 7034)
Sample and Experiments: A polished chip sample of NWA 7034 (7×10 mm) was prepared through dry polishing process for our STXM-based TEM/NanoSIMS microscopic in-situ analysis. Fine-textures of carbon-rich portions in the polished sample were observed using FE-SEM/EDS. The ultra-thin foils of selected carbon-rich portions were prepared by a FIB system for STXM, FE-TEM/STEM, and NanoSIMS analyses. FIB-assisted STXM analysis was conducted for C-, N-, O-, and Na-NEXAFS at PF BL13A (compact STXM). TEM/STEM observation using JEOL JEM-2100F was conducted for fine textural observations after the STXM measurements. Finally, C, O, and D isotope imaging using CAMECA NanoSIMS 50L was conducted to obtain their isotopic characteristics.
Results and Discussion: OMs distributed in the matrix of NWA 7034. From C-, N- and O-NEXAFS, OMs have C*=C (aromatic C), C=C-C*=O (vinyl-keto bond), C*≡N (nitrile), NHx (C*=O)C (amidyl), and OR(C*=O)C (carboxylic C). Although C-NEXAFS study for Matrian OMs is very limited, C-NEXAFS feature of OMs in matrix are similar to the C-NEXAFS features of opaque rim materials of carbonates in ALH 84001 [Flynn et al. (1998)] and OMs in Tissint meteorite fracture [this study]. In addition, C-NEXAFS features of OMs were different from terrestrial organic matters [Lehmann and Solomon. (2010)]. TEM/STEM observations revealed that OMs in NWA 7034 matrix is amorphous and partly be foamed. Fine-grained secondary minerals (e.g., halite and smectite group clay minerals) were incorporated into OMs. This result suggested that OMs formed aquatic condition or experienced aqueous alteration (not high temperature). δ13C value of OMs is 15 ± 7 ‰, which is different from the δ13C values of the OMs enclosed in minerals as inclusions in NWA 7034 (-23.4 ± 0.73 ‰) and bulk-rock NWA 7034 (-3.0 ± 0.16 ‰) within analytical error [Agee et al. (2013)]. The δ13C values range is consistent not only with those for Martian atmosphere but also those for Martian carbonate which may reflect atmospheric value of their precipitation periods [Niles et al. (2005); Niles et al. (2010)]. As for bulk-rock NWA 7034 (-3.0 ± 0.16 ‰), present result implies that the possibility of mixture contribution between magmatic components (δ13C = -20 to -30 ‰) and Martian (sub)surface-related component (e.g., 15 ± 7 ‰). The δD value of the OM was 373 ± 68 ‰ (partly near 1000 ‰: clay minerals embedded region) which is slightly higher than that of Martian mantle value (275 ‰), and approximately consistent with Martian subsurface ice/water [Usui et al. (2012)].
Conclusion: Through our STXM-based TEM/NanoSIMS microscopic in-situ analysis, OMs in NWA 7034 is Martian surface-related (high δ13C and δD) and include Martian surface-related smectite group clay minerals (high δD).