79th Annual Meeting of the Meteoritical Society (2016) 6334.pdf

CHROMIUM NITRIDE IN METAL OF THE CM CHONDRITE QUE 97990. M. I. F. Barth*, D. Harries and F. Langenhorst, Institut für Geowissenschaften, Friedrich-Schiller Universität Jena, Carl-Zeiss-Promenade 10, 07745 Jena, Germany. *E-Mail: [email protected]

Introduction: The interaction of volatiles such as H2O, H2S and nitrogen species with meteoritic minerals is of key interest to understand the origin and evolution of their reservoirs in the early solar system. Whether or not aque- ous alteration took place solely on C-chondritic parent bodies and/or also occurred as a nebula process is still under discussion. The consistent degree of alteration within bulk meteorites is argueing towards strong parent-body altera- tion processes. It is most likely that if constituents of CM chondrites underwent alteration prior to the final parent- body accretion, most of it has been overprinted by subsequent parent-body processes. The chances to observe fea- tures of possible pre-accretionary processes are highest within CM chondrites displaying low degrees of parent-body alteration. QUE 97990 is among the least altered CM chondrites, having been classified as a CM 2.6 subtype [1]. In a recent study, chromium nitride (carlsbergite) assemblages were detected as part of phosphorous- and chro- mium bearing sulfide assemblages within the antarctic Y-791198 and Y-793321 CM chondrites [2]. This first report on the occurrence of carlsbergite within chondritic meteorites is of importance for deciphering the sources of Earth’s nitrogen. The N-isotopic ratio of the carlsbergite is close to that of terrestrial nitrogen. It has been suggested that ammonia-bearing ices in the early solar system played a role as a reservoir of reactive nitrogen, that was also in- volved in the formation of these chromium nitrides. We have sampled Cr- and P-bearing metal from the QUE 97990 meteorite by FIB-TEM in order to decipher processes that occurred prior to parent body formation. Results and Discussion: The nearly spherical metal grain from which the TEM-sections were prepared is kama- cite with Ni-concentration of ~4.4 wt.%, which is in accordance with reported Ni-contents in QUE 97990 from pre- vious studies [3]. Cr-content of the kamacite is extremely low, based on STEM-EDX measurements. Carlsbergite occurs in the central part of the metal grain as platelets with sizes up to ~2 µm in length and 25 nm in thickness. The identity of this mineral was confirmed by both selected area electron diffraction (SAED) patterns as well as STEM- EDX analyses. A second lamella from the outer part and the surrounding matrix of the metal does not contain any carlsbergite. Based on this information, a rough measurement of nitrogen-abundance within the metal was possible. Considering that all the carlsbergite crystals occur solely in the FIB-sampled, inner part of the metal grain, a mini- mum concentration of 100 ppm N in the bulk metal can be assumed. In a solar gas with a total pressure of 0.1 mbar about 0.03 ppm N2 would dissolve in molten metal [4]. Hence, even the minimum value implied by our observations is more than a thousand times higher than the result of the interaction of a gas of solar composition with a molten metal droplet. Therefore, there must have been a specific process that led to the incorporation of nitrogen into the metal. The orientation of these platelets is along two perpendicular directions. SAED of carlsbergite and the surround- ing kamacite confirm a crystallographic Baker-Nutting orientation relationship between bcc-metal and nitride [5]. The orientation relationship also indicates the carlsbergite grew at relatively low temperatures (<800 °C), at which the metal alloy had attained the bcc structure instead of the fcc structure at higher temperatures, where a different orientation relationship would be expected [6]. A scenario involving ammonia-bearing gas is most plausible to explain the strong enrichment of nitrogen in the metal, similar to what was suggested for carlsbergite in CM sulfides, where the formation from a metal phase was hypothesized [2]. The new observations strengthen the previous interpretation that a ammonia-enriched, pre- accretionary environment existed. However, no oxides and only a very thin sulfide layer occur on the metal grain observed in this study, suggesting that different formation conditions than in the previously reported case of carls- bergite-bearing sulfides must have prevailed. Possibly the involved temperature and the ratios of hydrogen to vola- tiles (i.e., H2O, H2S, NH3) were higher. The formation of carlsbergite depends also on the abundance of Cr in the original metal phase. Out of 29 metal grains in QUE 97990 measured by EPMA, only two grains had Cr-contents below the Cr content calculated for the grain sampled. Initial studies of Y-791198 have shown that many Cr-rich metal grains do not contain CrN [2], and further studies will evaluate whether this implies different histories of ex- posure or non-exposure to NH3-enriched environments. References: [1] Rubin A. E. et al. 2007. Geochimica et Cosmochimica Acta 71:2361–2382. [2] Harries D. et al. 2015. Nature Geoscience 8:97-101. [3] Kimura M. et al. 2011. Meteoritics and Planetary Science 46:431-442. [4] Kunze H.-D. et al. 1970. Metallurgical Transactions 1:281-290 [5] Sennour M. et al. 2004. Journal of Materials Science 39:4521-4531. [6] Simmons et al. 1992. Journal of Materials Science 27:6105-6115. Acknowledgements: We acknowledge the DFG for project support within SPP 1385 (grant HA7187/1-1) and funding of the FIB-TEM facilities (grant LA830/14-1). ANSMET and the Meteorite Working Group are acknowl- edged for providing the sample of QUE 97990.