The Absence of Nickel Isotopic Anomaly in Iron Meteorite Metal and Sulfide

1753. 38th LPSC. 5031 THE ASSEMBLAGE NATIVE COPPER, COBALTIAN KAMACITE, AND TROILITE IN ORDINARY CHONDRITES: DISSOCIATION PRODUCTS NOT RELATED TO A SHOCK EVENT H. Chennaoui Aoudjehane1, A. El Goresy2, and A. Jambon3. 1Universite Hassan II Aln Chock, Laboratoire Geosciences, BP 5366 MaarifCasablanca, Morocco. E-mail: [email protected]. 2Bayerisches Geoinstitut, UniversiUit Bayreuth, 95447 Bayreuth, Germany. 3Universite Pierre et Marie Curie-Paris6 Laboratoire MAGIE, Case II 0, France. 4Place Jussieu, 75252 Paris, France. Introduction: Native Cu in chondritic meteorites is an abundant accessory phase associated with troilite and metal. Ramdohr [I, 2] interpreted its occurrence is the result of exsolution from taenite during cooling. In a recent report, [3] claimed metallic Cu to have formed by shock. No evidence was presented supporting the shock origin nor a chemical mass balance to explain the association of metallic copper with troilite symplectites. We investigated this assemblage in the LL6 chondrite Benguerir to clarify the origin of the assemblage. Results: SEM and quantitative electron microprobe analyses conducted on the four-phase assemblage, native Cu + troilite + kamacite + taenite, revealed chemical compositions that cannot be produced by exsolution from taenite during a shock event. The native copper grains enclosed in FeNi are usually surrounded by aNi-poor but Co-rich kamacite. In Usti Nad Orlici the Ni-poor kamacite contains up to 1.77 wt% Co. No depletion in the adjacent taenite was detected in profiles across taenitekamacite-copper thus negating formation of both native Cu and cobaltian kamacite by exsolution from taenite. In the LL6 chondrite Benguerir a similar assemblage was encountered. The cobaltian kamacite surrounding the Cu grains were found to be enormously enriched in Co (up to 14.3 wt%) and depleted in Ni. This cannot be explained by exsolution from taenite during cooling nor by a shock-induced heat event. The symplectitic texture of FeS, the unusual enrichment of the kamacite engulfing the native Cu specks in Co, and the lack of Ni diffusion profiles in the neighboring taenite strongly suggests formation of the assemblage through breakdown of a meta-stable complex sulfide species containing Fe, Cu, and Co as major constituents. We envisage a pentlandite-type mineral as a precursor that broke down to this assemblage according to the idealized reaction: (Fe0.833Co0.33Cu0.33) 9S8 --+ 8FeS + 0.33Cu + 0.66(FeCo) Both meteorites belong to the shock stage S2 [ 4] and no evidence for melting veins or pockets, no recrystallization of troilite and no maskelynite. Furthermore, both troilite and ilmenite grains encountered depict twin lamellae // (I 0-21) and (I 0-11) respectively, indicating formation at P < 2 GPa and low post-shock temperature (<200 °C). Conclusions: This assemblage did not form by exsolution from taenite by shock but rather through decomposition of a meta-stable complex sulfide during low-temperature cooling in the parent body. References: [I] Ramdohr P. 1963. Journal of Geophysical Research 68:2011-2036. [2] Ramdohr P. 1973. The opaque minerals in stony meteorites. Elsevier. p. 244. (3] Rubin A. E. 1994. Meteoritics & Planetary Science 29:93-98. [4] El Goresy A. 2006. Meteoritics & Planetary Science 41:A204.