Physics
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufm.v13e..01w&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #V13E-01
Physics
1015 Composition Of The Core, 1026 Composition Of The Moon, 1027 Composition Of The Planets, 1041 Stable Isotope Geochemistry (0454, 4870), 4870 Stable Isotopes (0454, 1041)
Scientific paper
We present Fe isotope data for metal and troilite fractions separated from iron meteorites, to investigate isotope fractionation processes during planetary core segregation/crystallization processes. The Fe isotope compositions (δ^{57/54}Fe) of metal fractions separated from magmatic and non- magmatic irons range from -0.07 to +0.32‰). The δ^{57/54}Fe values of troilites (FeS) separated from these irons range from -0.60 to -0.12‰, defining metal-troilite fractionation factors of +0.04 to +0.79‰. These fractionation factors could have resulted from kinetic or equilibrium fractionation processes. Kinetic fractionation could operate during processes such as initial troilite nucleation at the Fe-FeS eutectic (c.a. 990° C) or subsolidus diffusive exchange between metal and troilite during cooling. The latter process is likely to be more important and may well overprint any isotopic signatures resulting from the former as it takes place at lower temperatures where differences in the diffusion rates of heavy and light isotopes are enhanced. We investigated kinetic fractionation during subsolidus diffusion using mass balance models in conjunction with our Fe isotope data and Ni stable isotope data for the same samples (Quitt é et al, EPSL, submitted). Our models suggest that kinetic fractionation processes cannot explain the Fe isotope data. However, fractionation factors display a positive correlation with kamacite bandwidth, i.e. the most slowly-cooled meteorites, which should be closest to diffusive equilibrium, show the greatest difference in metal and sulphide δ^{57/54}Fe values. The maximum fractionation factors seen in each iron meteorite group are extremely similar: 0.79, 0.63, 0.76 and 0.74‰ for the IIAB, IIIAB, IAB and IIICD irons, respectively. These observations suggest that the largest fractionation factors approach isotopic equilibrium, and that the range in fractionation factors is produced in a scenario where sulphides initially nucleate out of isotopic equilibrium with the metal, and equilibrate to different degrees during cooling. A fractionation factor of c.a. 0.80‰ between metal and troilite is consistent with available high- temperature experimental data (e.g. Roskosz et al., EPSL 2006; Schuessler et al., GCA, 2005, A211). However, it is surprising in the light of theoretical studies, which predict sulphides to be isotopically heavy with respect to native Fe (Polyakov and Mineev, GCA, 2000; Schauble et al., GCA, 2001). Possible causes of isotopic fractionation include the contrast between the metallic bonding of Fe in kamacite and taenite with the ionic nature of Fe-S bonds in troilite. Alternatively, S2- may behave in a similar fashion to Cl-, a relatively large anion with small force-constants, which could favour the concentration of light Fe isotopes in species where Fe is bound to S2-anions (E. Schauble, pers comm.). Models using this fractionation factor can explain the relatively heavy δ^{57/54}Fe values of IIAB metals as a function of sequestering of isotopically light iron into the S-bearing parts of planetary cores and the large amounts of S inferred to be present in the core of the IIAB parent body (Chabot, GCA 2004).
Halliday Alex
Levasseur Sylvain
Markowski Agnès
Quitté Ghylaine
Teutsch Nadya
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