HR: 1340h
AN: P33A-1000    [Abstracts]
TI: Iron Isotope Fractionation in Iron Meteorites: New Insights into Metal-Sulfide Segregation and Core Crystallization
AU: * Williams, H M
EM: williams@erdw.ethz.ch
AF: ETH-Zurich, Sonneggstr. 5, Zurich, CH8092 Switzerland
AU: Halliday, A N
EM: halliday@erdw.ethz.ch
AF: ETH-Zurich, Sonneggstr. 5, Zurich, CH8092 Switzerland
AU: Teutsch, N
EM: teutsch@erdw.ethz.ch
AF: ETH-Zurich, Sonneggstr. 5, Zurich, CH8092 Switzerland
AU: Levasseur, S
EM: levasseur@erdw.ethz.ch
AF: ETH-Zurich, Sonneggstr. 5, Zurich, CH8092 Switzerland
AB: Recent studies have demonstrated that substantial iron isotope fractionation occurs between pallasite metal, troilite and olivine [1,2] and that smaller variations exist in the iron isotope compositions ($\delta$$^{57/54}$Fe) of bulk meteorites [3-5]. Interpreting such isotopic variations in terms of planetary formation processes is hampered by a lack of knowledge regarding the behavior of iron isotopes during accretion and core-mantle differentiation. Many iron meteorites are considered to be remnants of asteroidal cores and may be used to place preliminary constraints on the behavior of iron isotopes during planetary core formation and crystallization. We present iron isotope data obtained using standard MC-ICPMS methods [6] for metal and sulfide fractions extracted from iron meteorites. The metal fractions have $\delta$$^{57/54}$Fe values ranging from 0.02$\permil$ to 0.27$\permil$. Replicate large samples (10-15g) of the metal fractions of several meteorites have $\delta$$^{57/54}$Fe values within 0.02$\permil$ of each other. There do not appear to be any strong relationships between the $\delta$$^{57/54}$Fe values of the metal phases and the trace element compositions of the meteorites studied. However, considerable variation exists in the $\delta$$^{57/54}$Fe values of the troilites. These range from -0.40$\permil$ to 0.29$\permil$. In most cases, the troilites have $\delta$$^{57/54}$Fe values that are lighter than those of the corresponding metal fractions by $\sim$ 0.5$\permil$. Given the slow cooling rates inferred for iron meteorites it is likely that these phases are in isotopic equilibrium. If the isotopic fractionation between metal and troilite is representative of the fractionation between sulfide and melt during core crystallization, then the large differences recently proposed for the initial S contents of the cores of the different iron meteorite parent bodies [7] could be reflected in the $\delta$$^{57/54}$Fe values of bulk iron meteorites. This hypothesis will be evaluated in the light of further data. 1 F. Poitrasson et al., Lunar and Planetary Sciences XXXV,1634-1635, 2004. 2 X.K. Zhu et al., EPSL 200(1-2), 47-62, 2002. 3 F. Poitrasson et al., EPSL 223(3-4), 253-266, 2004. 4 X.K. Zhu et al., Nature 412, 311-313, 2001. 5 K. Kehm et al., GCA 67(15), 2879-2891, 2003. 6 H.M. Williams et al., Science 304, 1656-1659, 2004. 7 N.L. Chabot, GCA 68, 3607-3618, 2004.
DE: 6205 Asteroids and meteoroids
DE: 3672 Planetary mineralogy and petrology (5410)
DE: 1040 Isotopic composition/chemistry
DE: 1060 Planetary geochemistry (5405, 5410, 5704, 5709, 6005, 6008)
DE: 1015 Composition of the core
SC: Planetary Sciences [P]
MN: 2004 AGU Fall Meeting