HR: 0800h
AN: V51E-0831    [Abstracts]
TI: Experimental determination of Fe isotope fractionation between liquid metal, silicate and sulfide at high pressures and temperatures
AU: * Williams, H M
EM: helenw@earth.ox.ac.uk
AF: Department of Earth Sciences, Oxford University, Parks Road, Oxford, OX1 3PR, United Kingdom
AU: Wood, B J
EM: berniew@earth.ox.ac.uk
AF: Department of Earth Sciences, Oxford University, Parks Road, Oxford, OX1 3PR, United Kingdom
AU: Halliday, A N
EM: alex.halliday@mpls@earth.ox.ac.uk
AF: Department of Earth Sciences, Oxford University, Parks Road, Oxford, OX1 3PR, United Kingdom
AB: There is evidence for significant equilibrium Fe isotope fractionation (≤0.26‰/amu) between metal and troilite (FeS) in iron meteorites (Williams et al., EPSL (250) 2006) and a smaller fractionation (<0.1‰/amu) between metal and olivine in pallasites (Zhu et al., EPSL (200) 2002; Weyer et al., EPSL (240) 2005). Theory suggests that differences in iron oxidation state and coordination between metal, silicate and FeS will result in stable isotope fractionation (Polyakov and Mineev, GCA (64) 2000; Schauble et al., GCA (65) 2001). However, it is not yet clear if the apparent observed fractionations can be extrapolated to the pressure and temperature conditions of planetary core formation. We have investigated Fe isotope fractionation between silicate melt and liquid Fe-S alloys and between liquid iron and basaltic melt at pressure and temperature conditions of 2-2.5GPa and 1920-2150K using piston-cylinder partitioning experiments from previous studies (Kilburn and Wood EPSL (152) 1997; Gessmann and Wood, EPSL (200) 2002; Wood et al., EPSL (in revision) 2007). Metal, sulfide and silicate fractions were separated from mounted and sectioned experimental charges using a computer-controlled micromill (New Wave-Merchantek). Sample dissolution, Fe purification and isotopic analysis followed established procedures (Williams et al., EPSL (235) 2005). In agreement with another preliminary high-pressure experimental study (Poitrasson and Roskosz, LPSC XXXVIII 2007) we find no appreciable fractionation between liquid iron metal and basaltic melt. However, there is a resolvable Fe isotope fractionation between silicate melt and Fe-S alloy which ranges from 0.12±0.04 to 0.15±0.04‰/amu for separate experiments (errors are propagated based on the 2 SD errors of replicate analyses). The Fe isotope compositions of coexisting phases from these experiments define a positive linear relationship with a slope that is, within error, equal to unity, implying isotopic equilibrium. No relationship between apparent fractionation factor and pressure or temperature is detectable within the range covered by the experiments. The fractionation factors determined from our experiments overlap with the average equilibrium fractionation factor obtained between silicate melt and pyrrhotite (Fe_1-xS) of 0.18±0.02‰/amu at 0.5GPa and 1114-1274K (Schuessler et al., GCA (71) 2007) and are also broadly consistent with silicate-FeS fractionation factors inferred indirectly from iron meteorites and pallasites which range from ~0.16 to 0.24‰/amu. Taken together these observations suggest that resolvable stable isotope fractionation between Fe-S alloys and silicate melts can take place at extreme pressure and temperature conditions and that isotopically light Fe can be sequestered into the S-bearing parts of planetary cores.
DE: 1015 Composition of the core
DE: 1027 Composition of the planets
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1060 Planetary geochemistry (5405, 5410, 5704, 5709, 6005, 6008)
DE: 3924 High-pressure behavior
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting