The effect of core composition on iron isotope fractionation between planetary cores and mantles

摘要

We have conducted high-pressure, high-temperature isotope exchange experiments between molten silicate and molten Fe-Si-C-alloys to constrain the effect of Si on equilibrium Fe isotope fractionation during planetary core formation. The values of Delta Fe-57(Metal-Silicate) at 1850 degrees C and 1 GPa determined by high-resolution MC-ICP-MS in this study range from -0.013 +/- 0.054 parts per thousand (2SE) to 0.072 +/- 0.085 parts per thousand with 1.34-8.14 atom % Si in the alloy, respectively. These results, although not definitive on their own, are consistent with previous experimental results from our group and a model in which elements that substitute for Fe atoms in the alloy structure (i.e., Ni, S, and Si) induce a fractionation of Fe isotopes between molten silicate and molten Fe-alloys during planetary differentiation. Using in situ synchrotron X-ray diffraction data for molten Fe-rich alloys from the literature, we propose a model to explain this fractionation behavior in which impurity elements in Fe-alloys cause the nearest neighbor atomic distances to shorten, thereby stiffening metallic bonds and increasing the preference of the alloy for heavy Fe isotopes relative to the silicate melt. This fractionation results in the bulk silicate mantles of the smaller terrestrial planets and asteroids becoming isotopically light relative to chondrites, with an enrichment of heavy Fe isotopes in their cores, consistent with magmatic iron meteorite compositions. Our model predicts a bulk silicate mantle delta Fe-57 ranging from -0.01 parts per thousand to -0.12 parts per thousand for the Moon, -0.06 parts per thousand to -0.33 parts per thousand for Mars, and -0.08 parts per thousand to B -0.33 parts per thousand for Vesta. Independent estimates of the delta Fe-57 of primitive mantle source regions that account for Fe isotope fractionation during partial melting agree well with these ranges for all three planetary bodies and suggest that Mars and Vesta have cores wit