Molybdenum fractionation in iron meteorites and oxidized conditions of their parent bodies

Abundances of many siderophile elements have been used to classify iron meteorites and their parent bodies, as well as to demonstrate the effects of fractional crystallization and mixing in asteroidal cores. The partitioning behaviour of molybdenum (Mo) in iron meteorites remains unclear however. We report Mo abundances for 23 magmatic iron meteorites from Groups IC, IIAB, IIIAB, IIIF and IVA, as measured by isotope dilution. Molybdenum correlates with Ga, which is also slightly chalcophile, during crystallisation of magmatic iron cores. Combining our data, literature data and experimental data, the initial Mo and W contents in the molten cores of some iron meteorites groups can be modelled. These initial contents of siderophile elements in the molten cores of iron meteorite parent bodies are controlled by their partition coefficients during core-mantle differentiation. It has been demonstrated experimentally that Mo and W partition coefficients are a function of oxygen fugacity. Combining our estimates for the initial Mo and W contents of the cores, it can be shown that conditions were relatively oxidizing (fO₂ ranging from IW+0.8 to +2.5 ) during core-mantle differentiation of the IIIAB and IVA groups. The parent body of IVB group appear to have formed in a relatively reduced environment (fO₂ = IW-9.7).