Inter-mineral iron isotope fractionation in San Carlos mantle xenoliths

Iron isotopic compositions of mantle minerals can provide powerful tracers for geochemical processes in the mantle, such as partial melting, metasomatism, and oxidation. Predictions of equilibrium Fe-isotope fractionation from theory based on crystal structure and Mössbauer data conflict, making interpretation of Fe isotope fractionation uncertain. Independent constraints can be provided by characterization of inter-mineral fractionations in well understood localities. We studied inter-mineral iron isotopic fractionation of minerals from three mantle xenoliths from San Carlos, Arizona. The samples were selected for their contrasting modal abundance of constituent minerals, and include a lherzolite, a dunite, and a websterite. Our results show that there is a measureable and systematic fractionation in Fe isotope ratios between minerals that are consistent with qualitative predictions based on equilibrium fractionation theory. After the purity of the mineral separates was guaranteed by checking the composition of each grain on a scanning electron microscope, the samples were dissolved in a Parr bomb at 240°C. Iron was extracted and purified by anionic exchange chromatography, and isotope measurements were carried out by MC-ICPMS. Iron isotope compositions are reported relative to IRMM-14. For the dunite and the websterite, the measured order of ^57}Fe/{54Fe is spinel > orthopyroxene ≈ olivine > clinopyroxene. For the lherzolite the observed order from highest to lowest ^57}Fe/{54Fe is spinel > clinopyroxene > orthopyroxene ≈ olivine. This observation agrees with Mg isotope data from the same xenolith (Young et al., 2009), suggesting that the clinopyroxene is out of equilibrium with the rest of the rock. This is most likely the result of partial melting and/or metasomatism by a liquid/fluid phase. The order of fractionation of the other minerals is consistent with our understanding of bond strengths in these phases, validating the use of bond strengths as a means for interpreting Fe isotope inter-mineral fractionation in igneous systems.