Comment on “Iron isotope fractionation during planetary differentiation” by S. Weyer et al., Earth Planet. Sci. Lett. V240, pages 251–264

A review of published Fe isotope compositions measured for mantle xenoliths and massif-type peridotites demonstrates that the Fe isotope composition of the “mantle” is highly heterogeneous as compared to the narrow Fe isotope range measured for basaltic rocks. The greatest isotopic heterogeneity exists for samples derived from the lithospheric mantle, whereas basalts largely record the isotopic compositions of the asthenospheric mantle. These published Fe isotope data show that the variability measured in mantle samples is too large to be used to accurately determine a planetary Fe isotope composition, and, contrary to the assertions of Weyer et al. [S. Weyer, A. Anbar, G. Brey, C. Münker, K. Mezger, A. Woodland, Iron isotope fractionation during planetary differentiation. Earth Planet. Sci. Lett. 240 (2005) 251–264], there is no evidence that the bulk terrestrial mantle has a different Fe isotope composition relative to terrestrial basaltic magmas. We contend that similar rock types must be compared (i.e., basalt–basalt) when considering possible inter-planetary Fe isotope differences.