On the iron isotope homogeneity level of the continental crust

International audienceAssessing the level of homogeneity of different planetary reservoirs is an essential step to establish a new isotopic tracer. In recent studies, we concluded that most igneous rocks from the continental crust were homogeneous to similar to 71 wt.% SiO2) that we concluded were significantly heavier than less silicic rocks from the crust on the basis of Fe isotopic analyses of seven bulk rock samples from different geological settings. Beard and Johnson [Beard, B.L., Johnson, C.M., 2006. Comment on "Heavy iron isotope composition of granites determined by high resolution MC-ICP-MS" by F. Poitrasson and R. Freydier, Chemical Geology, volume 222, pages 132-147. Chem. Geol., this issue.] challenge this finding on the basis of previously "published" data. It is shown here that this controversy stems from the confusion between whole-rock and mineral iron isotope analyses, since this isotopic tracer, like many others, is more heterogeneous at the mineral scale than the bulk rock scale. Thus, minerals cannot be used in place of bulk rock Fe isotope compositions. The controversy also arises from different levels of analytical uncertainties. If only the best Fe isotope probes of planetary reservoirs-bulk rocks - are considered, then the conclusion that igneous rocks with SiO2 > 71 wt% are significantly heavier than the mean delta Fe-57 of the continental crust [Poitrasson, F., Freydier, R., 2005. Heavy iron isotope composition of granites determined by high resolution MC-ICP-MS. Chem. Geol., 222, 132-147.] remains valid. Nonetheless, given that granites represent only a minor fraction of the continental crust, this finding does not change the bulk igneous Fe isotope composition estimate previously defined for the Earth