Comment on “Heavy iron isotope composition of granites determined by high resolution MC-ICP-MS”, by F. Poitrasson and R. Freydier [Chem. Geol. 222 132–147

Abstract

Scrutiny of published Fe isotope data for igneous rocks that have > 71 wt.% SiO 2 reveals that most samples have the same Fe isotope composition as mafic igneous rocks, contrary to the conclusions of Poitrasson and Freydier [Poitrasson, F. and Freydier, R., 2005. Heavy iron isotope composition of granites determined by high resolution MC-ICP-MS. Chem. Geol. 222, 132–147]. The only silicic igneous rocks that have Fe isotope compositions that are significantly different than mafic igneous rocks are 4 granitic samples that have SiO 2 > 75 wt.% analyzed by Poitrasson and Freydier [Poitrasson, F. and Freydier, R., 2005. Heavy iron isotope composition of granites determined by high resolution MC-ICP-MS. Chem. Geol. 222, 132–147]; three silicic igneous rocks analyzed by Beard et al. [Beard, B.L, Johnson, C.M., Skulan, J.L., Nealson, K.H., Cox, L., Sun, H., 2003a. Application of Fe isotopes to tracing the geochemical and biological cycling of Fe. Chem. Geol. 195 87–117] (SiO 2 > 75 wt.%) have Fe isotope compositions that are identical to mafic igneous rocks, but these samples were ignored. Moreover, the Fe isotope composition of upper continental crust as deduced from the 58 sedimentary clastic rocks (shales, loess, modern marine sediments, and suspended load of rivers), reported by Beard et al. [Beard, B.L., Johnson, C.M., von Damm, K.L. and Poulson, R.L, 2003b. Iron isotope constraints on Fe cycling and mass balance in oxygenated Earth oceans. Geology 31 629–632], is the same as the average of igneous rocks. These data suggest that granitic rocks which deviate from the igneous average may be rare.