There is an extreme constancy of the isotopic composition of igneous rocks. The mean value of δFe of terrestrial rocks is 0.00 ± 0.05‰. More precise isotopic measurements indicate that the small deviations from 0.00‰ may reflect a slight mass-dependent fractionation. This mass fractionation has been proposed to be F = 0.039 ± 0.008‰ per atomic mass unit relative to IRMM-014. There may also be slight isotopic variations in igneous rocks depending on their composition and process of formation. The average value of δFe for ultramafic igneous rocks is -0.06‰, whereas the average value of δFe for mid-ocean ridge basalts (MORB) is +0.03‰. Sedimentary rocks exhibit slightly larger variations in δFe, with values between -1.6‰ and +0.9‰ relative to IRMM-014. Banded iron formations δFe span the entire range observed on Earth, from -2.5‰ to +1‰.