Does planetary differentiation really fractionate iron isotopes?

Abstract

The difference in the mean Fe isotope composition of samples from the Earth, Moon, Mars and Vesta has been recently interpreted as tracking contrasted planetary accretion mechanisms [F. Poitrasson, A.N. Halliday, D.C. Lee, S. Levasseur, N. Teutsch, Iron isotope differences between Earth, Moon, Mars and Vesta as possible records of contrasted accretion mechanisms, Earth Planet. Sci. Lett. 223 (2004) 253–266]. Using newly produced Fe isotopic data on terrestrial and lunar samples, pallasites, eucrites and Martian meteorites, Weyer et al. [S. Weyer, A.D. Anbar, G.P. Brey, C. Munker, K. Mezger, A.B. Woodland, Iron isotope fractionation during planetary differentiation, Earth Planet. Sci. Lett. 240 (2005) 251–264] reinterpreted these data as fingerprinting planetary differentiation. In particular, these authors suggested that partial melting in the terrestrial and lunar mantles produced melts isotopically heavy. It is shown here that the inference of Weyer et al. [S. Weyer, A.D. Anbar, G.P. Brey, C. Munker, K. Mezger, A.B. Woodland, Iron isotope fractionation during planetary differentiation, Earth Planet. Sci. Lett. 240 (2005) 251–264] is strongly biased by the sampling approach taken. Notably, these authors used olivine in place of the host bulk peridotites δ 57Fe signatures despite this mineral has been shown to be frequently isotopically lighter than coexisting phases, and they analyzed lunar samples heavily affected chemically by the meteoritic bombardment, a process known to alter Fe isotope signatures. Their pallasite metal–silicate fractionation data are also likely biased by the approach adopted to estimate the iron isotope composition of the different mineral phases. In fact, their conclusion of Fe isotopic fractionation during basalt extraction from planetary mantles is invalidated by the observation that basaltic shergottites and eucrites have δ 57Fe indistinguishable from those of chondrites. Therefore, the heavier Fe isotopic composition of the Moon relative to the Earth, itself heavier than most chondrites and achondrites remains best explained by loss of light iron isotopes during the high temperature event accompanying the interplanetary impact that led to the formation of the Moon [F. Poitrasson, A.N. Halliday, D.C. Lee, S. Levasseur, N. Teutsch, Iron isotope differences between Earth, Moon, Mars and Vesta as possible records of contrasted accretion mechanisms, Earth Planet. Sci. Lett. 223 (2004) 253–266., F. Poitrasson, S. Levasseur, N. Teutsch, Significance of iron isotope mineral fractionation in pallasites and iron meteorites for the core–mantle differentiation of terrestrial planets, Earth Planet. Sci. Lett. 234 (2005) 151–164].