Abyssal peridotites reveal the near-chondritic Fe isotopic composition of the Earth

Abstract

Terrestrial oceanic and continental basalts are enriched by approximately +0.1δ in ⁵⁶Fe/⁵⁴Fe ratio relative to primitive, undifferentiated meteorites (chondrites). The δ⁵⁶Fe values of terrestrial basalts are also distinct from those of basalts from Mars and asteroid Vesta, which have chondritic Fe isotopic compositions. The processes responsible for the isotopic enrichment of terrestrial basalts are debated, in part because the Fe isotopic composition of the mantle source of terrestrial basalts is unknown. Here we report Fe isotopic measurements of abyssal peridotites, which are the residues of limited melting at oceanic ridges and are thus the best proxies for the composition of the convective portion of the mantle. Our data show that abyssal peridotites have a mean δ⁵⁶Fe value of +0.010±0.007δ (relative to IRMM-014), which is indistinguishable from chondrites. After correcting this data for seafloor weathering and mantle melting, we estimate the average Fe isotopic composition of the terrestrial mantle to be δ⁵⁶Fe=+0.025±0.025δ, which is also indistinguishable from chondrites, within current analytical precision. We determine that the maximum shift in δ⁵⁶Fe for peridotite residues during partial mantle melting is 0.01δ. Our results argue against isotopic fractionation during core-mantle differentiation or iron vaporization during the Moon-forming giant impact, because both processes would yield a bulk mantle δ⁵⁶Fe value that is non-chondritic. In addition, our results suggest that disproportionation of mantle Fe²⁺-Fe³⁺ in perovskite and Fe⁰ metal and segregation of metal to the core could not have been a driver for Fe isotopic fractionation in the silicate mantle. Instead, the different iron isotopic compositions of abyssal peridotites and MORBs support mounting evidence for iron isotopic fractionation of melts but not residues during the formation of oceanic and continental crust. © 2013 Elsevier B.V.