Iron isotope fractionation during sulfide-rich felsic partial melting in early planetesimals

New Fe isotope data of feldspar-rich meteorites Graves Nunataks 06128 and 06129 (GRA 06128/9) reveal that they are the only known examples of crustal materials with isotopically light Fe isotope compositions ( δ Fe 56 = − 0.08 ± 0.06 ‰ ; δ56Fe is defined as the per mille deviation of a sampleʹs 56Fe/54Fe ratio from the IRMM-014 standard) in the Solar System. In contrast, associated brachinites, as well as brachinite-like achondrites, have Fe isotope compositions ( δ Fe 56 = + 0.01 ± 0.02 ‰ ) that are isotopically similar to carbonaceous chondrites and the bulk terrestrial mantle. In order to understand the cause of Fe isotope variations in the GRA 06128/9 and brachinite parent body, we also report the Fe isotope compositions of metal, silicate and sulfide fractions from three ordinary chondrites (Semarkona, Kernouve, Saint-Séverin). Metals from ordinary chondrites are enriched in the heavier isotopes of Fe (average δ Fe 56 = 0.15 ‰ ), sulfide fractions are enriched in the lighter isotopes of Fe (average δ Fe 56 = − 0.14 ‰ ), and the δ56Fe values of the silicates are coincident with that of the bulk rock (average δ Fe 56 = 0.03 ‰ ). richment of light isotopes of Fe isotopes in GRA 06128/9 is consistent with preferential melting of sulfides in precursor chondritic source materials leading to the formation of Fe–S-rich felsic melts. Conceptual models show that melt generation to form a GRA 06128/9 parental melt occurred prior to the onset of higher-temperature basaltic melting (<1200 °C) in a volatile-rich precursor and led to the generation of buoyant felsic melt with a strong Fe–S signature. These models not only reveal the origin of enrichment in light isotopes of Fe for GRA 06128/9, but are also consistent with petrological and geochemical observations, experimental studies for the origin of Fe–S-rich felsic melts, and for the cessation of early melting on some asteroidal parent bodies because of the effective removal of the major radioactive heat-source, 26Al. The mode of origin for GRA 06128/9 contrasts strongly with crust formation on Earth, the Moon, Mars and other asteroids, where mantle differentiation and/or oxygen activity are the major controls on crustal Fe isotope compositions.