Kun Wang^a, James M.D. Day^b, Randy L. Korotev^a, Ryan A. Zeigler^c and Frédéric Moynier^a,d

^aDepartment of Earth and Planetary Sciences and McDonnell Center for the Space Sciences, Washington University in St. Louis, One Brookings Drive, St. Louis, MO 63130, USA
^bGeosciences Research Division, Scripps Institution of Oceanography, La Jolla, CA 92093-0244, USA
^cAstromaterials Research and Explorations Science Directorate, Acquisition and Curation, NASA Johnson Space Center, 2101 NASA Road 1, Houston, TX 77058, USA
^dInstitut de Physique du Globe de Paris, Université Paris Diderot, Sorbonne Paris Cité, 1 rue Jussieu, 75238, Paris Cedex 05, France

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  ⁵⁶Fe is defined as the per mille   deviation of a sample’s 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 () 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 ), sulfide fractions are enriched in the lighter isotopes of Fe (average ), and the δ  ⁵⁶Fe values of the silicates are coincident with that of the bulk rock (average ).
The enrichment 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.

Reference
Wang K, Day JMD, Korotev RL, Zeigler RA and Moynier F (2014) Iron isotope fractionation during sulfide-rich felsic partial melting in early planetesimals. Earth and Planetary Science Letters 392:124–132.
[doi:10.1016/j.epsl.2014.02.022]
Copyright Elsevier

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