Iron isotope fractionation during planetary differentiation

Stefan Weyer, Ariel Anbar, Gerhard P. Brey, Carsten Münker, Klaus Mezger, Alan B. Woodland

Research output: Contribution to journalArticlepeer-review

177 Scopus citations

Abstract

The Fe isotope composition of samples from the Moon, Mars (SNC meteorites), HED parent body (eucrites), pallasites (metal and silicate) and the Earth's mantle were measured using high mass resolution MC-ICP-MS. These high precision measurements (δ56Fe ≈ ±0.04‰, 2 S.D.) place tight constraints on Fe isotope fractionation during planetary differentiation. Fractionation during planetary core formation is confined to <0.1‰ for δ56Fe by the indistinguishable Fe isotope composition of pallasite bulk metal (including sulfides and phosphides) and olivine separates. However, large isotopic variations (≈ 0.5‰) were observed among pallasite metal separates, varying systematically with the amounts of troilite, schreibersite, kamacite and taenite. Troilite generally has the lightest (δ56Fe ≈ -0.25‰) and schreibersite the heaviest (δ56Fe ≈ +0.2‰) Fe isotope composition. Taenite is heavier then kamacite. Therefore, these variations probably reflect Fe isotope fractionation during the late stage evolution and differentiation of the S- and P-rich metal melts, and during low-temperature kamacite exsolution, rather than fractionation during silicate-metal separation. Differentiation of the silicate portion of planets also seems to fractionate Fe isotopes. Notably, magmatic rocks (partial melts) are systematically isotopically heavier than their mantle protoliths. This is indicated by the mean of 11 terrestrial peridotite samples from different tectonic settings (δ56Fe=+0.015 ± 0.018‰), which is significantly lighter than the mean of terrestrial basalts (δ56Fe=+0.076 ± 0.029‰). We consider the peridotite mean to be the best estimate for the Fe isotope composition of the bulk silicate Earth, and probably also of bulk Earth. The terrestrial basaltic mean is in good agreement with the mean of the lunar samples (δ56Fe=+0.073 ± 0.019‰), excluding the high-Ti basalts. The high-Ti basalts display the heaviest Fe isotope composition of all rocks measured here (δ56Fe ≈ +0.2‰). This is interpreted as a fingerprint of the lunar magma ocean, which produced a very heterogeneous mantle, including the ilmenite-rich source regions of these basalts. Within uncertainties, samples from Mars (SNC meteorites), HED (eucrites) and the pallasites (average olivine + metal) have the same Fe isotope compositions as the Earth's mantle. This indicates that the solar system is very homogeneous in Fe isotopes. Its average δ56Fe is very close to that of the IRMM-014 standard.

Original languageEnglish (US)
Pages (from-to)251-264
Number of pages14
JournalEarth and Planetary Science Letters
Volume240
Issue number2
DOIs
StatePublished - Dec 1 2005

Keywords

  • Core formation
  • Iron isotopes
  • Magma ocean
  • Moon
  • Solar system
  • Terrestrial planets

ASJC Scopus subject areas

  • Geophysics
  • Geochemistry and Petrology
  • Earth and Planetary Sciences (miscellaneous)
  • Space and Planetary Science

Fingerprint Dive into the research topics of 'Iron isotope fractionation during planetary differentiation'. Together they form a unique fingerprint.

Cite this