TY - JOUR
T1 - Iron isotope fractionation during planetary differentiation
AU - Weyer, Stefan
AU - Anbar, Ariel
AU - Brey, Gerhard P.
AU - Münker, Carsten
AU - Mezger, Klaus
AU - Woodland, Alan B.
N1 - Funding Information:
We thank the Natural History Museum of London, the Natural History Museum of Vienna and the Ward collection at the University of Rochester for providing pallasite samples. Asish Basu (University of Rochester) is thanked for providing peridotite samples. We are grateful to Rama Chakrabarti and Gail Arnold for their support in sample preparation. Helen Williams is thanked for providing an in-house standard and for fruitful discussions. Mark Rehkämper and one anonymous reviewer are thanked for their constructive and helpful reviews. This study was supported by the DFG (Deutsche Forschungsgemeinschaft; Project: WE 2850/1-1).
PY - 2005/12/1
Y1 - 2005/12/1
N2 - 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.
AB - 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.
KW - Core formation
KW - Iron isotopes
KW - Magma ocean
KW - Moon
KW - Solar system
KW - Terrestrial planets
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U2 - 10.1016/j.epsl.2005.09.023
DO - 10.1016/j.epsl.2005.09.023
M3 - Article
AN - SCOPUS:28044448285
SN - 0012-821X
VL - 240
SP - 251
EP - 264
JO - Earth and Planetary Science Letters
JF - Earth and Planetary Science Letters
IS - 2
ER -