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

作者：Craddock Paul R^*; Warren Jessica M; Dauphas Nicolas

来源：Earth and Planetary Science Letters, 2013, 365: 63-76.

DOI：10.1016/j.epsl.2013.01.011

• iron
• isotope
• abyssal peridotite
• mantle
• chondritic

Terrestrial oceanic and continental basalts are enriched by approximately +0.1 parts per thousand in Fe-56/Fe-54 ratio relative to primitive, undifferentiated meteorites (chondrites). The delta Fe-56 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 delta Fe-56 value of +0.010 +/- 0.007 parts per thousand (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 delta Fe-56 = +0.025 +/- 0.025 parts per thousand, which is also indistinguishable from chondrites, within current analytical precision. We determine that the maximum shift in delta Fe-56 for peridotite residues during partial mantle melting is 0.01 parts per thousand. 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 delta Fe-56 value that is non-chondritic. In addition, our results suggest that disproportionation of mantle Fe2+-Fe3+ in perovskite and Fe-0 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.

• 出版日期2013-3-1