Silicon Isotope Evidence Against an Enstatite Chondrite Earth

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Science  01 Mar 2012:
DOI: 10.1126/science.1219509


The compositions of Earth materials are strikingly similar to those of enstatite chondrite meteorites in many isotope systems. Although this suggests that Earth largely accreted from enstatite chondrites, definitive proof of this model has been lacking. By comparing the Si isotope signatures of several extraterrestrial materials with terrestrial samples, we show that they cannot be explained by core formation scenarios involving a bulk Earth of enstatite chondrite composition. Si isotope similarities between the bulk silicate Earth and the Moon preclude the existence of a hidden reservoir in the lower mantle, a necessary condition of the enstatite chondrite model, and require an equilibrium process after the Moon-forming impact. A three-endmember chondritic mixing model for the Earth reconciles the Si isotope similarities between enstatite chondrites and Earth.