Earth’s bulk composition has elemental and isotopic characteristics that cannot be fully reconciled with a mixture of known primitive meteorite compositions1–3. One potential explanation for this is that the proto-Earth accreted materials with isotopic signatures distinct from those accreted after the Moon-forming giant impact. Here we report high-precision mass-independent potassium isotopic measurements from thermal ionization mass spectrometry of terrestrial rocks from various ancient and modern sources in the crust and mantle that we argue are consistent with this explanation. Specifically, we found that some mafic Archaean rocks derived from the Hadean–Eoarchaean mantle (including samples from Isua, Nuvvuagittuq and the Kaapvaal Craton) and certain modern ocean island basalts (from La Réunion Island and Kama’ehuakanaloa volcano, Hawaii) exhibit an average 40K deficit of 65 parts per million compared to all other terrestrial samples analysed. The deficit distinguishes these samples from the bulk silicate Earth and any known meteorite group and cannot result from magmatic processes. Therefore, we propose this 40K deficit represents primitive proto-Earth mantle domains that largely escaped mantle mixing after the giant impact and exist in the present-day deep mantle, contributing to some modern hotspot volcanism.
