Helium in Earth’s early core

The observed escape of the primordial helium isotope, 3 He, from the Earth’s interior indicates that primordial helium survived the energetic process of planetary accretion and has been trapped within the Earth to the present day. Two distinct reservoirs in the Earth’s interior have been invoked to account for variations in the 3 He/ 4 He ratio observed at the surface in ocean basalts: a conventional depleted mantle source and a deep, still enigmatic, source that must have been isolated from processing throughout Earth history. The Earth’s iron-based core has not been considered a potential helium source because partitioning of helium into metal liquid has been assumed to be negligible. Here we determine helium partitioning in experiments between molten silicates and iron-rich metal liquids at conditions up to 16 GPa and 3,000 K. Analyses of the samples by ultraviolet laser ablation mass spectrometry yield metal–silicate helium partition coefficients that range between 4.7×10 −3 and 1.7×10 −2 and suggest that significant quantities of helium may reside in the core. Based on estimated concentrations of primordial helium, we conclude that the early core could have incorporated enough helium to supply deep-rooted plumes enriched in 3 He throughout the age of the Earth.