Core origin of seismic velocity anomalies at Earth’s core–mantle boundary

Seismic studies have found fine-scale anomalies at the core–mantle boundary (CMB), such as ultralow velocity zones (ULVZs)^1,2 and the core rigidity zone^3,4. ULVZs have been attributed to mantle-related processes^5–10, but little is known about a possible core origin. The precipitation of light elements in the outer core has been proposed to explain the core rigidity zone³, but it remains unclear what processes can lead to such precipitation. Despite its importance for the outer core¹¹, the melting behaviour of Fe–Si–H at relevant pressure–temperature conditions is not well understood. Here we report observations of the crystallization of B2 FeSi from Fe–9wt%Si melted in the presence of hydrogen up to 125 GPa and 3,700 K by using laser-heated diamond anvil cells. Hydrogen dramatically increases the Si concentration in the B2 crystals to a molar ratio of Si:Fe ≈ 1, whereas it mostly remains in the coexisting Fe liquid. The high Si content in the B2 phase makes it stable in a solid form at the outermost core temperatures and less dense than the surrounding liquids. Consequently, the Si-rich crystallites could form, float and be sedimented to the underside of the CMB interface, and that well explains the core side rigidity anomalies^3,4. If a small amount of the FeSi crystals can be incorporated into the mantle, they would form dense low-velocity structures above the CMB, which may account for some ULVZs¹⁰. The B2 FeSi precipitation promoted by H in the outermost core provides a single core-driven origin for two types of anomalies at the CMB. Such a scenario could also explain the core-like tungsten isotope signatures in ocean island basalts¹², after the materials equilibrated with the precipitates are entrained to the uppermost mantle by the mantle plumes connected to ULVZs.