The D” layer at the base of the Earth’s mantle exhibits anomalous seismic properties, which are attributed to heat loss from and chemical interaction with the underlying molten Fe-rich outer core. Here we show that mass transfer due to temperature variations within the D” layer could lead to resolvable fractionation of iron isotopes. We constrain the degree of isotope fractionation by experiments on core-forming Fe alloy liquids at 2100–2300 K and 2 GPa, which demonstrate that heavy Fe isotopes preferentially migrate towards lower temperature and vice versa. We find that this isotope fractionation occurs rapidly due to the high mobility of iron, which reaches 0.013 ± 0.002‰ (2σ) per degree per amu at steady state. Numerical simulations of mantle convection capturing the evolution of a basal thermal boundary layer show that iron isotope fractionation immediately above the core–mantle boundary can reach measurable levels on geologic timescales and that plumes can entrain this fractionated material into the convecting mantle. We suggest that such a process may contribute to the heavy Fe isotope composition of the upper mantle inferred from mantle melts (basalts) and residues (peridotites) relative to chondrites. That being the case, non-traditional stable isotope systems such as Fe may constrain the interactions between the core and mantle.

地球地幔底部的D''层表现出异常的地震特性，这归因于来自下方熔融富铁外核的热损失以及与之发生的化学相互作用。在这里，我们表明，由于D''层内的温度变化而引起的质量转移可能导致可分离的铁同位素分离。我们通过在2100–2300 K和2 GPa的成芯铁合金液体上进行的实验来限制同位素的分级程度，这表明重铁同位素优先向较低温度迁移，反之亦然。我们发现，这种同位素分馏快速发生由于铁的高迁移率，达到0.013±0.002‰（2 σ每度全）AMU处于稳定状态。地幔对流捕获基础热边界层演化的数值模拟表明，岩心-地幔边界正上方的铁同位素分馏在地质时标上可以达到可测量的水平，并且羽状流可以将这种分馏的物质夹带到对流的地幔中。我们认为，这种过程可能有助于上地幔的重铁同位素组成，这是由相对于球粒陨石的地幔熔体（玄武岩）和残余物（橄榄岩）推断的。在这种情况下，非传统的稳定同位素系统（例如Fe）可能会限制岩心和地幔之间的相互作用。