Similar to Earth, many large planetesimals in the Solar System experienced planetary-scale processes such as accretion, melting and differentiation. As their cores cooled and solidified, substantial chemical fractionation occurred due to solid metal–liquid metal fractionation. Iron meteorites—core remnants of these ancient planetesimals—record a history of this process. Recent iron isotope analyses of iron meteorites found their ⁵⁷Fe/⁵⁴Fe ratios to be heavier than chondritic by approximately 0.1 to 0.2 per mil for most meteorites, indicating that a common parent body process was responsible. However, the mechanism for this fractionation remains poorly understood. Here we experimentally show that the iron isotopic composition of iron meteorites can be explained solely by core crystallization. In our experiments of core crystallization at 1,300 °C, we find that solid metal becomes enriched in the heavier iron isotope by 0.13 per mil relative to liquid metal. Fractional crystallization modelling of the IIIAB iron meteorite parent body shows that observed iridium, gold and iron compositions can be simultaneously reproduced during core crystallization. The model implies the formation of complementary sulfur-rich components of the iron meteorite parental cores that remain unsampled by meteorite records and may be the missing reservoir of isotopically light iron. The lack of sulfide meteorites and previous trace element modelling predicting substantial unsampled volumes of iron meteorite parent cores support our findings.

与地球相似，太阳系中的许多大型行星都经历了行星尺度的过程，例如吸积，融化和分化。当它们的核心冷却并凝固时，由于固态金属-液态金属的分馏，发生了实质性的化学分馏。铁陨石是这些古代小行星的核心残余物，记录了这一过程的历史。最近对铁陨石的铁同位素分析发现其⁵⁷ Fe / ⁵⁴对于大多数陨石，铁的比重比软骨粉重约0.1-0.2 / mil，这表明是常见的母体过程造成的。但是，这种分馏的机理仍然知之甚少。在这里，我们通过实验表明，陨铁的铁同位素组成只能由核心结晶来解释。在我们在1300°C下进行的核心结晶实验中，我们发现相对于液态金属，固态金属在重铁同位素中的富集度为每密耳0.13 / mil。IIIAB铁陨石母体的分步结晶模型表明，观察到的铱，金和铁成分可以在核心结晶过程中同时复制。该模型暗示了陨铁母体核心的互补富硫成分的形成，这些成分仍未通过陨石记录进行采样，并且可能是同位素轻铁缺少的储层。硫化物陨石的缺乏和先前的痕量元素模型预测了铁陨石母核的大量未采样量，这支持了我们的发现。