Chemical evolution of planetesimal cores during the early history of the solar system

地球是由许多小的10-100公里大小的微行星和少量的较大的月球到火星大小的胚胎聚集而成。根据目前的地核形成模型，早期在地球上增生的天体应该在高度还原的条件下经历了核幔分化，因此大量的硅、铬和钒被分配到它们的金属铁镍核中。铁陨石来自早期形成的微行星的金属核心，与预测相反，这些陨石中的Si和Cr浓度极低（例如<1 ppm）。该提案的目的是验证以下假设：熔融的星子核最初含有高浓度的弱亲铁元素Si和Cr，但在冷却过程中，这些元素变得越来越亲石，并分配回上覆的熔融地幔（岩浆海洋）。这一假设将首先通过实验（在1-25千兆帕斯卡）和理论（在更大的压力范围内使用分子动力学模拟）确定在熔融铁中的Si和Cr的化学扩散系数进行测试。其次，我们将开发模型的化学相互作用在星子核幔边界的现实冷却历史，假设液体的核心和覆盖岩浆海洋是在对流状态。化学演变的速率将由通过边界层的扩散控制，对于边界层，将需要扩散结果。

The Earth formed by the accretion of numerous small 10-100 km-sized planetesimals together with a smaller number of larger Moon- to Mars-sized embryos. According to current models of Earth's core formation, the bodies that accreted early to the Earth should have undergone core-mantle differentiation under highly reducing conditions, such that significant amounts of silicon, chromium and vanadium partitioned into their metallic Fe-Ni cores. Iron meteorites are derived from the metallic cores of early-formed planetesimals, and, in contrast to the predictions, the concentrations of Si and Cr in these meteorites are extremely low (e.g. <1 ppm). The aim of this proposal is to test the following hypothesis: Molten planetesimal cores originally contained high concentrations of the weakly siderophile elements Si and Cr but during cooling these elements became increasingly lithophile and partitioned back into the overlying molten mantle (magma ocean). This hypothesis will be tested by first determining the chemical diffusivities of Si and Cr in molten iron both experimentally (at 1-25 Gigapascals) and theoretically (using molecular dynamics simulations over a larger pressure range). Second we will develop models of chemical interaction at planetesimal core-mantle boundaries for realistic cooling histories, assuming that both the liquid core and overlying magma ocean are in a state of convection. The rate of chemical evolution will be controlled by diffusion through boundary layers for which the diffusion results will be required.