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

地球是由许多10-100公里大小的小星子和少量月球到火星大小的大胚胎聚集而成的。根据目前的地核形成模型，早期吸积到地球的天体应该在高度还原的条件下经历了地核-地幔分化，这样大量的硅、铬和钒就会分裂成金属铁-镍核。铁陨石来自早期形成的星子的金属核，与预测相反，这些陨石中Si和Cr的浓度极低（例如< 1ppm）。这一提议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，并重新分裂到覆盖的熔融地幔（岩浆海洋）中。这一假设将首先通过实验（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）确定铁液中Si和Cr的化学扩散率来验证。其次，我们将为现实的冷却历史建立星子核-地幔边界的化学相互作用模型，假设液体核心和上覆的岩浆海洋都处于对流状态。通过边界层的扩散将控制化学演化的速率，而边界层的扩散结果将是必需的。

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.