Melting Temperatures and Diffusion Coefficients of Iron to 130 GPa

地球内核的温度和粘度对于了解地球内部的状态和演化是至关重要的。在330 Gpa时，铁的熔化温度是固体内核和液体内核交界处温度的最佳一阶估计值。内核-外核边界温度的知识提供了对核的热状态和历史的限制，以及对地球磁发电机供电的线索。然而，在50~100 Gpa相对较低的压力下，对熔化曲线甚至是固相的看法很少。外推到内核表面的压力，这些差异解释了熔化温度超过3000K的不确定度。这些熔体测定中的许多问题都源于用现有方法检测熔体的困难。对内核观察到的显著的地震各向异性表明，它正在经历一个未知来源的动态过程。由于没有测量HCP铁的扩散性质，现有的关于内核各向异性发展机制的假设依赖于对类似材料的扩散数据的极端外推。本实验测量了铁在地球核心条件下的两个关键性质：熔化温度和自扩散系数。发展了一种测量高压下铁的熔化温度和自扩散系数的新方法。这种新方法结合了高精度的激光加热钻石压腔内压力温度测量和非原位激光烧蚀电感耦合质谱仪测量，以检测从激光加热钻石压腔中回收的同位素标记铁箔样品的变化。这样，铁同位素在样品表面的扩散或混合可以精确地区分样品中的熔融和扩散，从而在高压下产生高精度的熔化温度和扩散系数。

The temperature and viscosity of the Earth's inner core is fundamentally important to the understanding of the state and evolution of the Earth's interior. The melting temperature of iron at 330 GPa is the best first-order estimate of the temperature at the boundary between the solid inner core and the liquid outer core. Knowledge of the temperature of the inner core-outer core boundary provides a constraint on the thermal state and history of the core as well as clues to the powering of the Earth's magnetic dynamo. However, there is little agreement on the melting curve or even the solid phase at relatively low pressures between 50 and 100 GPa. Extrapolated to the pressure of the surface of the inner core, these discrepancies account for more than 3000 K uncertainty in the melting temperature. Much of the problem in these melting determinations stems from difficulty in detecting melt in available methods.Significant seismic anisotropy observed for the inner core suggests that it is undergoing a dynamic process of unknown origin. With no measurements of diffusion properties on hcp iron, existing hypotheses on the mechanism by which inner core anisotropy develops rely on extreme extrapolations of diffusion data on analogous materials.This experiment measures two key properties of iron at Earth's core conditions: its melting temperature and self-diffusion coefficient. A new approach has been developed to provide measurements of melting temperatures and self-diffusion coefficients of iron at high pressures. This new method combines high-precision temperature measurements at pressure in the laser-heated diamond anvil cell with ex-situ laser ablation inductively coupled mass spectrometry measurements to detect changes in isotopically marked iron foil samples recovered from the laser-heated diamond anvil cell. In this way, the diffusion or mixing of iron isotopes on the surface of the sample can precisely differentiate melting from diffusion in the sample, yielding high-precision melting temperatures and diffusion coefficients to high pressures.