Melting of compressed iron-alloys by monitoring atomic dynamics

地震和宇宙化学观测表明，地核的主要成分是铁，被认为是由~10wt%的镍和一些轻元素(如S、硅、氧、碳、氢)合金化而成的。地核由一个固体的内部区域组成，周围环绕着一个液态的外核。高压下铁的熔化温度为地核内部的温度分布提供了一个重要的参考点，并影响到与该区域有关的一些重要的地球物理量：穿过核-地幔边界的热流、核-地幔边界上方热边界层内的温度梯度、相关系和候选震相的预期地震波速。更具体地说，液态外核和固体内核交界处的铁的熔点提供了该界面温度的上限，因为到目前为止的研究表明，所有可能的外核液体与相应的内核合金固体在纯铁的熔点或以下共存。在此之前，对高压下的铁熔化的研究是通过冲击压缩、电阻和激光加热的方法，使用目测或同步辐射X射线衍射法和理论方法。然而，铁相对于其合金的熔化曲线仍然不确定，特别是在70 Gpa以上的压力下。PI开发了一种新的测量方法，可以使用57Fe同步加速器穆斯堡尔谱(SMS)在高压下检测含铁材料的固-液相边界，也称为核前向散射。带宽为1 meV的聚焦同步辐射穿过钻石顶压室内激光加热的57Fe样品。当熔化发生时，特征短信时间签名消失。这个过程由Lamb-Mössbauer因子描述，这个因子与铁原子的均方位移直接相关。因此，我们测量材料中原子的动力学，而不是静态衍射测量。由于这种方法监测原子的动态，因此，短消息技术为高压下材料的熔点测定提供了一种新的、独立的手段。我们已经成功地对高达82 Gpa的纯铁进行了这样的熔化研究。我们的数据定义了与以前的研究不同的熔化趋势，从而为进一步在更高压力下对铁和铁合金进行这种方法提供了重要的理论依据。熔化实验将在阿贡国家实验室高级光子源3-ID-B区进行，合金的压力至少为80 Gpa，纯铁的压力高于80 Gpa。通过约束这些铁合金在高PT条件下的相对熔化曲线，我们将对选择轻元素合金化对铁的影响提供一个新的约束--S熔化行为。这些结果将是朝着理解在陆地类型行星核心条件下熔化富铁合金的重要一步，从而为其各自内部提供必要的温度限制。

Seismological and cosmochemical observations indicate that the main constituent in Earth's core is iron, which is thought to be alloyed with ~10 wt% nickel and some light elements (e.g. S, Si, O, C, H). Earth's core consists of a solid inner region surrounded by a liquid outer core. The melting temperature of iron at high-pressure provides an important reference point for the temperature distribution within Earth's core and affects a number of important geophysical quantities related to this region: heat flow across the core-mantle boundary (CMB), the temperature gradient within the thermal boundary layer above the CMB, the phase relations, and expected seismic wave speeds of candidate phases. More specifically, the melting point of iron at the boundary between the liquid outer core and solid inner core provides an upper bound of the temperature at that interface, because studies thus far indicate that all plausible outer core liquids coexist with corresponding inner core alloy solids at or below the melting point of pure iron. Previously, melting studies of iron at high-pressures were performed by shock-compression, resistive- and laser-heating in diamond anvil cells using visual observations or synchrotron x-ray diffraction, and theoretical methods. However, the melting curve of iron with respect to its alloys remains uncertain, especially at pressures above 70 GPa. The PI has developed a novel metric for detecting the solid-liquid phase boundary of iron-bearing materials at high-pressures using 57Fe synchrotron Mössbauer spectroscopy (SMS), also known as nuclear forward scattering. Focused synchrotron radiation with 1 meV bandwidth passes through a laser-heated 57Fe-bearing sample inside a diamond anvil cell. The characteristic SMS time signature vanishes when melting occurs. This process is described by the Lamb-Mössbauer factor, a quantity that is directly related to the mean-square displacement of the iron atoms. Therefore, we measure the dynamics of the atoms in the material, in contrast to a static diffraction measurement. As this method monitors the dynamics of the atoms, the SMS technique provides a new and independent means of melting point determination for materials under high-pressure. We have successfully performed such melting investigations on pure iron up to 82 GPa. Our data define a different melting trend compared with previous studies, thus providing important rationale to proceed further with this method on iron and iron-alloys at higher pressures. The melting experiments will be conducted to pressures of at least 80 GPa for the alloys and higher than 80 GPa for pure iron, at sector 3-ID-B of the Advanced Photon Source at Argonne National Laboratory. By constraining the relative melting curves of these iron-alloys at high-PT conditions, we will provide a new constraint on the effect of select light-element alloying to iron?s melting behavior. These outcomes will represent a significant step towards understanding the melting of iron-rich alloys at conditions of terrestrial-type planetary cores and thus provide necessary temperature constraints of their respective interiors.