The crystallisation sequence of Earth's core

地核在使地球活跃方面发挥着关键作用，例如向地幔提供热量以部分驱动构造过程，并通过为地球发电机提供动力来产生磁场，但人们对这些过程知之甚少。在这里，我们将通过研究不纯液体核心中的分离和结晶过程来阐明地球核心是如何运作的。据信，堆芯由铁合金组成，但含有少量杂质，但这些杂质对其性能有很大影响。这种所谓的“轻元素”含量解释了核心相对于纯铁的4%-7%的密度差，从而改变了核心材料的结晶温度和核心对流的动力方式。在提出的候选轻元素中，硅和氧可能存在于古老的核心中，这是地球历史早期岩浆海洋中金属-硅酸盐相互作用的结果，尽管它们各自的浓度尚未确定。该项目将确定(I)Fe-Si-O岩心液体的结晶顺序，通过构建它们在高压(P)和温度(T)条件下的行为的热力学模型，以及(Ii)岩心的化学演化及其对发电机动力的影响。限制地核的冷却过程是理解地核的起源、当前状态和演化的基础，通过产生磁场来保护地球大气层和生物圈免受太阳风和有害电离辐射的影响，地核与表面环境和生命联系在一起。这个项目将采用耦合的方法来(I)通过将Pi自己的新颖和非常精确的实验室测量与热力学模型相结合。我们将在我们研制的内阻加热金刚石压腔(DAC)中通过高P-T实验来研究Fe-Si-O系统中的共晶熔化和亚固相线相关系。这种技术产生了高达P=200 Gpa和T=4000K的极端条件，最值得注意的是，由于阻性加热(+/-50K)，温度精度比传统的激光加热系统(在3000K下约+/-200K)要好得多。样品分析将在同步加速器设施中进行原位X射线衍射测量，并在爱丁堡大学进行化学和织构观察。我们还将使用热力学计算，使用实验约束的共晶熔点来获得高P-T条件下液体的性质，这些条件不受实验的直接限制。使用内部电阻加热的数模转换器是精确限制液体热力学性质的关键，而这在现在是不可能的。根据所建立的热力学模型，我们将计算结晶相关系，然后确定在核心冷却时如何通过沉淀SiO_2或分离来发生部分化学差异。我们将使用为结晶过程获得的物理参数进行地球动力学计算来检查(Ii)，作为其中的一部分，我们将通过比较由此产生的发电机历史和磁场强度的古地磁记录来测试铁的拟议热导率的广泛范围以及古代(即开始的)核心的硅/氧比。因此，我们将报告一组一致的古岩芯中的硅氧比、铁电导率和记录的古地磁数据的场强。然后，我们将根据古岩芯中的硅/氧比确定岩心形成的物理化学条件，包括岩浆海洋的氧逸度。约束的电导率值也将为核幔边界的能量通量及其对地幔对流的影响提供一个新的估计。这种对核心性质的新洞察推动了对地球基本过程的理解，并将是理解地球如何运作的关键，包括其表面环境及其维持生命的能力。

Earth's core plays a key role in making the planet active, such as supplying heat to the mantle to partially drive tectonic processes and generating magnetic fields by powering the geodynamo, but the processes are poorly known and understood. Here we will elucidate how Earth's core functions by examining the processes of demixing and crystallisation in an impure liquid core. The core is believed to consist of Fe alloy, but with small amounts of impurities that nevertheless have a strong effect on its properties. This so-called 'light element' content accounts for the core's 4-7 % density deficit relative to pure iron, changing the crystallising temperature of core materials and how core convection is powered. Among proposed candidate light elements, silicon and oxygen are likely present in the ancient core as a consequence of metal-silicate interaction in the magma ocean early in Earth's history, although their respective concentrations are yet to be determined. This project will determine (i) the crystallisation sequence of Fe-Si-O core liquids by constructing a thermodynamic model for their behaviour under high pressure (P) and temperature (T) conditions, and (ii) the chemical evolution of the core and its influences on powering the geodynamo. Constraining cooling process of the core is fundamental for understanding the origin, current state, and evolution of Earth's core, which are linked to surface environment and life through generating magnetic fields that protect the Earth's atmosphere and biosphere from the solar wind and harmful ionising radiation.This project will take a coupled approach to (i) by combining PI's own novel and very precise laboratory measurements with thermodynamic modelling. We will examine eutectic melting and subsolidus phase relations in the system Fe-Si-O by high-P-T experiment in internally resistive-heated diamond anvil cells (DAC) that we have developed. This technique produces extreme conditions up to P = 200 GPa and T = 4000 K and most notably the temperature precision is much better due to resistive heating (+/-50 K) than in the conventional laser-heating system (ca. +/-200 K at 3000 K). Sample analysis will be made at synchrotron facilities for in-situ X-ray diffraction measurements and University of Edinburgh for chemical and textural observations. We will also employ thermodynamic calculations using the experimentally constrained eutectic melting points to obtain properties of liquids under high-P-T conditions which are not directly constrained by experiment. Use of the internally resistive-heated DAC is key to accurately constraining the thermodynamic properties of liquids, which was not possible until now. From the constructed thermodynamic model, we will calculate the crystallising phase relations and then determine how fractional chemical differentiation occurs upon core cooling either by precipitating SiO2 or demixing. We will employ geodynamic calculations using obtained physical parameters for crystallisation process to examine (ii), as part of which, we will test the wide range of proposed thermal conductivity of iron and the Si/O ratio of the ancient (i.e., starting) core by comparing the resulting dynamo history and paleomagnetic record of field intensity. Thus, we will report a consistent set of the Si/O ratio in the ancient core, iron conductivity, and recorded paleomagnetic data of field intensity. We will then determine the physicochemical conditions of core formation including the oxygen fugacity of the magma ocean from the Si/O ratio in the ancient core. The constrained conductivity value will also provide a new estimate for the energy flux across the core-mantle boundary and hence its influence on mantle convection. This new insight into the nature of the core drives understanding of fundamental Earth processes through time, and will be pivotal to understanding how the Earth functions, including its surface environment and its ability to sustain life.