The crystallisation sequence of Earth's core

地核的结晶序列

• 批准号: NE/W005832/1
• 负责人: Tetsuya Komabayashi
• 金额: $ 51.67万
• 依托单位: University of Edinburgh
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2022
• 资助国家: 英国
• 起止时间: 2022 至 无数据
• 项目状态: 未结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FW005832%2F1
• 关键词: crystallisation; sequence; Earth; core

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.

地核在使地球活跃方面起着关键作用，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，但这些过程鲜为人知。在这里，我们将阐明如何地球的核心功能，通过检查过程中的分层和结晶在一个不纯的液体核心。核心被认为是由铁合金组成，但含有少量的杂质，但对它的性能有很大的影响。这种所谓的“轻元素”含量占核心相对于纯铁的4- 7%密度赤字，改变核心材料的结晶温度和核心对流的动力。在提出的候选轻元素中，硅和氧可能存在于古老的地核中，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核心液体的结晶顺序，方法是为它们在高压（P）和高温（T）条件下的行为构建热力学模型，以及（ii）核心的化学演变及其对地球发电机的影响。限制地核的冷却过程是理解地核起源、现状和演化的基础，通过产生磁场，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，从而与地表环境和生命联系在一起。通过结合PI自己的新颖和非常精确的实验室测量与热力学建模。我们将利用自行研制的内循环加热金刚石对顶砧装置，通过高温高压实验研究Fe-Si-O系的共晶熔化和亚固相线相关系。这种技术产生的极端条件高达P = 200 GPa和T = 4000 K，最值得注意的是，由于电阻加热（+/-50 K），温度精度比传统的激光加热系统（约100 K）好得多。+/-200 K at 3000 K）。样品分析将在同步加速器设施进行现场X射线衍射测量，并在爱丁堡大学进行化学和结构观察。我们还将采用热力学计算，使用实验约束的共晶熔点，以获得高P-T条件下的液体的性质，而不是直接由实验约束。使用内部连续加热的DAC是精确限制液体热力学性质的关键，这在以前是不可能的。从构建的热力学模型，我们将计算结晶相关系，然后确定分数化学分化发生在核心冷却沉淀二氧化硅或分层。我们将采用地球动力学计算，使用获得的结晶过程的物理参数来检查（ii），作为其中的一部分，我们将测试铁的热导率和古硅氧比（即，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，因此，我们将报告一组一致的硅/O比在古代核心，铁导电性，并记录古地磁场强度的数据。然后，我们将确定核形成的物理化学条件，包括氧逸度的岩浆海洋从硅/O比在古老的核心。约束电导率值也将提供一个新的估计的能量通量通过核幔边界，从而对地幔对流的影响。这种对地核性质的新见解推动了对地球基本过程的理解，并将对理解地球如何运作，包括其表面环境及其维持生命的能力至关重要。