NSFGEO-NERC:Integrated Experimental and Dynamical Modeling of Top-down Crystallization in Terrestrial Cores:Implications for Core Cooling in the Earth

地球的磁场是在我们脚下近3000公里的液态铁核中通过一种被称为地球发电机的过程产生的。磁场保护地表环境和低轨道卫星免受太阳辐射;因此，磁场至少在过去35亿年中的存在对生命的存在和现代全球通信的运作具有广泛的影响。描述地球发电机起源的标准模型假定，磁场是通过固体地幔下的液体地核缓慢冷却和固体内核自下而上的逐渐冻结来维持的。在第一次计算了铁合金在地核条件下的热导率之后，这个模型不再成立，它预测了地球历史早期的快速冷却，年轻的内核和下地幔的普遍熔化。在这种情况下，目前还不清楚地球发电机是如何在大约5 - 10亿年前形成内核之前提供动力的。最近的研究认为，古老的核心可能是自上而下结晶的。这个联合实验-理论项目的中心目标是了解自上而下的结晶是否以及如何产生磁场并影响地核的热化学演化。该项目由两个相互关联的主要部分组成：核心类似物的实验和核心进化的理论模型。相平衡实验将在UCSD-SIO的多砧装置中使用NSF-COMPRES组件在高达30 GPa的压力和高达2200 C的温度下进行。我们将考虑Fe-S-Mg（-O）和Fe-S-O（-Si）系统，建立在我们最近在Fe-S-O系统的实验工作。淬火产品的化学分析将用于确定相的化学性质、液相线曲线和所研究系统的共晶温度。结果将应用于地球的压力和温度条件下使用严格的热力学外推，也将直接适用于小型类地行星。与此同时，我们将开发一个新的理论模型，该模型使用最近用于研究火星核心的技术来描述地球核心顶部两相区域的热演化和化学演化。该模型将预测两相区的性质和磁场的演化，这可以使用各种观测进行测试，因此将提供过去35亿年来地核演化的连贯描述。该提议的一个新颖方面是实验和理论模型之间的不断相互作用。将使用基于化学的方法来改进模型，然后将使用数值结果来激励特定组合物的新实验。这项拟议中的研究将大大提高目前对地球以及水星和火星等其他行星核心结晶的理解。

Earth's magnetic field is generated almost 3000 km below our feet in the liquid iron core by a process known as the geodynamo. The field protects the surface environment and low-orbiting satellites from solar radiation; its existence for at least the last 3.5 billion years therefore has broad implications for the presence of life and the operation of modern global communications. The standard model describing the origin of the geodynamo posits that the field is maintained by slow cooling of the liquid core below a solid mantle and gradual bottom-up freezing of the solid inner core. This model is no longer tenable following the first calculations of the thermal conductivity of iron alloys at core conditions, which predict rapid cooling, a young inner core and pervasive melting of the lower mantle early in Earth's history. In this scenario it is presently unclear how the geodynamo was powered before the inner core formed some 0.5-1 billion years ago. Recent studies have argued that the ancient core could have crystallized from the top down. The central aim of this joint experimental-theoretical project is to understand if and how top-down crystallization generates magnetic fields and influences the thermochemical evolution of Earth's core. The project consists of two major interlinked components: experiments on core analogues and theoretical models of core evolution. Phase equilibria experiments will be carried out at pressure up to 30 GPa and temperature up to 2200 C in the multi-anvil apparatus at UCSD-SIO using NSF-COMPRES assemblies. We will consider the Fe-S-Mg(-O) and Fe-S-O(-Si) systems, building on our recent experimental work in the Fe-S-O system. Chemical analyses of quenched products will be used to determine the chemistry of phases, the liquidus curve and the eutectic temperature for the investigated systems. Results will be applied to the Earth's pressure and temperature conditions using rigorous thermodynamic extrapolation and will also be directly applicable to small terrestrial planets. In parallel we will develop a new theoretical model that describes the thermal and chemical evolution of two-phase regions at the top of Earth's core using techniques that were recently employed to study the Martian core. The model will predict the properties of the two-phase region and the evolution of the magnetic field, which can be tested using a variety of observations, and will therefore provide a coherent description of Earth's core evolution over the past 3.5 billion years.A novel aspect of this proposal is the constant interactions between experiments and theoretical models. Laboratory-based chemistry will be used to refine the models, and numerical results will then be used to motivate new experiments at specific compositions. The proposed study will significantly improve the current understanding of core crystallization in the Earth and also in other planets such as Mercury and Mars.