Latent-heat based measurements of the melting curve of iron to the pressure of Earth’s inner core boundary

该奖项全部或部分由《2021年美国救援计划法案》（公法117-2）资助。地核是一个巨大的金属球，位于我们脚下3000公里处。对其基本特性的进一步了解将有助于地球科学家更好地了解热量和物质在固体地球的许多层（内核、外核、地幔和地壳）内的运动方式。例如，外核液态金属的运动产生强大的磁场，保护地表的生命免受有害辐射，但驱动这种运动的力的大小是不确定的。另一个例子是，来自外核的热量可能会融化地幔底部岩石中的一些矿物质，但热流的数量和地幔底部的温度是未知的。造成这些不确定性的一个显著原因是，地核的一个基本事实仍然高度不确定：地核的温度。推断地核温度的最佳方法依赖于一个简单的想法。液态的外核，主要是铁，在地球冷却时冻结在固态的内核上，这意味着液-固界面的温度是冻结温度，或者相当于大部分是铁的液态物质的熔化温度。因此，了解地核温度及其对整个地球的影响的出发点，是准确地确定铁在受到存在于内核-外核边界（330万个大气压）的巨大压力时的熔化温度。即将到来的项目旨在通过使用一种新型的实验室测量来准确地确定这一关键温度。除了这些科学影响之外，该项目还将吸引本科生，并使研究高压相变的物理学家和材料科学家受益，这些相变涉及熵变——从磁转变到超导转变，再到熔化。该团队还将在视频杂志JoVE上公布样品制备的细节，以便向广泛的受众清楚地传播他们的方法。在快速加热铁和硫化铁样品时，将进行实验以确定潜热的吸收。样品将在金刚石砧细胞中被压缩到高达330万大气压的压力，并通过流过样品的短脉冲电流进行加热。这种脉冲焦耳加热程序可以通过限制从样品中逸出的热量来进行潜热检测，并且还可以通过限制样品熔化的总时间来限制化学反应，大约为1到100微秒。该项目旨在(a)通过实现高达约100万大气压的正/负50 K精度和高达330万大气压的正/负200 K精度，显着提高高达330万大气压的铁熔化温度数据的准确性，(b)通过测量高达100万大气压的铁（0.9）S（0.1）的固体温度，以正/负50 K精度证明潜热技术可以扩展到铁合金的部分熔化。(c)将新的测量结果吸收到地核绝热体模型和磁场形成和演化的水动力模型中。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

This award is funded in whole or in part under the American Rescue Plan Act of 2021 (Public Law 117-2).The Earth’s core is a massive ball of metal located 3000 km beneath our feet. Improved knowledge of its basic properties will help Earth scientists better understand the way heat and material move inside the solid Earth’s many layers (inner core, outer core, mantle, and crust). For example, motion of liquid metal in the outer core generates a powerful magnetic field that shields life at the surface from harmful radiation, but the magnitude of forces driving the motion are uncertain. Another example is that heat from the outer core might melt some minerals in the rocks at the base of the mantle, but the amount of heat flow and the temperature at the base of the mantle are unknown. One notable reason for these uncertainties is that a basic fact about the Earth’s core remains highly uncertain: its temperature. The best way to infer the core temperature relies on a simple idea. The liquid outer core, which is mostly iron, freezes onto the solid inner core as the Earth cools, meaning the temperature at the liquid-solid interface is the freezing temperature, or equivalently the melting temperature, of the mostly iron liquid material. Hence, the starting point for understanding the core’s temperature, and its effect on the entire Earth, is to accurately determine the melting temperature of iron when subjected to the enormous pressure that exists at the inner core-outer core boundary: 3.3 million atmospheres. The upcoming project aims to accurately determine this crucial temperature by using a new type of laboratory measurement. In addition to these scientific impacts, the project will engage undergraduate students, and benefit physicists and materials scientists studying high-pressure phase transitions that involve entropy changes - from magnetic transitions, to superconducting transitions, to melting. The team will also publish the sample preparation details in the video journal JoVE to clearly disseminate their methods to a wide audience. Experiments will be performed to identify the absorption of latent heat while samples of iron and iron-sulfide are rapidly heated. The samples will be compressed in diamond anvil cells to pressures as high as 3.3 million atmospheres, and heated with short pulses of electricity that flow through the samples. This pulsed Joule heating procedure allows latent heat detection by limiting the amount of heat escaping from samples, and also limits chemical reactions by restricting the total time during which the sample is molten to roughly 1 to 100 microseconds. The project aims to (a) dramatically improve the accuracy of the melting temperature data for iron up to 3.3 million atmospheres by achieving plus/minus 50 K accuracy up to approximately 1 million atmospheres and plus/minus 200 K accuracy up to 3.3 million atmospheres, (b) to prove that the latent-heat technique can be extended to partial melting of iron alloys by measuring the solidus temperature of Fe(0.9)S(0.1) up to 1 million atmospheres with plus/minus 50 K accuracy, and (c) assimilate the new measurements into models of Earth core adiabats and hydrodynamic models of magnetic field formation and evolution.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.