Thermochemical remanent magnetisations: How do they affect ancient magnetic field intensities from the Earth and Solar System?

热化学剩磁：它们如何影响地球和太阳系的古代磁场强度？

• 批准号: NE/V001388/1
• 负责人: Adrian Muxworthy
• 金额: $ 83.68万
• 依托单位: Imperial College London
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2021
• 资助国家: 英国
• 起止时间: 2021 至 无数据
• 项目状态: 未结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FV001388%2F1
• 关键词: Thermochemical; remanent; magnetisations; How; do

Ancient records of magnetic fields stored in rocks and meteorites hold the key to answering some of the most fundamental questions in Earth and Planetary Sciences including the evolution of the Earth's Core and geodynamo, and the formation of the Solar System. In particular, it is the estimates of ancient field intensities that allows us to solve many of these questions, from constraining theories of Solar evolution, to ideas that link the start of the geodynamo to the beginning of life on Earth.To recover ancient field intensities, we study igneous rocks that have recorded thermoremanent magnetisations (TRM) during cooling. A TRM is the remanent magnetisation recorded by magnetic minerals as they cool from above the Curie temperature (~600 C) in weak magnetic fields like the Earth's. The Curie temperature is a key parameter that defines the maximum temperature at which a material exhibits magnetisation. During TRM acquisition it is assumed that the magnetic minerals are chemically stable, and do not physically or chemically alter during cooling. Such TRMs can be stable for times greater than the age of the Universe.The magnetic mineral in igneous rocks, particularly basalts, is usually titanomagnetite Fe2.4Ti0.6O4. Basalts are ubiquitous on Earth, for example, most of the top of the ocean crust (70% of the Earth's surface) is basalt. It has been known for many decades that as Fe2.4Ti0.6O4 cools it unmixes (exsolves) into a magnetic magnetite phase (Fe3O4) and a non-magnetic ulvöspinel phase (Fe2TiO4). The unmixing has been extensively studied since the 1950s and has been shown to occur at temperatures above and below the Curie temperature. The exact temperature at which unmixing stops depends on many factors like the cooling rate, with slower cooling rates more likely to give rise to exsolution structures at low temperatures.For many years palaeomagnetists who study ancient field intensities have assumed that exsolution processes stop at temperatures above the Curie temperature, and that rocks acquire TRMs; however, there is growing evidence to suggest that the minerals continue to unmix below the Curie temperature, thereby chemically alerting and recording another type of magnetic remanent magnetisation termed a thermochemical remanent magnetisation (TCRM). This is a problem, as methods for ancient magnetic field intensity determination assume that rocks carry a TRM not a TCRM.The Earth Science community maintains a database of global ancient field intensities. Analysis for this proposal indicates at least ~51% of the 4293 intensity estimates (site-level) in the database collected over the last 60 years, could be compromised by the incorrect assumption that the magnetisation is a TRM when it is in fact a TCRM. This maybe the reason for the large scatter found in the database.Hitherto little attempt has been made to determine the effect of TCRM on ancient field intensity determination, primarily because of the complexity of the problem. In recent years the PI, CoIs, Visiting Fellow and Project Partners, have developed new nanometric imaging, numerical algorithms (MERRILL) and magnetic measurement protocols to study TRM acquisition, that now make the TCRM problem tractable. We aim to nanometrically image magnetic structures in Ti-rich iron oxides during unmixing at temperature, to allow us to understand how the magnetisation is affected by the unmixing process. We will combine this information with nanometric chemical mapping to build numerical models, using a new multiphase addition to MERRILL. The numerical model will allow us to: (1) make predictions which we will ground-truth against magnetic measurements, (2) determine the stability of TCRM on geological timescales, and (3) to determine the contribution of TCRM to ancient magnetic field intensity determinations. We will use the results to develop new ancient field intensity estimations protocols and provide corrections to legacy data.

储存在岩石和陨石中的磁场的古代记录是回答地球和行星科学中一些最基本问题的关键，包括地核和地球发电机的演变以及太阳系的形成。特别是，它是古老的磁场强度的估计，使我们能够解决许多这些问题，从太阳演化的约束理论，到地球发电机的开始联系到地球上生命的开始的想法。为了恢复古老的磁场强度，我们研究了在冷却过程中记录了热对流磁化（TRM）的火成岩。TRM是磁性矿物从居里温度（~600 ℃）以上冷却时在弱磁场（如地球磁场）中记录的剩磁。居里温度是定义材料表现出磁化的最高温度的关键参数。在TRM采集期间，假定磁性矿物是化学稳定的，并且在冷却期间不会发生物理或化学变化。这种TRM可以稳定的时间比宇宙的年龄更长。火成岩中的磁性矿物，特别是玄武岩，通常是钛磁铁矿Fe2.4Ti0.6O4。玄武岩在地球上无处不在，例如，海洋地壳的顶部（地球表面的70%）大部分是玄武岩。几十年来，人们已经知道，随着Fe2.4Ti0.6O4冷却，它会分解（溶出）成磁性磁铁矿相（Fe 3 O 4）和非磁性尖晶石相（Fe 2 TiO 4）。自20世纪50年代以来，人们已经广泛地研究了非混合现象，并且已经证明在居里温度以上和以下的温度下都会发生这种现象。不混合停止的确切温度取决于许多因素，如冷却速率，冷却速率越慢，越容易在低温下形成出溶结构。多年来，研究古磁场强度的古地磁学家一直假设出溶过程在居里温度以上停止，岩石获得TRM;然而，有越来越多的证据表明，低于居里温度，矿物继续不混合，从而化学报警和记录另一种类型的磁共振磁化称为热化学共振磁化（TCRM）。这是一个问题，因为确定古代磁场强度的方法假设岩石携带的是TRM而不是TCRM。地球科学界维护着一个全球古代磁场强度的数据库。对该提案的分析表明，在过去60年收集的数据库中的4293个强度估计值（场地级）中，至少有~51%可能受到磁化强度是TRM的错误假设的影响，而实际上它是TCRM。这可能是在数据库中发现的大分散的原因。迄今为止，很少有人尝试确定TCRM对古场强测定的影响，主要是因为问题的复杂性。近年来，PI，CoIs，客座研究员和项目合作伙伴开发了新的纳米成像，数值算法（梅里尔）和磁测量协议来研究TRM采集，现在使TCRM问题易于处理。我们的目标是纳米图像的磁性结构在富钛铁氧化物在温度下的解混过程中，让我们了解磁化是如何影响的解混过程。我们将联合收割机结合这些信息与纳米化学测绘建立数值模型，使用一个新的多相除了梅里尔。数值模型将使我们能够：（1）进行预测，我们将地面真理对磁性测量，（2）确定TCRM的稳定性在地质时标，（3）确定TCRM的贡献古磁场强度测定。我们将使用这些结果来开发新的古代场强估计协议，并对遗留数据进行校正。

项目成果

Magnetic characterisation of London's airborne nanoparticulate matter

• DOI: 10.1016/j.atmosenv.2022.119292
• 发表时间: 2022-10
• 期刊: Atmospheric Environment
• 影响因子: 5
• 作者: A. Muxworthy;Claire Lam;David Green;Alison Cowan;B. Maher;T. Gonet

Interpreting high-temperature magnetic susceptibility data of natural systems

• DOI: 10.3389/feart.2023.1171200
• 发表时间: 2023-05
• 期刊: Physical review letters
• 影响因子: 8.6
• 作者: A. Muxworthy;Jack N Turney;L. Qi;Evelyn B. Baker;Joseph R. Perkins;M. Abdulkarim

Micromagnetic determination of the FORC response of paleomagnetically significant magnetite assemblages - supplementary data

具有古地磁意义的磁铁矿组合的 FORC 响应的微磁测定 - 补充数据

• DOI: 10.5281/zenodo.10529804
• 发表时间: 2024
• 作者: Nagy L