Predicting the reliability with which the geomagnetic field can be recorded in igneous rocks

预测火成岩中记录地磁场的可靠性

• 批准号: NE/J020508/1
• 负责人: Adrian Muxworthy
• 金额: $ 29.01万
• 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: 2012
• 资助国家: 英国
• 起止时间: 2012 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FJ020508%2F1
• 关键词: Predicting; reliability; geomagnetic; field; recorded

Palaeomagnetic recordings in ancient rocks and meteorites hold the key to answering some of the most fundamental questions in Earth Sciences. Theories regarding the evolution of the geodynamo, the thermal evolution of the Earth's core, plate tectonics and palaeogeography, and the formation of the solar system, are all constrained by observations of the ancient fields trapped in rocks that are hundreds or even thousands of millions of years old. However, not all palaeomagnetic observations are reliable, because the magnetic signal carried by most rocks and meteorites is dominated by a poorly understood thermoremanent magnetisation (TRM) in grains with non-uniform magnetic structures.Most palaeomagnetic interpretations are based on the assumption that such TRMs are carried by magnetically uniform, single domain (SD) particles, whose behaviour is well described by Néel's SD TRM theories. However, slightly larger grains with non-uniform magnetic structures are ubiquitous in nature. These are termed pseudo-SD (PSD) as they display some characteristics to SD grains (such as a large magnetic remanence), but can have a significantly different recording fidelity. Presently there is no physical model for PSD TRM acquisition therefore we have no means of assessing the stability and reliability of many palaeomagnetic signals. This proposal will address the urgent need to quantify the fundamental behaviour of PSD TRM. In particular we aim to address two key issues that can affect palaeomagnetic fidelity: (a) PSD stability as a function of time and temperature, and (b) their TRM dependence on cooling rates. This will be achieved by developing a three-dimensional numerical model that incorporates the effects of thermal-fluctuations. It will then be possible to model PSD TRM acquisition and assess the accuracy with which PSD domain states can record a geomagnetic field.A key aspect of the numerical modelling is validation of the predicted domain structures, as a function of grain size and temperature, against direct nano-metric-scale experimental observations. This will be achieved using a remarkable set of highly characterised artificial samples (produced by an electron lithography process in a previous NERC-funded study) and using the advanced transmission electron microscope (TEM) technique of off-axis electron holography, which is able to image the magnetisation on a nano-metric scale. Experiments will also be conducted on bulk samples, including a suite of already collected lavas.Once validated, the numerical model will be used to explore the fidelity of TRM recordings and palaeointensity (ancient geomagnetic field intensity) determinations in a range of grain geometries applicable to natural samples containing PSD domain states.The research will result in a comprehensive understanding of TRM acquisition for PSD grains of magnetite, which are thought to the dominant carrier of palaeomagnetic recordings, and identify how accurately PSD grains can record the ancient field. The predictive micromagnetic model we develop will be able to directly address a number of key issues, for example:(1) Palaeointensity estimates from PSD magnetites are used to constrain models of the Earth's core dynamics and the Solar System's formation. We will be able to determine whether these palaeointensities are likely to under or over estimate the true value of the ancient field.(2) Archaen palaeointensity estimates are often determined from PSD magnetite crystals, embedded with in single-silicate crystals extracted from gabrros. The model will allow us to quantify the effect of long-term cooling-rates on TRM intensity, something which cannot be done experimentally.With increased accuracy of palaeomagnetic observations, a much clearer picture will emerge of the past behaviour of the geomagnetic field, and hence a far better hope of unravelling the true nature of the early universe and the evolution and behaviour of the Earths deep interior.

古岩石和陨石中的古地磁记录是回答地球科学中一些最基本问题的关键。关于地球发电机的演化、地核的热演化、板块构造和古地理学以及太阳系的形成的理论，都受到对被困在岩石中的古老磁场的观测的限制，这些岩石有数百万年甚至数千万年的历史。然而，并不是所有的古地磁观测都是可靠的，因为大多数岩石和陨石携带的磁信号都是由不均匀磁性结构的颗粒中的热释光磁化（TRM）控制的。大多数古地磁解释都是基于这样的假设，即这种TRM是由磁性均匀的单畴（SD）颗粒携带的，其行为由Néel的SD TRM理论很好地描述。然而，具有不均匀磁性结构的稍大晶粒在自然界中普遍存在。这些被称为伪SD（PSD），因为它们显示SD晶粒的一些特性（例如大的剩磁），但是可以具有显著不同的记录保真度。目前还没有PSD TRM采集的物理模型，因此我们无法评估许多古地磁信号的稳定性和可靠性。该提案将解决量化私营部门司TRM基本行为的迫切需要。特别是，我们的目标是解决两个关键问题，可以影响古地磁保真度：（a）PSD的稳定性作为时间和温度的函数，（B）TRM依赖于冷却速率。这将通过建立一个三维数值模型来实现，该模型将考虑到热波动的影响。这将是可能的模型PSD TRM收购和评估的准确性，PSD域状态可以记录地磁场。数值模拟的一个关键方面是验证预测的域结构，作为晶粒尺寸和温度的函数，对直接纳米尺度的实验观察。这将使用一组显著的高度特征化的人工样品（在先前NERC资助的研究中通过电子光刻工艺生产）并使用离轴电子全息的先进透射电子显微镜（TEM）技术来实现，该技术能够在纳米尺度上成像磁化。实验还将在大量样品上进行，包括一套已经收集的熔岩。一旦验证，数值模型将用于探索TRM记录和古强度的保真度（古地磁场强度）测定。该研究将导致对磁铁矿PSD颗粒的TRM采集的全面理解，这些颗粒被认为是古地磁记录的主要载体，并确定PSD颗粒记录古磁场的准确性。我们开发的预测微磁模型将能够直接解决一些关键问题，例如：（1）PSD磁铁矿的古强度估计用于约束地球核心动力学和太阳系形成的模型。我们将能够确定这些古强度是否可能低估或高估了古代磁场的真实价值。(2)通常从PSD磁铁矿晶体中确定古强度估计，PSD磁铁矿晶体嵌入从辉长岩中提取的单硅酸盐晶体中。该模型将使我们能够量化长期冷却速率对TRM强度的影响，这是实验无法做到的。随着古地磁观测精度的提高，对地磁场过去行为的了解将更加清晰，因此更有希望揭开早期宇宙的真实性质以及地球内部深处的演化和行为。

项目成果

Paleomagnetic field reconstruction from mixtures of titanomagnetites

• DOI: 10.1016/j.epsl.2017.02.033
• 发表时间: 2017-05
• 期刊: Earth and Planetary Science Letters
• 影响因子: 5.3
• 作者: T. Berndt;R. Ramalho;Miguel A. Valdez-Grijalva;A. Muxworthy

From Nano to Micro: Evolution of Magnetic Domain Structures in Multidomain Magnetite

• DOI: 10.1029/2019gc008319
• 发表时间: 2019-06-01
• 期刊: GEOCHEMISTRY GEOPHYSICS GEOSYSTEMS
• 影响因子: 3.5
• 作者: Nagy, Lesleis;Williams, Wyn;Muxworthy, Adrian R.
• 通讯作者: Muxworthy, Adrian R.

Paleomagnetic recording fidelity of nonideal magnetic systems.

• DOI: 10.1002/2014gc005249
• 发表时间: 2014-06
• 期刊: Geochemistry, geophysics, geosystems : G(3)
• 作者: Muxworthy AR;Krása D;Williams W;Almeida TP
• 通讯作者: Almeida TP

Micromagnetic simulations of first-order reversal curve (FORC) diagrams of framboidal greigite

• DOI: 10.1093/gji/ggaa241
• 发表时间: 2020-05
• 期刊: Geophysical Journal International
• 影响因子: 2.8
• 作者: Miguel A. Valdez-Grijalva;L. Nagy;A. Muxworthy;W. Williams;A. Roberts;D. Heslop

First-order reversal curve (FORC) diagrams of nanomagnets with cubic magnetocrystalline anisotropy: A numerical approach

具有立方磁晶各向异性的纳米磁体的一阶反转曲线（FORC）图：数值方法

• DOI: 10.1016/j.jmmm.2018.09.086
• 发表时间: 2019
• 期刊: Journal of Magnetism and Magnetic Materials
• 影响因子: 2.7
• 作者: Valdez-Grijalva M