Determining ancient magnetic field strengths from the Earth and Solar System

确定地球和太阳系的古代磁场强度

• 批准号: NE/S001018/1
• 负责人: Adrian Muxworthy
• 金额: $ 76.75万
• 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: 2019
• 资助国家: 英国
• 起止时间: 2019 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FS001018%2F1
• 关键词: Determining; ancient; magnetic; field; strengths

Palaeomagnetic recordings in ancient rocks and meteorites hold the key to answering some of the most fundamental questions in Earth and Planetary Sciences including the evolution of the Core and geodynamo, plate tectonics and palaeogeography, and the formation of the Solar System. Recently, our fundamental understanding of how rocks record the geomagnetic field has been challenged. Hitherto palaeomagnetists regarded ideal recorders as those particles that are both uniformly magnetized and thermally stable, and referred to these as single-domain (SD) grains. However, it has long been recognised that the SD range size range for most grain shapes (expressed as the diameter of their equivalent volume sphere) is extremely narrow, only existing in particles between approximately 30 to 80 nm in size. Most palaeomagnetic samples are dominated by larger grains that contain non-uniform magnetic domain structures referred to a pseudo-single-domain (PSD), which reflects the ambiguity with which their magnetic properties are known. The origin of PSD grains' magnetization and apparently high stability has remained largely a mystery; palaeomagnetic protocols designed for SD behaviour often have very high failure rates (as much as 90 %). These high failure rates are then usually attributed to the presence of PSD grains. The advent of numerical micromagnetic modelling and nanometric magnetic imaging, has given us the ability to understand these complex systems. A remarkable recent discovery, from a collaboration between the PI, CoI and named PDRA, who showed that these highly magnetised PSD grains usually exist in a single magnetic vortex domain state, which has both a high magnetic signal and a much higher recording stability than even the 'ideal' uniformly magnetised SD particles. With these recent developments in numerical modeling, we are now in position to construct a full model of the many millions of magnetic particles contained within a real palaeomagnetic sample. This will be achieved through the development of a micromagnetic database that contains the full domain structure and magnetic characteristics as a function of grain size, shape, temperature and external field strength, for particles in the most palaeomagnetic significant size range of 30 - 1000 nm in magnetite. This database will form a unique resource from which we can mine the data to: 1) Provide a comprehensive and fundamental new understanding of PSD domain state characteristics. This is essential in order to provide a means of linking laboratory rock-magnetic observations to the thermal stability and magnetic blocking temperatures that control a samples ability to retain an accurate recording of the geomagnetic field over many millions of years. 2) Reconstruct the recording fidelity of palaeomagnetic samples. For any distribution of grain sizes we will be able to predict the ability of a sample to acquire a natural remanent magnetisation through a simulated cooling from above its Curie temperature. The effect of subsequent partial remagnetization in laboratory magnetic fields and cooling rates can then be determined to determine the impact of the experimental process on our ability to extract the correct value of the ancient geomagnetic field. 3) Establish a non-heating method of obtaining reliable palaeointensity values. A major drawback in current methods of paleointensity determinations is that the magnetic minerals often undergo chemical alteration during laboratory re-heating. By establishing the relationship between domain state thermal and magnetic field stability (blocking temperatures and coercivities) it will be possible to provide a theoretical basis for replacing laboratory heating by room-temperature isothermal or anhysteretic magnetization measurements.

古岩石和陨石中的古地磁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化，板块构造和古地理学以及太阳系的形成。最近，我们对岩石如何记录地磁场的基本理解受到了挑战。古地磁学家认为理想的记录器是那些既均匀磁化又热稳定的颗粒，并将其称为单畴（SD）颗粒。然而，人们早就认识到，大多数颗粒形状的SD范围尺寸范围（表示为它们的等效体积球的直径）非常窄，仅存在于尺寸在约30至80 nm之间的颗粒中。大多数古地磁样品以较大的颗粒为主，这些颗粒包含称为伪单畴（PSD）的非均匀磁畴结构，这反映了其磁性已知的模糊性。PSD颗粒的磁化和明显的高稳定性的起源在很大程度上仍然是一个谜;为SD行为设计的古地磁协议通常具有非常高的失败率（高达90%）。这些高故障率通常归因于PSD晶粒的存在。数值微磁建模和纳米磁成像的出现，使我们有能力理解这些复杂的系统。最近PI、CoI和PDRA之间的合作发现了一个引人注目的发现，他们表明这些高度磁化的PSD颗粒通常存在于单个磁涡旋畴状态中，其具有高磁信号和比“理想”均匀磁化的SD颗粒更高的记录稳定性。随着这些最新的发展，在数值模拟，我们现在能够构建一个完整的模型，数百万的磁性粒子包含在一个真实的古地磁样本。这将通过开发一个微磁性数据库来实现，该数据库包含磁铁矿中30 - 1000纳米的最古磁性有效尺寸范围内的颗粒的完整磁畴结构和磁性特征，这些结构和磁性特征是粒度、形状、温度和外部场强的函数。该数据库将形成一个独特的资源，我们可以从中挖掘数据：1）提供对PSD域状态特征的全面和基本的新理解。这是必不可少的，以便提供一种手段，将实验室岩石磁性观测与热稳定性和磁阻挡温度联系起来，这些温度控制着样品保持数百万年来地磁场准确记录的能力。2)重建古地磁样品的记录保真度。对于任何粒度分布，我们将能够预测样品通过从其居里温度以上的模拟冷却获得自然回复磁化的能力。然后可以确定实验室磁场和冷却速率中随后的部分再磁化的效果，以确定实验过程对我们提取古地磁场正确值的能力的影响。3)建立一种获得可靠的古强度值的非加热方法。目前测定古地磁强度的方法的一个主要缺点是磁性矿物在实验室重新加热过程中经常发生化学蚀变。通过建立畴态热稳定性和磁场稳定性（阻断温度和磁化率）之间的关系，将有可能为用室温等温或无滞后磁化测量代替实验室加热提供理论基础。

项目成果

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

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.

Size Ranges of Magnetic Domain States in Tetrataenite

透辉石中磁畴态的尺寸范围

• DOI: 10.1002/essoar.10512266.1
• 发表时间: 2022
• 作者: Mansbach E

The meaning of maxima and minima in first order reversal curves: Determining the interaction between species in a sample

• DOI: 10.1016/j.jmmm.2022.170042
• 发表时间: 2022-10
• 期刊: Journal of Magnetism and Magnetic Materials
• 影响因子: 2.7
• 作者: R. Moreno;W. Williams;A. Muxworthy;G. Paterson;D. Heslop