A new non-heating method for determining the ancient geomagnetic field intensity

一种测定古地磁场强度的非加热新方法

• 批准号: NE/D000351/1
• 负责人: Adrian Muxworthy
• 金额: $ 6.31万
• 依托单位: University of Southampton
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2006
• 资助国家: 英国
• 起止时间: 2006 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FD000351%2F1
• 关键词: new; non; heating; method; determining

One key feature of the earth that is not currently well understood is the origin of the earth's magnetic field. That is not to say we understand nothing about it. A great deal is known, however, we are unable to predict how it will change in the future. The earth's magnetic field is commonly thought of as a giant dipole magnet, like a bar magnet, which is tilted slightly off the earth's rotation axis. As a first approximation this is generally true, however, in reality the field is more complicated than this. There are other much smaller field components that vary gradually with time, like slow-moving waves on the surface of the ocean. Knowing where these variations are and how they will change in the future is important for satellites and other navigation systems. To understand these variations it is important to understand how the earth's magnetic field is generated in the earth's core. The earth's core contains liquid iron, which circulates with the rotating earth. This circulating motion acts like a dynamo, causing the earth's magnetic field to be generated. However, it is not as simple as a bicycle dynamo. Instead, it is some sort of coupled dynamo in which the magnetic field it generates can spontaneously reverse. The behaviour of this dynamo is not fully understood. To be able to understand the behaviour of the dynamo and to predict the future we need to know how the earth's magnetic fields varied in the past. To do this we need to examine rocks. Magnetic minerals such as those in lodestone, i.e., iron oxides, are abundant in rocks, and they record the magnetic field in a similar manner to music tapes or videos. However, the actual magnetic signal recorded by the minerals can be rather confusing, and it takes geological knowledge as well as a good understanding of physics to be able to unravel their magnetic signal and to interpret it meaningfully. The magnetic minerals not only record the direction of the magnetic signal, but also the intensity (palaeointensity) of the field at the time of rock formation. The direction is easier to unravel than the intensity; however, the intensity is critical if we are to completely describe the field and how it is generated. There have been several methods that claim to determine the palaeointensity, however, they have met with limited success. These methods work by comparing the magnetic remanence of rock with a similar one induced in the laboratory. There are two problems with this approach; first, the method only works for the small grains (sub-micron in diameter) which obey certain rules; larger grains do not obey these rules. Magnetically interacting grains do not obey these rules either. Second, to replicate the natural remanence in the laboratory it is necessary to heat the rocks to over 600 C, which commonly causes unwanted chemical alteration in the samples. What is required is a non-heating method of palaeointensity determination, which allows for both smaller and larger grains. I propose to develop a new completely different method that fulfils these criteria. If we know the type of magnetic grains present, then theoretically they must have a unique field-dependent magnetic intensity. Typically rocks have wide distributions of grains, so what is required is a method that can magnetically characterise the entire distribution. This can be done by measuring a Preisach distribution. This is a two-dimensional distribution, which can be plotted like a map. In the simplest interpretation of the distribution, one direction is related to grain size, whilst the other direction is related to the degree of magnetic interactions within the system. If we measure the Preisach distribution of a sample and conduct some other tests, it is theoretically possible to determine the palaeointensity. I propose to implement and test this method experimentally. If successful, this should have a major impact on our ability to understand the long term behaviour of the geomagnetic field.

地球的一个关键特征是目前还没有很好地了解地球磁场的起源。这并不是说我们对它一无所知，我们知道的很多，但是我们无法预测它在未来会如何变化。地球的磁场通常被认为是一个巨大的偶极磁铁，就像一个条形磁铁，稍微偏离地球的旋转轴。作为第一近似，这通常是正确的，然而，在现实中，该领域比这更复杂。还有其他小得多的场分量随时间逐渐变化，就像海洋表面缓慢移动的波浪一样。了解这些变化在哪里以及它们在未来将如何变化对卫星和其他导航系统非常重要。要了解这些变化，重要的是要了解地球磁场是如何在地核中产生的。地核中含有液态铁，随地球旋转而循环。这种循环运动就像发电机一样，导致地球磁场的产生。然而，它并不像自行车发电机那么简单。相反，它是某种耦合发电机，其中它产生的磁场可以自发逆转。这个发电机的行为还没有完全弄清楚。为了能够理解发电机的行为并预测未来，我们需要知道地球磁场在过去是如何变化的。要做到这一点，我们需要检查岩石。磁性矿物，如天然磁石中的磁性矿物，即，铁氧化物在岩石中含量丰富，它们以类似于音乐磁带或录像带的方式记录磁场。然而，矿物记录的实际磁信号可能相当令人困惑，需要地质知识以及对物理学的良好理解才能解开它们的磁信号并对其进行有意义的解释。磁性矿物不仅记录了磁信号的方向，而且记录了岩石形成时磁场的强度（古强度）。方向比强度更容易解开;然而，如果我们要完整地描述场及其如何产生，强度是至关重要的。已经有几种方法声称可以确定古强度，但是，它们都遇到了有限的成功。这些方法的工作原理是将岩石的剩磁与实验室中感应的类似剩磁进行比较。这种方法存在两个问题;首先，该方法仅适用于遵守某些规则的小颗粒（直径为亚微米）;较大的颗粒不遵守这些规则。磁性相互作用的颗粒也不遵守这些规则。其次，为了在实验室中复制天然剩磁，必须将岩石加热到600 ℃以上，这通常会导致样品中不必要的化学变化。所需要的是一种非加热的古强度测定方法，它允许更小和更大的颗粒。我建议开发一种完全不同的新方法，以满足这些标准。如果我们知道存在的磁性颗粒的类型，那么理论上它们必须具有唯一的磁场依赖性磁场强度。通常，岩石的颗粒分布很广，因此需要一种方法，可以对整个分布进行磁性测量。这可以通过测量Preisach分布来完成。这是一个二维分布，可以像地图一样绘制。在对分布的最简单解释中，一个方向与晶粒大小有关，而另一个方向与系统内磁相互作用的程度有关。如果我们测量样品的Preisach分布，并进行一些其他的测试，在理论上是可能的，以确定古强度。我建议实施和实验测试这种方法。如果成功，这将对我们理解地磁场长期行为的能力产生重大影响。

项目成果

Thermal fluctuation fields in basalts

玄武岩中的热波动场

• DOI: 10.1186/bf03352890
• 发表时间: 2009
• 期刊: Earth, Planets and Space
• 作者: Muxworthy A

Evaluation of the multispecimen parallel differential pTRM method: a test on historical lavas from Iceland and Mexico

• DOI: 10.1111/j.1365-246x.2008.03740.x
• 发表时间: 2008-05
• 期刊: Geophysical Journal International
• 影响因子: 2.8
• 作者: D. Michalk;A. Muxworthy;H. Böhnel;J. Maclennan;N. Nowaczyk

A Preisach method for estimating absolute paleofield intensity under the constraint of using only isothermal measurements: 1. Theoretical framework

• DOI: 10.1029/2010jb007843
• 发表时间: 2011-04
• 期刊: Journal of Geophysical Research
• 作者: A. Muxworthy;D. Heslop

Revisiting a domain-state independent method of palaeointensity determination

• DOI: 10.1016/j.pepi.2010.01.003
• 发表时间: 2010-03
• 期刊: Physics of the Earth and Planetary Interiors
• 影响因子: 2.3
• 作者: A. Muxworthy

A Preisach method for estimating absolute paleofield intensity under the constraint of using only isothermal measurements: 2. Experimental testing

• DOI: 10.1029/2010jb007844
• 发表时间: 2011-04
• 期刊: Journal of Geophysical Research
• 作者: A. Muxworthy;D. Heslop;G. Paterson;D. Michalk