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

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。地球动力学的演化、地核的热演化、板块构造和古地理以及太阳系的形成等理论，都受到了对岩石中数百年甚至数亿年历史的古磁场的观测的制约。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主要是由具有非均匀磁性结构的颗粒中的一种鲜为人知的热磁化（TRM）控制的。大多数古地磁解释都是基于这样的假设，即这些TRM是由磁性均匀的单畴（SD）粒子携带的，其行为被nsamel的SD TRM理论很好地描述了。然而，具有不均匀磁性结构的稍大颗粒在自然界中普遍存在。这些被称为伪SD (PSD)，因为它们显示了SD颗粒的一些特征（例如大的磁剩余物），但可能具有显着不同的记录保真度。目前还没有用于PSD TRM采集的物理模型，因此我们没有办法评估许多古地磁信号的稳定性和可靠性。该建议将解决对PSD TRM的基本行为进行量化的迫切需要。特别是，我们的目标是解决可能影响古地磁保真度的两个关键问题：(a) PSD稳定性作为时间和温度的函数，以及(b)它们的TRM依赖于冷却速率。这将通过开发一个包含热波动影响的三维数值模型来实现。然后，就有可能对PSD TRM采集进行建模，并评估PSD域状态记录地磁场的准确性。数值模拟的一个关键方面是验证预测的区域结构，作为晶粒尺寸和温度的函数，反对直接的纳米尺度实验观察。这将通过使用一组显著的高度表征的人造样品（在之前nerc资助的研究中由电子光刻工艺产生）和使用先进的透射电子显微镜（TEM）离轴电子全息技术来实现，该技术能够在纳米尺度上成像磁化。实验还将在大量样本上进行，包括一套已经收集到的熔岩。一旦验证，该数值模型将用于探索TRM记录的保真度和古强度（古地磁场强度）测定，适用于包含PSD域状态的自然样品的一系列谷物几何形状。该研究将有助于全面了解磁铁矿PSD颗粒的TRM采集，并确定PSD颗粒对古地磁记录的准确程度。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核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领域的真实价值。(2)太古时代的古强度估计通常是由PSD磁铁矿晶体确定的，这些磁铁矿晶体镶嵌在从长石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，地磁场过去的行为将呈现出一幅更加清晰的画面，从而更有希望揭示早期宇宙的真实本质以及地球深处的演化和行为。

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.