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。关于地球发电机的演化、地核的热演化、板块构造和古地理以及太阳系形成的理论，都受到对困在数亿年甚至数十亿年前岩石中的古场的观察的制约。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是由不均匀磁结构的颗粒中的热剩余磁化(TRM)所主导的。大多数古地磁解释是基于这样的TRM由磁均匀的单域(SD)粒子携带的假设，其行为由奈尔的SD TRM理论很好地描述。然而，自然界中普遍存在具有不均匀磁结构的稍大的颗粒。这些被称为伪SD(PSD)，因为它们表现出SD颗粒的某些特征(例如大的剩磁)，但可以具有显著不同的记录保真度。目前还没有用于PSD TRM采集的物理模型，因此我们无法评估许多古地磁信号的稳定性和可靠性。这项提议将解决对私营部门可持续发展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)作为时间和温度的函数的PSD稳定性，以及(B)它们的TRM对冷却速度的依赖。这将通过开发一个结合了热波动影响的三维数值模型来实现。然后，就可以对PSD TRM采集进行建模，并评估PSD磁区状态记录地磁场的准确性。数值建模的一个关键方面是根据直接的纳米级实验观测，验证预测的磁区结构作为晶粒度和温度的函数。这将使用一组极具特色的人造样品(由NERC资助的前一项研究中的电子光刻过程产生)和先进的离轴电子全息透射电子显微镜技术来实现，该技术能够在纳米尺度上拍摄磁化过程的图像。实验还将在大量样品上进行，包括一套已经采集的熔岩。一旦得到验证，数值模型将用于探索一系列适用于包含PSD域态的自然样品的TRM记录和古地磁强度(古地磁场强度)测定的保真度。研究将导致对被认为是古地磁记录主要载体的磁铁矿PSD颗粒的TRM采集有一个全面的了解，并确定PSD颗粒记录古场的准确性。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)来自PSD磁铁矿的古强度估计被用来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古场的真值。(2)太古代古强度的估计通常来自PSD磁铁矿晶体，嵌在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度对TRM强度的影响，这是无法在实验中完成的。随着古地磁观测精度的提高，将出现关于地球磁场过去行为的更清晰的图景，因此更有希望揭开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.