The effect of chemical alteration on the fidelity of palaeomagnetic pseudo-single-domain recorders

我们对地核、其形成和地质时间尺度上的地球发电机的理解源于对古地磁的研究。同样，古地磁资料在古地理、古气候学、构造学、火山学等许多地质领域都发挥着关键作用。这些研究依赖于岩石组成的磁性矿物记录有意义且可破译的剩磁的能力。为了可靠地解释古地磁数据，我们需要了解诱发剩磁并随后改变剩磁的机制。虽然一些机制，如热剩余磁化(TRM)获取已被熟知，但有一大类剩磁获取或改变机制称为化学(或结晶)剩余磁化(CRM)或化学蚀变，这些机制知之甚少，但在自然界中却很常见，因此通常有助于古地磁观测。CRM是指对岩石的磁性矿物进行物理或化学改变的任何过程。它可以采取多种形式，例如：(1)它可以是在环境温度下生长时在磁性颗粒中诱导和保持的剩磁，即生长CRM，或者(2)它可以是矿物通过氧化或还原而发生化学变化时其剩磁的结果变化，其中新相可以更强或更弱。问题更加复杂，因为通常很难区分TRM和CRM，但这两种剩磁信号的来源和解释完全不同。例如，如果古地磁场强度是一个增长的地磁场强度，但却错误地假设为起源的TRM，这将导致对古代地磁场强度的高估。由于问题的广泛性和复杂性，以及难以在实验上量化这些过程，理论上对CRM和化学蚀变的处理一直受到限制。理论模型只适用于最小的磁性颗粒，称为单磁区(SD)，因为它们的磁化是均匀的。较大的颗粒具有复杂的磁区图案，被称为多磁区(MD)；略高于SD阈值尺寸的小MD颗粒往往表现出一些SD特征，被称为伪SD(PSD)。这种PSD颗粒往往主导岩石的信号，但对于PSD CRM还没有严格的理论理解。这项建议的目的是应用最先进的实验和数值技术来理解和量化SD和PSD样品中的CRM和化学蚀变过程。我们将采用包括电子全息技术在内的最新先进的电子显微镜技术，当矿物在受控的氧化/还原气氛下发生变化时，我们可以实时成像原子尺度上的磁化作用。为了将透射电子显微镜图像与块体磁性联系起来，我们将使用我们最近开发的多相微磁模型，使我们能够将磁性矿物学的纳米级化学变化与测量的块体磁性联系起来。我们将量化不同类型的化学蚀变如何影响古方向和古强度信息。特别是，我们将在实验和数值上研究：(1)颗粒生长/溶解；(2)低温氧化，例如，钛磁铁矿在&lt；150℃下氧化成钛磁铁矿。

Our understanding of the Earth's core, its formation and the geodynamo on geological timescales is derived from palaeomagnetic studies. Similarly, palaeomagnetic data plays a key role in the fields of palaeogeography, palaeoclimatology, tectonics, volcanology and many other geological areas. These studies rely on the ability of a rock's constituent magnetic minerals to record a meaningful and decipherable magnetic remanence. To reliably interpret palaeomagnetic data we need to understand the mechanisms that induce magnetic remanence and can subsequently alter it. Whilst some mechanisms, e.g., thermoremanent magnetisation (TRM) acquisition, are well understood, there is a broad class of remanence acquiring or altering mechanisms termed chemical (or crystallisation) remanent magnetisation (CRM) or chemical alteration, that are poorly understand yet are frequent in nature and so commonly contribute to palaeomagnetic observations. CRM refers to any process that physically or chemically alters the magnetic minerals of a rock. It can take many forms, for example: (1) it can be a remanence induced and retained in magnetic grains as they grow at ambient temperature, i.e. a growth CRM, or (2) it can be the resultant change in a mineral's magnetic remanence as it chemically alters through oxidation or reduction, where the new phase can be either magnetically stronger or weaker. The problem is further complicated in that it is often difficult to distinguish between say a TRM and CRM, yet the origin and interpretation of the two remanence signals is completely different. For example, if the ancient geomagnetic field intensity (palaeointensity) is a growth CRM yet wrongly assumed to be a TRM in origin, this will lead to an over-estimate of the ancient geomagnetic field strength. Theoretical treatment of CRM and chemical alteration has been limited due to the broadness and complexity of the problem, and the difficulty in quantifying these processes experimentally. Theoretical models only exist for the smallest magnetic grains, termed single domain (SD) as their magnetisation is uniform. Larger grains have complex domain patterns and are termed multidomain (MD); small MD grains just above the SD threshold size tend display some SD characteristics and are termed pseudo-SD (PSD). Such PSD grains tend to dominate the signal of rocks, yet no rigorous theoretical understanding exists for PSD CRM. The aim of this proposal is to apply state-of-the-art experimental and numerical techniques to the understanding and quantification of the CRM and chemical alteration processes in SD and PSD samples. We will employ the latest advanced transmission electron microscopy (TEM) techniques including electron holography that allows us to image magnetisation on atomic scales in real-time as the minerals alter under controlled oxidising/reducing atmospheres. To link the TEM images to the bulk magnetic properties we will use our recently developed multiphase micromagnetic models, allowing us to relate nanometre sized chemical changes to the magnetic mineralogy to the measured bulk magnetic properties. We will quantify how different types of chemical alteration affect both palaeo-directional and palaeointensity information. In particular we will examine experimentally and numerically: (1) grain growth/dissolution and (2) low-temperature oxidation, e.g., the oxidation of titanomagnetite to titanomaghemite at temperatures < 150 C.