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

我们对地核、地核的形成和地质时间尺度上的地球动力学的认识来源于古地磁研究。同样，古地磁资料在古地理、古气候学、构造学、火山学和许多其他地质领域也起着关键作用。这些研究依赖于岩石组成的磁性矿物记录有意义和可破译的磁残余的能力。为了可靠地解释古地磁数据，我们需要了解产生剩磁并随后改变剩磁的机制。虽然一些机制，如热磁化（TRM）获取已经被很好地理解，但还有一类广泛的剩余物获取或改变机制，称为化学（或结晶）剩余物磁化（CRM）或化学蚀变，人们对这些机制知之甚少，但它们在自然界中很常见，因此通常有助于古地磁观测。CRM指的是物理上或化学上改变岩石磁性矿物的任何过程。它可以采取多种形式，例如：(1)它可以是磁性颗粒在环境温度下生长时产生的剩余物，即生长CRM，或者(2)它可以是矿物的磁性剩余物在氧化或还原的化学变化中产生的结果变化，其中新相的磁性可以更强或更弱。问题更复杂的是，通常很难区分TRM和CRM，但这两种剩余信号的来源和解释是完全不同的。例如，如果古地磁场强度（古强度）是一个增长型地磁场强度，但在起源上被错误地假设为一个TRM，这将导致对古地磁场强度的高估。由于问题的广泛性和复杂性，以及在实验上量化这些过程的困难，对CRM和化学变化的理论处理受到限制。理论模型只存在于最小的磁性颗粒，称为单畴（SD），因为它们的磁化是均匀的。较大的晶粒具有复杂的畴型，称为多畴型；略高于SD阈值大小的小MD颗粒往往表现出一些SD特征，被称为伪SD （PSD）。这类PSD颗粒往往主导岩石信号，但对PSD CRM还没有严格的理论认识。本提案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和化学变化过程。我们将采用最新的先进透射电子显微镜（TEM）技术，包括电子全息术，使我们能够实时成像原子尺度上的磁化，因为矿物在受控的氧化/还原气氛下发生变化。为了将TEM图像与体磁性联系起来，我们将使用我们最近开发的多相微磁模型，使我们能够将纳米级的化学变化与磁性矿物学与测量的体磁性联系起来。我们将量化不同类型的化学蚀变如何影响古定向和古强度信息。特别是，我们将通过实验和数值研究：(1)晶粒生长/溶解和(2)低温氧化，例如，在< 150℃的温度下，钛磁铁矿氧化为钛磁铁矿。

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.