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

化学蚀变对古地磁伪单域记录器保真度的影响

• 批准号: NE/H00534X/1
• 负责人: Adrian Muxworthy
• 金额: $ 38.2万
• 依托单位: Imperial College London
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2010
• 资助国家: 英国
• 起止时间: 2010 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FH00534X%2F1
• 关键词: effect; chemical; alteration; fidelity; palaeomagnetic

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.

我们对地核、地核的形成和地质年代尺度上的地球发电机的认识来自古地磁研究。同样，古地磁数据在古地理学、古气候学、构造学、火山学和许多其他地质领域中起着关键作用。这些研究依赖于岩石的磁性矿物成分记录有意义和可解释的剩磁的能力。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。除了热剩磁磁化（TRM）采集被很好地理解之外，还存在被称为化学（或结晶）剩磁磁化（CRM）或化学蚀变的广泛类别的剩磁采集或改变机制，这些机制被很好地理解，但在性质上是频繁的，因此通常有助于古地磁观测。CRM是指物理或化学改变岩石中磁性矿物的任何过程。它可以采取许多形式，例如：（1）它可以是磁性颗粒在环境温度下生长时在磁性颗粒中诱导和保留的剩磁，即生长CRM，或者（2）它可以是矿物的剩磁的结果变化，因为它通过氧化或还原进行化学改变，其中新相可以是磁性更强或更弱。问题进一步复杂化，因为通常难以区分例如TRM和CRM，然而两种剩磁信号的起源和解释完全不同。例如，如果古地磁场强度（palaeointensity）是一个增长CRM，但错误地假设为TRM的起源，这将导致高估古地磁场强度。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，CRM和化学蚀变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），因为它们的磁化强度是均匀的。较大的晶粒具有复杂的畴模式，称为多畴（MD）;略高于SD阈值尺寸的小MD晶粒往往显示一些SD特征，称为伪SD（PSD）。这种PSD颗粒往往主导岩石的信号，但没有严格的理论理解PSD CRM存在。本提案的目的是应用国家的最先进的实验和数值技术的理解和量化的CRM和SD和PSD样品中的化学蚀变过程。我们将采用最先进的透射电子显微镜（TEM）技术，包括电子全息术，使我们能够在受控的氧化/还原气氛下改变矿物时实时成像原子尺度上的磁化强度。为了将TEM图像与体磁性质联系起来，我们将使用我们最近开发的多相微磁模型，使我们能够将纳米尺寸的化学变化与磁性矿物学和测量的体磁性质联系起来。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，我们将通过实验和数值方法研究：（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成钛磁赤铁矿。

项目成果

Effect of maghemization on the magnetic properties of nonstoichiometric pseudo-single-domain magnetite particles

• DOI: 10.1002/2015gc005858
• 发表时间: 2015-09-01
• 期刊: GEOCHEMISTRY GEOPHYSICS GEOSYSTEMS
• 影响因子: 3.5
• 作者: Almeida, Trevor P.;Muxworthy, Adrian R.;Dunin-Borkowski, Rafal E.
• 通讯作者: Dunin-Borkowski, Rafal E.

Critical single-domain grain sizes in elongated iron particles: implications for meteoritic and lunar magnetism

• DOI: 10.1093/gji/ggv180
• 发表时间: 2015-07-01
• 期刊: GEOPHYSICAL JOURNAL INTERNATIONAL
• 影响因子: 2.8
• 作者: Muxworthy, Adrian R.;Williams, Wyn
• 通讯作者: Williams, Wyn

Visualized effect of oxidation on magnetic recording fidelity in pseudo-single-domain magnetite particles.

• DOI: 10.1038/ncomms6154
• 发表时间: 2014-10-10
• 期刊: NATURE COMMUNICATIONS
• 影响因子: 16.6
• 作者: Almeida, Trevor P.;Kasama, Takeshi;Muxworthy, Adrian R.;Williams, Wyn;Nagy, Lesleis;Hansen, Thomas W.;Brown, Paul D.;Dunin-Borkowski, Rafal E.
• 通讯作者: Dunin-Borkowski, Rafal E.

Formation of magnetic minerals at hydrocarbon-generation conditions

• DOI: 10.1016/j.marpetgeo.2015.10.003
• 发表时间: 2015-12
• 期刊: Marine and Petroleum Geology
• 影响因子: 4.2
• 作者: R. Abubakar;A. Muxworthy;M. Sephton;P. Southern;J. Watson;A. Fraser;T. Almeida

Observing thermomagnetic stability of nonideal magnetite particles: Good paleomagnetic recorders?

• DOI: 10.1002/2014gl061432
• 发表时间: 2014-10-28
• 期刊: GEOPHYSICAL RESEARCH LETTERS
• 影响因子: 5.2
• 作者: Almeida, Trevor P.;Kasama, Takeshi;Dunin-Borkowski, Rafal E.
• 通讯作者: Dunin-Borkowski, Rafal E.