The effect of chemical alteration on the fidelity of palaeomagnetic pseudo-single-domain recorders
Lead Research Organisation:
University of Edinburgh
Department Name: Sch of Geosciences
Abstract
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.
Organisations
Publications
Almeida T
(2015)
Effect of maghemization on the magnetic properties of nonstoichiometric pseudo-single-domain magnetite particles
in Geochemistry, Geophysics, Geosystems
Almeida T
(2014)
Magnetic characterization of synthetic titanomagnetites: Quantifying the recording fidelity of ideal synthetic analogs
in Geochemistry, Geophysics, Geosystems
Almeida T
(2014)
Hydrothermal synthesis, off-axis electron holography and magnetic properties of Fe 3 O 4 nanoparticles
in Journal of Physics: Conference Series
Almeida TP
(2014)
Visualized effect of oxidation on magnetic recording fidelity in pseudo-single-domain magnetite particles.
in Nature communications
Devienne J
(2021)
Micromagnetic simulation of taenite particles
Ge K
(2014)
Effects of the core-shell structure on the magnetic properties of partially oxidized magnetite grains: Experimental and micromagnetic investigations
in Geochemistry, Geophysics, Geosystems
Krása D
(2011)
Room- and low-temperature magnetic properties of 2-D magnetite particle arrays Magnetic properties of 2-D magnetite particle
in Geophysical Journal International
Li J
(2013)
A strong angular dependence of magnetic properties of magnetosome chains: Implications for rock magnetism and paleomagnetism
in Geochemistry, Geophysics, Geosystems
Muxworthy A
(2013)
Critical single domain grain sizes in chains of interacting greigite particles: Implications for magnetosome crystals
in Geochemistry, Geophysics, Geosystems
Nagy L
(2021)
Estimating the Effect of Cooling Rate on the Acquisition of Magnetic Remanence
in Geophysical Research Letters
Description | We developed a micro magnetic model that is able to determine the stable magnetic domain structure for minerals that contain more than one magnetic phase. Thus, in the case where minerals in rocks have been chemically alerted, we are now able to determine if the original magnetic recording has survived or not. We have also established for the first time the domain structure and magnetic recording stability in gregite as a function of grain size. |
Exploitation Route | The multi-phas micro magnetic model is very versatile, and will be used by other in materials science and rock magnetism to predict the properties of a complex magnetic minerals. |
Sectors | Education Environment Other |