Magnetic Interactions 2014 – Meeting Report
On January 8th and 9th 2014 the NPM group hosted the Magnetic Interactions 2014 meeting – the annual gathering of researchers from the UK (and beyond) working on all aspects of magnetism in the Earth sciences. The meeting was held at St Catharine’s college in their recently opened McGrath conference centre and was made possible by generous sponsorship from:
Mineral Physics Group, Mineralogical Society
Day 1
The magnetic interactions meeting is hosted by a different UK research group each year, with the tone of the meeting often reflecting the particular research focus of the hosts. Hence no surprise that this year’s meeting began with a day dedicated to fundamental and applied rock and mineral magnetism. The first talk was an invited talk from Michael Carpenter from the Mineral Physics group in Cambridge, who presented an overview of new experimental and theoretical approaches to studying magnetoelastic coupling in magnetic materials. As rock magnetists we are all familiar with the important concept of magnetostriction (the change in shape of a specimen as it becomes magnetised). Carpenter explained how magnetostriction has its origins in the coupling between magnetic ordering and elastic strain. The strength of this coupling can vary enormously from material to material, but wherever present it leads to changes in the elastic constants (e.g. shear and bulk modulus) – changes that can now be measured with high precision as a function of temperature and applied magnetic field using a recently developed national facility in Cambridge. This facility opens up many new opportunities for studying magnetoelastic coupling in natural magnetic materials, including for the first time the chance to measure magnetoelastic effects in the nanoparticles that are most relevant to rock magnetism.
Next, Dr. Trevor Almeida from Imperial presented the results of his pioneering study of oxidation of pseudo single domain (PSD) magnetite particles. Using a combination of in-situ high-temperature oxidation in an environmental TEM and electron holography, Almeida was able to show directly for the first time that the non-uniform magnetisation state of a PSD particle can change dramatically as the particle surface becomes oxidised. Wyn Williams from Edinburgh continued the theme of oxidation, but this time from a modelling perspective. He presented micromagnetic models of magnetite cubes with a surface shell of maghemite. The calculations demonstrate that exchange coupling between the hard core of magnetite and softer maghemite shell is necessary to explain experimental observations. He also showed how the micromagnetic state of the core and shell change as the core shrinks and the shell grows during progressive oxidation. NPM PhD student James Bryson presented his work applying new X-ray methods for imaging magnetism at the nanometre scale in meteoritic metal. Bryson explained how X-rays can reveal information about magnetism via the X-ray magnetic circular dichroism (XMCD) effect. The absorption of X-rays is made to become spin dependent by using a circularly polarised beam of X-rays generated by a synchrotron. There are several different ways of spatially resolving the XMCD signal, but the most promising from a paleomagnetic perspective is X-ray photoemission electron microscopy (X-PEEM), which can be performed on a polished surface while maintaining the sample’s natural remanent state. This technique revealed the complex nanomagnetic signals recorded by pallasite meteorites, providing further evidence that these rocks were exposed to a magnetic field on their parent body.
Richard Harrison demonstrated some of the new capabilities of FORCinel v. 2.0, focussing on the incorporation of Ramon Egli’s VARIFORC smoothing algorithm. This method provides several advantages over conventional smoothing algorithms, especially for samples containing non-interacting single domain particles. PhD student Rabiu Abubakar from Imperial described how rock magnetism can be used as a tool for the exploration of oil and gas. Magnetic minerals are produced by burial and/or contact with migrating crude oils. He outlined attempts to reproduce the phenomenon in the laboratory by heating and pressurising organic rich sediments from the Wessex basin, demonstrating that magnetic minerals could be formed with grain sizes that would allow them to migrate with the oil and contribute to magnetic anomalies. Finally Nono Lascu from the NPM group in Cambridge presented the work he did on speleothem magnetism during his previous post at the Institute for Rock Magnetism in Minnesota. Lascu explained how stalagmites can provide continuous high resolution paleomagnetic records that can also be radiometrically dated to high precision. As an example he showed how speleothem magnetism was able to constrain the duration of the Laschamp geomagnetic field excursion to a maximum of 3000 years.
Day 2
Laura Roberts-Artal in the new McGrath centre
Day 2 began with a demonstration of the soon-to-be-released GEOMAGIA50 database by Max Brown from GFZ, Potsdam, which now incorporates global sedimentary magnetic records in addition to acheomagnetic records. The open access database will provide easy access to published paleomagnetic and chronological data for the past 50 ka, via a convenient web-based interface. Andy Herries (La Trobe University) provided a fascinating example of magnetic records in speleothems from South African paleocaves containing early hominid fossils. Magnetism helped to constrain the ages of these ~ 2 million year old fossils through the identification of geomagnetic field excursions and reversals in finely banded flow stones. Laura Roberts-Artal from Liverpool presented her work on the paleomagnetism of 3.47 billion year old rocks from the Barberton Greenstone Belt. Analysing rocks that are this old is one of the most challenging, yet most important, tasks in our field. Explaining how the core dynamo was driven during this early part of Earth’s history is currently presenting a major challenge to geophysicists and geomagnetists, so it is crucial to put the paleomagnetic evidence on a firm footing. Roberts-Artal presented new data from previously unstudied cores, that allowed her demonstrate the reliability of remanence recorded in rocks that were at least 3.2 billion years old. Imperial PhD student Jay Shah presented new paleointensity data from Tristan da Cunha, an island situated within the South Atlantic geomagnetic Anomaly (SAA) – a large region where the intensity of the Earth’s dynamo field is anomalously low (i.e. 25 μT, compared to the expected range of > 35 μT). His analysis demonstrated that the SAA is a persistent feature of the geomagnetic field up to 81 ka.
Neil Suttie from Liverpool described his use of measurements of relative field intensity made by the explorer and naturalist Alexander von Humboldt during his voyage to South America in the years 1798 to 1803 to constrain field intensities prior to 1840 – the date when accurate measurements of the absolute field intensity began. von Humboldt was able to make measurements of variations in relative field intensity with latitude by counting the number of vibrations of a magnetised needle over a 10 minute period. Suttie showed that this data is consistent with field models for the period but that variations in intensity inferred from archaeomagnetic data cannot be reconciled with this model. John Piper from Liverpool described the three styles of tectonic behaviour that are expected to characterise Earth-like planetary bodies (exoplanets): (i) stagnant lids; (ii) episodic lids and (iii) mobile lids in constant motion able to generate zones of subduction and spreading (Plate Tectonics). The palaeomagnetic record from Earth’s continental crust provides a means of testing for tectonic style throughout the history of the crust. Piper argued that poles closely conform to a single position over very long intervals of geological time (2.7-2.2 Ga, 1.5-1.25 Ga and 0.75-0.6 Ga) suggesting that lid tectonics may have dominated the behaviour of the Earth’s lithosphere for significant parts of its history. Tony Morris from Plymouth presented an overview of the detachment-mode of sea-floor spreading, which is characteristic of slow- and ultraslow-spreading ridges. The detachment mode of seafloor spreading differs greatly from the classical mechanism of magmatic accretion, and involves the large-scale tectonic rotation around a ridge-parallel axis, exposing upper mantle and lower crustal rocks on the seafloor. The rotation can be detected by paleomagnetic analysis of samples drilled from present-day spreading ridges. Morris, however was able to find compelling evidence for the same rotation signatures in the Mirdita ophiolite of Albania, demonstrating that this mode of spreading was operating at least as far back as the Jurassic. Finally, Andy Biggin from Liverpool provided a compelling case for formalised reliability criteria in assessing paleointensity of Earth’s magnetic field. Furthermore, by examining the output of typical geodynamo simulations, Biggin was able to get a better understanding of how paleosecular variation manifests itself in paleomagnetic studies. Applying these methods, Biggin’s work provided fresh support for the reality of a Mesozoic dipole low.
In addition to the talks there were 16 fantastic posters presented by researchers at all possible stages of their careers (undergraduates, masters students, PhD students and post docs and academics). For abstracts of all talks and posters, see the full programme. A big thanks to all who presented, and here’s to next year’s meeting in Leeds!
Richard Harrison
Nono Lascu
James Bryson
Comments are closed.