Publications
339 results found
Berndt T, Muxworthy AR, 2016, Magnetic dating of storm floods, jökulhlaups and tsunamis, EGU 2016
Di Chiara A, Muxworthy AR, Trindade R, 2016, Paleointensity of Proterozoic magmatic rocks from South America (poster), EGU 2016
Williams W, Nagy L, Fabian K, et al., 2016, Evolution of magnetic domain structures from Pseudo-Single-Domain to Multidomain (poster), EGU 2016
Almeida T, Muxworthy AR, Kovacs A, et al., 2016, Direct visualization of the thermomagnetic behaviour of pseudo-single-domain magnetite particles, Science Advances, Vol: 2, Pages: 1-5, ISSN: 2375-2548
The study of the palaeomagnetic signal recorded by rocks allows scientists to understand the Earth’s past magnetic field and the formation of the geodynamo. The magnetic recording fidelity of this signal is dependent on the magnetic domain state it adopts. The most prevalent example found in nature is the pseudo-single-domain (PSD) structure, yet its recording fidelity is poorly understood. Here, the thermo-remanent behaviour of PSD magnetite (Fe3O4) particles, which dominate the magnetic signatures of many rock lithologies, is investigated using electron holography. This study provides spatially resolved magnetic information from individual Fe3O4 grains as a function of temperature, which has been previously inaccessible. A small exemplar Fe3O4 grain (~ 150 nm) exhibits dynamic movement of its magnetic vortex structure above 400˚C, recovering its original state upon cooling, whilst a larger exemplar Fe3O4 grain (~ 250 nm) is shown to retain its vortex state on heating to 550˚C, close to the Curie temperature of 580˚C. Hence, it is demonstrated that Fe3O4 grains containing vortex structures are indeed reliable recorders of palaeo-directional and -intensity information; and the presence of PSD magnetic signals does not preclude the successful recovery of palaeomagnetic signals.
Shah J, Koppers AP, Leitner M, et al., 2016, Palaeomagnetic evidence for the persistence or recurrence of geomagnetic main field anomalies in the South Atlantic, Earth and Planetary Science Letters, Vol: 441, Pages: 113-124, ISSN: 1385-013X
We present a dataset of a full-vector palaeomagnetic study of Late Pleistocene lavas from the island Tristan da Cunha in the South Atlantic Ocean. The current day geomagnetic field intensity in this region is approximately 25 μT, compared to an expected value of ∼43 μT; this phenomenon is known as the South Atlantic geomagnetic Anomaly (SAA). Geomagnetic field models extending back to the last 10 ka find no evidence for this being a persistent feature of the geomagnetic field, albeit, all models are constructed from data which is particularly sparse in the southern hemisphere. New 40Ar/39Ar incremental heating dating indicates the studied lavas from Tristan da Cunha extruded between 90 and 46 ka. Palaeointensity estimations of eight lava flows made using the Thellier method yield an average palaeointensity of 18±6 μT and virtual axial dipole moment (VADM) of 3.1±1.2×1022 Am2. The lava flows demonstrate four time intervals comparable to the present day SAA, where the average VADM of the Tristan da Cunha lavas is weaker than the global VADM average. This suggests a persistent or recurring low intensity anomaly to the main geomagnetic field similar to the SAA existed in the South Atlantic between 46 and 90 ka.
Shah J, Muxworthy AR, Almeida T, et al., 2016, Hot Holography: Magnetic recording fidelity of dusty olivine (poster), 13th UK Planetary Forum Early Career Scientists’ Meeting
Berndt T, Muxworthy AR, Fabian K, 2016, Does size matter? Statistical limits of paleomagnetic field reconstruction from small rock specimens, Journal of Geophysical Research: Solid Earth, Vol: 121, Pages: 15-26, ISSN: 2169-9356
As samples of ever decreasing sizes are being studied paleomagnetically, care has to be taken that the underlying assumptions of statistical thermodynamics (Maxwell-Boltzmann statistics) are being met. Here we determine how many grains and how large a magnetic moment a sample needs to have to be able to accurately record an ambient field. It is found that for samples with a thermoremanent magnetic moment larger than 10−11Am2 the assumption of a sufficiently large number of grains is usually given. Standard 25 mm diameter paleomagnetic samples usually contain enough magnetic grains such that statistical errors are negligible, but “single silicate crystal” works on, for example, zircon, plagioclase, and olivine crystals are approaching the limits of what is physically possible, leading to statistic errors in both the angular deviation and paleointensity that are comparable to other sources of error. The reliability of nanopaleomagnetic imaging techniques capable of resolving individual grains (used, for example, to study the cloudy zone in meteorites), however, is questionable due to the limited area of the material covered.
Almeida TP, Muxworthy AR, Williams W, et al., 2016, Direct visualization of CRM and TRM of pseudo-single-domain magnetite particles (invited), Magnetic Interaction 2016
Supakulopas R, Muxworthy AR, Døssing A, et al., 2016, The palaeomagnetic field properties at high northern latitudes: preliminary results from Eyjafjardardalur valley, Magnetic Interactions 2016
Mac Niocaill C, Muxworthy AR, Vowles K, et al., 2016, Palaeomagnetic dates of Irish Zn-Pb deposits record regional remagnetization related to Variscan fluid flow, Magnetic Interactions 2016
Abubakar R, Muxworthy AR, Sephton M, et al., 2016, Mapping Petroleum Migration Pathways in Wessex Basin Using Magnetics and Seismic Mapping (poster), Magnetic Interactions 2016
Maidment S, Muxworthy AR, 2016, A chronostratigraphic framework for the Morrison formation, western USA: A sequence stratigraphic and magnetostratigraphic approach (poster), Magnetic Interactions 2016
Muxworthy AR, 2016, How Well Does Preisach Theory Predict Pseudo-Single-Domain Behavior? (poster), Magnetic Interactions 2016
Valdez-Grijalva MA, Muxworthy AR, 2016, Simulated 'early onset' PSD behaviour: Size and shape effects in non-interacting greigite nanoparticles, Magnetic Interactions 2016
Almeida T, Muxworthy AR, Kasama T, et al., 2016, Effect of maghemization on the magnetic properties of non-stoichiometric pseudo-single-domain magnetite particles (poster), Magnetic Interactions 2016
Shah J, Muxworthy AR, Almeida TP, et al., 2016, In-situ heating holography of chondrule dusty olivine, Magnetic Interactions
Badejo, Muxworthy AR, Fraser A, 2016, Application of magnetic techniques to lateral hydrocarbon migration - Lower Tertiary reservoir systems, UK North Sea (poster), Magnetic Interactions 2016
Almeida T, Muxworthy AR, Kasama T, et al., 2015, Effect of maghemization on the magnetic properties of non-stoichiometric pseudo-single-domain magnetite particles (poster), AGU Fall 2015
Almeida T, Muxworthy AR, Kasama T, et al., 2015, Direct visualisation of CRM and TRM of pseudo-single-domain magnetite grains and the implications for reliable palaeomagnetic signal acquisition (invited), AGU Fall 2015
Di Chiara A, Muxworthy AR, Trindade R, 2015, Paleointensity of Proterozoic magmatic rocks from South America, preliminary results, CRES 2015
Shah J, Muxworthy AR, Almeida T, et al., 2015, Electron Holography of Chondrule Dusty Olivine (poster), Meteorites and Solar System formation workshop
Berndt T, Muxworthy AR, Paterson GA, 2015, Determining the magnetic attempt time τ0, its temperature dependence and the grain-size distribution from magnetic viscosity measurements, Journal of Geophysical Research. Solid Earth, Vol: 120, Pages: 7322-7336, ISSN: 2169-9313
A new method to determine the atomic attempt time τ0 of magnetic relaxation of fine particles, which is central to rock and soil magnetism and paleomagnetic recording theory, is presented, including the determination of its temperature dependence, and simultaneously the grain-size distribution of a sample. It is based on measuring a series of zero-field magnetic viscous decay curves for saturation isothermal remanent magnetization at various different temperatures, that are later joined together on a single grain-size scale from which the grain-size distribution and attempt time are determined. The attempt time was determined for three samples containing non-interacting, single-domain titanomagnetites of different grain-sizes for temperatures between 27 K and 374 K. No clear temperature-dependent trend was found, however, values varied significantly from one sample to the other: from 10⁻¹¹ s to 10⁻⁸ s; in particular, the sample containing multiple magnetic phases had an effective attempt time significantly lower than the more homogeneous samples, thereby questioning the applicability of the simple Néel-Arhennius equation for magnetic relaxation for composite materials.
Almeida TP, Muxworthy AR, Kovács A, et al., 2015, Visualisation of high temperature magnetisation states in magnetite grains using off-axis electron holography, Journal of Physics: Conference Series, ISSN: 1742-6588
Abubakar R, Muxworthy AR, Southern P, et al., 2015, Formation of magnetic minerals in hydrocarbon-generation conditions, Marine and Petroleum Geology, ISSN: 1873-4073
In this paper, we report the pyrolysis and formation of magnetic minerals in three source rock samples from the Wessex Basin in Dorset, southern England. The experimental conditions in the laboratory recreated the catagenesis environment of oil source rocks. Magnetic analysis of both the heated and the unheated samples at room temperature and at very low temperatures (5 K), coupled with transmission electron-microscopy imaging and X-ray analysis, revealed the formation of nanometre-sized (<10 nm), magnetic particles that varied across the rock samples analysed, but more importantly across the pyrolysis temperature range. Magnetic measurements demonstrated the formation of these magnetic minerals peaked at 250 °C for all rock samples and then decreased at 300 °C before rising again at 320 °C. The newly formed magnetic minerals are suggested to be primarily pyrrhotite, though magnetite and greigite are also thought to be present. The sizes of the magnetic minerals formed suggest a propensity to migrate together with oil potentially explaining the magnetic anomalies observed above and within oil fields.
Muxworthy AR, 2015, How well does Preisach Theory predict Pseudo-Single-Domain Behavior? (poster), AGU Fall 2015
Abubakar R, Muxworthy AR, Sephton M, et al., 2015, Mapping Petroluem Migration Pathways Using Magnetics and Seismic Interpretations (poster), AGU Fall 2015
Muxworthy AR, 2015, Investigation of magnetic particulate matter inside animals' lung tissue: preliminary results, STUDIA GEOPHYSICA ET GEODAETICA, Vol: 59, Pages: 628-634, ISSN: 0039-3169
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Almeida TP, Muxworthy AR, Kasama T, et al., 2015, Effect of maghemization on the magnetic properties of nonstoichiometric pseudo-single-domain magnetite particles, Geochemistry Geophysics Geosystems, Vol: 16, Pages: 2969-2979, ISSN: 1525-2027
The effect of maghemization on the magnetic properties of magnetite (Fe3O4) grains in the pseudo-single-domain (PSD) size range is investigated as a function of annealing temperature. X-ray diffraction and transmission electron microscopy confirms the precursor grains as Fe3O4 ranging from ~ 150 nm to ~ 250 nm in diameter, whilst Mössbauer spectrometry suggests the grains are initially near-stoichiometric. The Fe3O4 grains are heated to increasing reaction temperatures of 120 – 220 ºC to investigate their oxidation to maghemite (γ-Fe2O3). High-angle annular dark field imaging and localized electron energy loss spectroscopy reveals slightly oxidized Fe3O4 grains, heated to 140 ºC, exhibit higher oxygen content at the surface. Off-axis electron holography allows for construction of magnetic induction maps of individual Fe3O4 and γ-Fe2O3 grains, revealing their PSD (vortex) nature, which is supported by magnetic hysteresis measurements, including first order reversal curve analysis. The coercivity of the grains is shown to increase with reaction temperature up to 180 ºC, but subsequently decreases after heating above 200 ºC; this magnetic behavior is attributed to the growth of a γ-Fe2O3 shell with magnetic properties distinct from the Fe3O4 core. It is suggested there is exchange coupling between these separate components that results in a vortex state with reduced vorticity. Once fully oxidized to γ-Fe2O3, the domain states revert back to vortices with slightly reduced coercivity. It is argued that due to a core/shell coupling mechanism during maghemization, the directional magnetic information will still be correct, however, the intensity information will not be retained.
Muxworthy AR, Bland PA, Collins G, et al., 2015, MAGNETIC FABRICS IN ALLENDE: IMPLICATIONS FOR MAGNETIC REMANENCE ACQUISITION., 78th Annual Meeting of the Meteoritical-Society, Publisher: WILEY, ISSN: 1086-9379
Shah J, Muxworthy AR, Almeida TP, et al., 2015, VISUALIZING THE MAGNETIC BEHAVIOR OF CHONDRULE DUSTY OLIVINE USING ELECTRON HOLOGRAPHY., 78th Annual Meeting of the Meteoritical-Society, Publisher: WILEY-BLACKWELL, ISSN: 1086-9379
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