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  • Journal article
    Døssing A, Riishuus MS, MacNiocaill C, Muxworthy AR, Maclennan Jet al., 2020,

    Late Miocene to late Pleistocene geomagnetic secular variation at high northern latitudes

    , Geophysical Journal International, Vol: 222, Pages: 86-102, ISSN: 0956-540X

    We report a palaeomagnetic study of Icelandic lavas of late Miocene to late Pliocene age to test the geocentric axial dipole hypothesis at high northern latitudes. Cores were sampled from 125 sites in the Fljótsdalur valley in eastern Iceland, and hand samples were taken for 17 new incremental heating 40Ar/39Ar age determinations. 96 per cent of the cores were oriented using both a Brunton compass and a sun compass. Comparison of the magnetic and sun azimuths reveals deviations of ±5°, ±10° and ±20°, respectively, for 42, 16 and 3 per cent of the data points, indicating that core sampling intended for palaeosecular variation (PSV) studies at high northern latitudes should be oriented by sun. A total of 1279 independent specimens were subjected to AF- and thermal-demagnetization for palaeodirectional analysis, and well-grouped site mean directions were obtained for 123 sites of which 113 were found to be independent sites. Applying a selection criteria of k > 50 and N ≥ 5 (Nmean = 9.5), we obtain a combined grand mean direction for 46 normal and 53 reverse (for VGPlat > ±45°) polarity sites of declination = 5.6° and inclination = 77.5° that is not significantly different from that expected from a GAD field. The corresponding palaeomagnetic pole position (VGPlat = 86.3°N, VGPlon = 21.2°E, dp/dm = 4.0°/4.3°) is coincident with the North Pole within the 95 per cent confidence limits. An updated age model is constructed based on the 40Ar/39Ar ages, showing that the majority of the Fljótsdalur lavas fall within 2–7 Ma. We combine the Fljótsdalur data with existing data from the nearby Jökuldalur valley. The 154 palaeodirections are well-dispersed between 1 and 7 Ma and constitute a high-quality data set for PSV analysis. Our results partly support previous conclusions of a generally higher dispersion during reverse polarity intervals. However

  • Journal article
    Valdez Grijalva M, Nagy L, Muxworthy A, Williams W, Roberts AP, Heslop Det al., 2020,

    Micromagnetic simulations of first-order reversal curve (FORC) diagrams of framboidal greigite

    , Geophysical Journal International, Vol: 222, Pages: 1126-1134, ISSN: 0956-540X

    Greigite is a sensitive environmental indicator and occurs commonly in nature as magnetostatically interacting framboids. Until now only the magnetic response of isolated non-interacting greigite particles have been modelled micromagnetically. We present here hysteresis and first-order reversal curve (FORC) simulations for framboidal greigite (Fe3S4), and compare results to those for isolated particles of a similar size. We demonstrate that these magnetostatic interactions alter significantly the framboid FORC response compared to isolated particles, which makes the magnetic response similar to that of much larger (multidomain) grains. We also demonstrate that framboidal signals plot in different regions of a FORC diagram, which facilitates differentiation between framboidal and isolated grain signals. Given that large greigite crystals are rarely observed in microscopy studies of natural samples, we suggest that identification of multidomain-like FORC signals in samples known to contain abundant greigite could be interpreted as evidence for framboidal greigite.

  • Journal article
    Harrison RJ, Zhao X, Hu P, Sato T, Heslop D, Muxworthy A, Oda H, Kuppili VSC, Roberts APet al., 2019,

    Simulation of remanent, transient, and induced first-order reversal curve (FORC) diagrams for interacting particles with uniaxial, cubic, and hexagonal anisotropy

    , G3: Geochemistry, Geophysics, Geosystems: an electronic journal of the earth sciences, ISSN: 1525-2027
  • Journal article
    Maidment S, Muxworthy A, 2019,

    A chronostratigraphic framework for the Upper Jurassic Morrison Formation, western USA

    , Journal of Sedimentary Research, Vol: 89, Pages: 1017-1038, ISSN: 1527-1404

    The fluvial, overbank, and lacustrine deposits of the Upper Jurassic Morrison Formation of the Western Interior, U.S.A. have been intensively studied due to their diverse and well-preserved dinosaurian fauna, and the presence of economic quantities of uranium and vanadium ores. The formation crops out over 12 degrees of latitude and 1.2 million km2, and is an excellent case study for the examination of paleoecology, community structure, and evolutionary dynamics at a time in Earth’s history when the climate was significantly warmer than today. However, paleoecological studies have been hampered by lack of correlation across the formation. Assuming a primarily tectonic control on fluvial architecture, we propose the first chronostratigraphic framework of the formation, which is based on sequence stratigraphy, magnetostratigraphy, and radiometric dating. The formation can be divided into three sequences each represented by a period of degradation followed by aggradation. This chronostratigraphic framework indicates that the formation youngs to the north, and was deposited over about 7 million years during the late Kimmeridgian and Tithonian. This framework provides a foundation for future sedimentological, stratigraphic, and paleobiological studies of the iconic dinosaurian fauna known from the Morrison.

  • Journal article
    Zhang Y, Muxworthy AR, Jia D, Wei G, Xia B, Wen B, Wang M, Liu W, Brzozowski MJet al., 2019,

    Identifying and dating the destruction of hydrocarbon reservoirs using secondary chemical remanent magnetization

    , Geophysical Research Letters, Vol: 46, Pages: 11100-11108, ISSN: 0094-8276

    Destructive processes are thought to be common in pre‐Cenozoic oil‐gas reservoirs. The timing, mechanism, and even identification of these processes, however, are difficult to clearly characterize, which obscures the evolution of such systems and the assessment of oil and gas reserves. Here, we reveal a new link between secondary chemical remanent magnetization acquisition and tectonically driven destruction of hydrocarbon reservoirs, which can be used to date the destructive processes and identify their tectonic controls. We performed a detailed paleomagnetic analysis of rocks from a typical destroyed reservoir (Majiang reservoir, China) and combined these data with scanning electronic microscope imaging and strontium isotope, total organic carbon, and clay analysis. We found that the Late Triassic syntilting secondary chemical remanent magnetizations of source and reservoir rocks resulted from the destructive processes driven by the Indosinian orogeny. We therefore argue that palaeomagnetic methods can be used to constrain destructive events within hydrocarbon reservoirs worldwide.

  • Journal article
    Roberts AP, Hu P, Harrison RJ, Heslop D, Muxworthy AR, Oda H, Sato T, Tauxe L, Zhao Xet al., 2019,

    Domain state diagnosis in rock magnetism: evaluation of potential alternatives to the Day diagram

    , Journal of Geophysical Research: Solid Earth, Vol: 124, Pages: 5286-5314, ISSN: 2169-9313

    The Day diagram is used extensively in rock magnetism for domain state diagnosis. It has been shown recently to be fundamentally ambiguous for 10 sets of reasons. This ambiguity highlights the urgency for adopting suitable alternative approaches to identify the domain state of magnetic mineral components in rock magnetic studies. We evaluate 10 potential alternative approaches here and conclude that four have value for identifying data trends, but, like the Day diagram, they are affected by use of bulk parameters that compromise domain state diagnosis in complex samples. Three approaches based on remanence curve and hysteresis loop unmixing, when supervised by independent data to avoid nonuniqueness of solutions, provide valuable component‐specific information that can be linked by inference to domain state. Three further approaches based on first‐order reversal curve diagrams provide direct domain state diagnosis with varying effectiveness. Environmentally important high‐coercivity hematite and goethite are represented with variable effectiveness in the evaluated candidate approaches. These minerals occur predominantly in noninteracting single‐domain particle assemblages in paleomagnetic contexts, so domain state diagnosis is more critical for ferrimagnetic minerals. Treating the high‐coercivity component separately following normal rock magnetic procedures allows focus on the more vexing problem of diagnosing domain state in ferrimagnetic mineral assemblages. We suggest a move away from nondiagnostic methods based on bulk parameters and adoption of approaches that provide unambiguous component‐specific domain state identification, among which various first‐order reversal curve‐based approaches provide diagnostic information.

  • Journal article
    Nagy L, Williams W, Tauxe L, Muxworthy ARet al., 2019,

    From nano to micro: evolution of magnetic domain structures in multi‐domain magnetite

    , Geochemistry, Geophysics, Geosystems, Vol: 20, Pages: 2907-2918, ISSN: 1525-2027

    Reliability of magnetic recordings of the ancient magnetic field is strongly dependent on the magnetic mineralogy of natural samples. Theoretical estimates of long‐term stability of remanence were restricted to single‐domain (SD) states, but micromagnetic models have recently demonstrated that the so‐called single‐vortex (SV) domain structure can have even higher stability that SD grains. In larger grains ( urn:x-wiley:ggge:media:ggge21913:ggge21913-math-000110 μm in magnetite) the multidomain (MD) state dominates, so that large uniform magnetic domains are separated by narrow domain walls. In this paper we use a parallelized micromagnetic finite element model to provide resolutions of many millions of elements allowing us, for the first time, to examine the evolution of magnetic structure from a uniform state, through the SV state up to the development of the domain walls indicative of MD states. For a cuboctahedral grain of magnetite, we identify clear domain walls in grains as small as ∼3 μm with domain wall widths equal to that expected in large MD grains; we therefore put the SV to MD transition at ∼3 μm for magnetite and expect well‐defined, and stable, SV structures to be present until at least ∼1 μm when reducing the grain size. Reducing the size further shows critical dependence on the history of domain structures, particularly with SV states that transition through a so‐called “unstable zone” leading to the recently observed hard‐aligned SV states that proceed to unwind to SD yet remain hard aligned.

  • Journal article
    Ku J, Valdez-Grijalva M, Deng R, Zuo W, Chen Q, Lin H, Muxworthy Aet al., 2019,

    Modelling external magnetic fields of magnetite particles: From micro- to macro-scale

    , Geosciences, Vol: 9, ISSN: 2076-3263

    We determine the role of particle shape in the type of magnetic extraction processes used in mining. We use a micromagnetic finite element method (FEM) to analyze the effect of external magnetic fields on the magnetic structures of sub-micron magnetite particles. In non-saturating fields, the magnetite particles contain multiple possible non-uniform magnetization states. The non-uniformity was found to gradually disappear with increasing applied field strength; at 100 mT the domain structure became near uniform; at 300 mT the magnetic structure saturates and the magnetization direction aligned with the field. In magnetic separation techniques, we suggest that 100 mT is the optimal field for magnetite to maximize the magnetic field with the lowest energy transfer; larger particles, i.e., >1 µm, will likely saturate in smaller fields than this. We also examined the effect of external magnetic fields on a much larger irregular particle (L × W × H = 179.5 × 113 × 103 μm) that was too large to be examined using micromagnetics. To do this we used COMSOL. The results show the relative difference between the magnitude of magnetic flux density of the particle and that of a corresponding sphere of the same volume is <5% when the distance to the particle geometry center is more than five times the sphere radius. The ideas developed in this paper have the potential to improve magnetic mineral extraction yield.

  • Journal article
    Valdez-Grijalva MA, Muxworthy AR, 2019,

    First-order reversal curve (FORC) diagrams of nanomagnets with cubic magnetocrystalline anisotropy: a numerical approach

    , Journal of Magnetism and Magnetic Materials, Vol: 471, Pages: 359-364, ISSN: 0304-8853

    First-order reversal curve (FORC) diagrams are increasingly used as a material’s magnetic domain state fingerprint. FORC diagrams of noninteracting dispersions of single-domain (SD) particles with uniaxial magnetocrystalline anisotropy (MCA) are well studied. However, a large class of materials possess a cubic MCA, for which the FORC diagram properties of noninteracting SD particle dispersions are less understood. A coherent rotation model was implemented to study the FORC diagram properties of noninteracting ensembles of SD particles with positive and negative MCA constants. The pattern formation mechanism is identified and related to the irreversible events the individual particles undergo. Our results support the utility of FORC diagrams for the identification of noninteracting to weakly-interacting SD particles with cubic MCA.

  • Journal article
    Nagy L, Williiams W, Tauxe L, Muxworthy A, Ferreira Iet al., 2018,

    Thermomagnetic recording fidelity of nanometer sized iron: implications for planetary magnetism

    , Proceedings of the National Academy of Sciences, ISSN: 0027-8424
  • Conference paper
    Ku J, Liu X, Muxworthy AR, Miao Z, Valdez M, Chen H, Deng R, Zuo W, Yin Wet al., 2018,

    Simplified modelling of particle magnetic induction field based on FEM

    , AMICR 2018, Publisher: WILEY, Pages: 54-55, ISSN: 1742-7835
  • Journal article
    Valdez-Grijalva MA, Muxworthy AR, Williams W, ó Conbhui P, Nagy L, Roberts AP, Heslop Det al., 2018,

    Magnetic vortex effects on first-order reversal curve (FORC) diagrams for greigite dispersions

    , Earth and Planetary Science Letters, Vol: 501, Pages: 103-111, ISSN: 0012-821X

    First-order reversal curve (FORC) diagrams are used increasingly in geophysics for magnetic domain state identification. The domain state of a magnetic particle is highly sensitive to particle size, about which FORC diagrams provide valuable information. However, the FORC signal of particles with nonuniform magnetisations, which are the main carrier of natural remanent magnetisations in many systems, is still poorly understood. In this study, the properties of non-interacting, randomly oriented dispersions of greigite (Fe3S4) in the uniform single-domain (SD) to non-uniform single-vortex (SV) size range are investigated via micromagnetic calculations. Signals for SD particles () are found to be in excellent agreement with previous SD coherent-rotation studies. A transitional range from to is identified for which a mixture of SD and SV behaviour produces complex FORC diagrams. Particles have purely SV behaviour with the remanent state for all particles in the ensemble in the SV state. It is found that for SV ensembles the FORC diagram provides a map of vortex nucleation and annihilation fields and that the FORC distribution peak should not be interpreted as the coercivity of the sample, but as a vortex annihilation field on the path to saturation.

  • Journal article
    Evans ME, Muxworthy AR, 2018,

    Vaalbara Palaeomagnetism

    , Canadian Journal of Earth Sciences, Vol: 56, Pages: 912-916, ISSN: 0008-4077

    Vaalbara is the name given to a proposed configuration of continental blocks—the Kaapvaal craton (southern Africa) and the Pilbara craton (north-western Australia)—thought to be the Earth’s oldest supercraton assemblage. Its temporal history is poorly defined, but it has been suggested that it was stable for at least 400 million years, between 3.1 and 2.7 Ga. Here, we present an updated analysis that shows that the existence of a single supercraton between ∼2.9 and ∼2.7 Ga is inconsistent with the available palaeomagnetic data.

  • Journal article
    Zhang Y, Jia D, Muxworthy AR, Li Y, Xia B, Xie Z, Hu J, Zi J, Liu Wet al., 2018,

    The Chemical Remagnetization of Ediacaran Dolomite in the Taishan Paleo-Reservoir, South China

    , JOURNAL OF GEOPHYSICAL RESEARCH-SOLID EARTH, Vol: 123, Pages: 6161-6175, ISSN: 2169-9313
  • Journal article
    Roberts AP, Zhao X, Harrison RJ, Heslop D, Muxworthy AR, Rowan CJ, Larrasoana J-C, Florindo Fet al., 2018,

    Signatures of Reductive Magnetic Mineral Diagenesis From Unmixing of First-Order Reversal Curves

    , JOURNAL OF GEOPHYSICAL RESEARCH-SOLID EARTH, Vol: 123, Pages: 4500-4522, ISSN: 2169-9313

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