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Journal articleAbubakar R, Muxworthy A, Fraser A, et al., 2020,
Mapping hydrocarbon charge-points in the Wessex Basin using seismic, geochemistry and mineral magnetics
, Marine and Petroleum Geology, Vol: 111, Pages: 510-528, ISSN: 1873-4073This study reports a multidisciplinary approach to determining hydrocarbon charge-points and migration in the Wessex Basin, southern England. Geochemical analysis of reservoir core material (Bridport Sandstone and Inferior Oolite) using gas chromatography-mass spectrometry (GC-MS), suggests that the oil in the Wessex Basin is from a single source, and that small variations in environmentally sensitive biomarkers are likely due to small differences in maturity or depositional conditions during the formation of the oil over millions of years. Using seismic data, basin modelling revealed two potential hydrocarbon migration pathways from the hanging wall of the Purbeck fault into the Sherwood Sandstone reservoir at Wytch Farm. One of these potential pathways is represented by cores termed Creech and the other Bushey Farm. To try to distinguish between the two potential pathways, cores were studied using mineral magnetic techniques. The magnetic signature was characterised using low-temperature (<50 K) magnetic measurements; this is because much of the magnetic signature was dominated by nanoparticles < 30 nm, which are thermally activated at room temperature and magnetically “transparent”. Wells that contained considerable amounts of hydrocarbons were dominated by nanometric magnetite (<30 nm). Such particles are small enough to migrate with the oil, through pore spaces, which are of the order ~100 nm. Wells located at the fringes of large hydrocarbon accumulation had enhanced pyrrhotite-dominated magnetic signals. Of the two potential migration pathways, the mineral magnetic results suggest that the oil migrated through Creech rather than through Bushey Farm.
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Journal articleHarrison RJ, Zhao X, Hu P, et 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 articleMaidment 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: 1527-1404The 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.
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Journal articleZhang Y, Muxworthy AR, Jia D, et al., 2019,
Identifying and dating the destruction of hydrocarbon reservoirs using secondary chemical remanent magnetization
, Geophysical Research Letters, Vol: 46, Pages: 11100-11108, ISSN: 0094-8276Destructive 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.
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Journal articleRoberts AP, Hu P, Harrison RJ, et 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: 2169-9313The 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.
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Journal articleNagy L, Williams W, Tauxe L, et al., 2019,
From nano to micro: evolution of magnetic domain structures in multi‐domain magnetite
, Geochemistry, Geophysics, Geosystems, Vol: 20, Pages: 2907-2918, ISSN: 1525-2027Reliability 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.
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Journal articlePenny C, Muxworthy A, Fabian K, 2019,
Mean-field modelling of magnetic nanoparticles: The effect of particle size and shape on the Curie temperature
, Physical review B: Condensed matter and materials physics, Vol: 99, ISSN: 1098-0121A Heisenberg mean-field model is used to study the effect of size and shape on the Curie temperature of magnetic nanoparticles. Simple cubic, body-centered cubic, and magnetite nanoparticles are modelled as spheres, cubes, and needlelike particles. The Curie temperatures of particles of different shape, but with the same crystal structure and smallest dimension d, are found to differ. The range in the value of the Curie temperature between particles of different shape, ΔTC, is found to be ∼20% of the bulk value of TC in particles where d<10 atoms. As particle size increases, the value of ΔTC reduces rapidly and becomes negligible above a threshold size. This threshold size differs between systems and is controlled predominantly by crystal structure. All systems were fit to the finite-size scaling equation, with values of the scaling exponent ν found to lie between 0.46 and 0.55, in good agreement with the expected value of ν=0.5. No trend in the value of ν due to shape was found.
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Journal articleKu J, Valdez-Grijalva M, Deng R, et al., 2019,
Modelling external magnetic fields of magnetite particles: From micro- to macro-scale
, Geosciences, Vol: 9, ISSN: 2076-3263We 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.
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Journal articleValdez-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: 0304-8853First-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.
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Journal articleNagy L, Williiams W, Tauxe L, et al., 2018,
Thermomagnetic recording fidelity of nanometer sized iron: implications for planetary magnetism
, Proceedings of the National Academy of Sciences, ISSN: 0027-8424 -
Conference paperKu J, Liu X, Muxworthy AR, et al., 2018,
Simplified modelling of particle magnetic induction field based on FEM
, AMICR 2018, Publisher: WILEY, Pages: 54-55, ISSN: 1742-7835 -
Journal articleValdez-Grijalva MA, Muxworthy AR, Williams W, et 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: 0012-821XFirst-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.
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Journal articleEvans ME, Muxworthy AR, 2018,
Vaalbara Palaeomagnetism
, Canadian Journal of Earth Sciences, Vol: 56, Pages: 912-916, ISSN: 0008-4077Vaalbara 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.
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Journal articleZhang Y, Jia D, Muxworthy AR, et 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 articleRoberts AP, Zhao X, Harrison RJ, et 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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