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Journal articleTurney JN, Fraser A, Muxworthy AR, et al., 2024,
New insights from petroleum systems modelling and magnetic analyses on the charge, fill and spill history of the Wytch Farm oil field, Wessex Basin, UK
, Marine and Petroleum Geology, Vol: 167, ISSN: 0264-8172Basin and petroleum systems modelling of the Wessex Basin, UK has been conducted to identify the maturation and migration events that charged the principal Bridport Sands and Sherwood Sandstone reservoirs at the Wytch Farm oil field. Modelling results have been compared with recent observations of magnetic enhancements at oil-water contacts (OWCs) and possibly paleocontacts (PCs) in Wytch Farm reservoirs, to assess the use of magnetic OWCs to help calibrate petroleum systems models and provide insights into the migration history of the Wessex Basin. The model predicts the Blue Lias source rock only reached maturity to the south of the Purbeck Fault, with hydrocarbon generation initiating in the Late Jurassic and peaking in the Late Cretaceous, requiring lateral migration to Wytch Farm using the Bridport Sands as the main carrier bed. Cross-fault and northward migration occurred through conduits at Creech, Bushey Farm and in offshore areas, which charged the principal structures at Wytch Farm. A ∼20 km wide Late Cretaceous juxtaposition between the Bridport Sands and Sherwood Sandstone in the hangingwall and footwall of the Purbeck Fault, respectively, led to the charging of the Sherwood Sandstone reservoir. A basin-wide Cenozoic easterly tilt of ≤1⁰ caused a westerly hydrocarbon remigration, has shifted the Bridport Sands and Sherwood Sandstone Wytch Farm structures to the west, and drastically reduced the size of Bridport Sands accumulations. There is a strong correlation between the predicted depths of Late Cretaceous and present-day OWCs with magnetic enhancements in drill cores. Multiple magnetic enhancements above the OWC at the Wareham oil field indicate the Cenozoic tilting event was periodic, forming multiple stable OWCs, with migration modelling suggesting a spill from Wytch Farm.
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Journal articleMuxworthy AR, Riishuus MS, Supakulopas R, et al., 2024,
The palaeomagnetic field recorded in Eyjafjarðardalur basalts (2.6-8.0 Ma), Iceland: Are inclination-shallowing corrections necessary in time-average field analysis?
, Geophysical Journal International, Vol: 238, Pages: 764-782, ISSN: 0956-540XThe geocentric axial dipole (GAD) hypothesis is key to many palaeomagnetic applications, for example plate-tectonic reconstructions; however, the validity of this hypothesis at high latitudes is not fully resolved. To address this, in this paper we determined the palaeomagnetic directional data of 156 lava units in Eyjafjarðardalur, Iceland, with the aim of determining the validity of the GAD hypothesis at high latitudes using time-averaged field (TAF) analysis. In addition to the palaeomagnetic directional data, we constructed an age model for the sequences using new 40Ar/39Ar dates, magnetostratigraphy and field data. The sequence age range is 2.6–8.0 Ma. We show that the mean virtual geomagnetic pole (VGP) for our data does not agree with the GAD theory at 95 per cent confidence, when only the standard tilt and tectonic corrections are made; however, when inclination-shallowing processes are accounted for, for example thermoremanence (TRM) anisotropy and refraction effects, the mean VGP can align with GAD at 95 per cent confidence. These inclination-shallowing processes are shown to reduce the inclination by up to 14° for some of the basaltic units. Applying the inclination-shallowing correction also reduces VGP dispersion to levels that agree with global model predictions. We propose that much of the scatter within the palaeomagnetic directional databases are due to inclination-shallowing process effects, which become more important as the natural remanent magnetization (NRM) intensity is high, for example >2 A m−1. We propose that inclination-shallowing processes can be identified and corrected for by examining the NRM intensity and dispersion.
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Journal articleWiliams W, Moreno R, Muxworthy AR, et al., 2024,
Vortex magnetic domain state behavior in the day plot
, G3: Geochemistry, Geophysics, Geosystems: an electronic journal of the earth sciences, Vol: 25, ISSN: 1525-2027The ability of rocks to hold a reliable record of the ancient geomagnetic field depends on the structure and stability of magnetic domain-states contained within constituent particles. In paleomagnetic studies, the Day plot is an easily constructed graph of magnetic hysteresis parameters that is frequently used to estimate the likely magnetic recording stability of samples. Often samples plot in the region of the Day plot attributed to so-called pseudo-single-domain particles with little understanding of the implications for domain-states or recording fidelity. Here we use micromagnetic models to explore the hysteresis parameters of magnetite particles with idealized prolate and oblate truncated-octahedral geometries containing single domain (SD), single-vortex and occasionally multi-vortex states. We show that these domain states exhibit a well-defined trend in the Day plot that extends from the SD region well into the multi-domain region, all of which are likely to be stable remanence carriers. We suggest that although the interpretation of the Day plot and its variants might be subject to ambiguities, if the magnetic mineralogy is known, it can still provide some useful insights about paleomagnetic specimens' dominant domain state, average particle sizes and, consequently, their paleomagnetic stability.
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Journal articlePerkins J, Muxworthy AR, Fraser A, et al., 2024,
Quantifying the mineral magnetic signature of petroleum systems and their source rocks: A study on the Inner Moray Firth, UK North Sea
, Geophysical Journal International, ISSN: 0956-540X -
Journal articleNagy L, Moreno R, Muxworthy AR, et al., 2024,
Micromagnetic determination of the FORC response of paleomagnetically significant magnetite assemblages
, G3: Geochemistry, Geophysics, Geosystems: an electronic journal of the earth sciences, ISSN: 1525-2027 -
Journal articleTurney J, Muxworthy AR, Sims M, et al., 2024,
Quantifying the characteristics of magnetic oil-water contacts in mature hydrocarbon reservoirs and their capacity for understanding hydrocarbon remigration
, Geophysical Journal International, Vol: 237, Pages: 570-587, ISSN: 0956-540XIncreasing magnetization within mature hydrocarbon reservoirs provides a new technique in identifying oil–water contacts (OWCs) in cored wells with the potential to assess yield thereby reducing the need for further exploration. Authigenic precipitation of magnetic minerals at OWCs may also help locate palaeocontacts (PCs), where structural changes to the petroleum system have caused hydrocarbon remigration. This study determines the magnetic characteristics of magnetic enhancements at OWCs and possibly PCs in silliclastic and carbonate reservoirs at the Wytch Farm oil field, Wessex Basin, UK. Increases in saturation magnetization and susceptibility are observed at the OWC in 11 of the 12 analysed cored reservoirs owing to the increased presence of magnetite and vivianite. Geochemical analysis and shallow reservoirs suggest biogenic and inorganic mineral precipitation is extensive at the OWC depending on iron, sulphur and phosphorus availability. Similar magnetic characteristics have been observed in magnetic enhancements above the OWC in numerous wells which may represent OWCs before a basin-wide easterly tilt caused hydrocarbon remigration in the Cenozoic. Multiple magnetic enhancements above the OWC in westerly onshore wells, suggest this remigration may have occurred as numerous phases.
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Journal articleDøssing A, Kolster ME, Silva ELS, et al., 2024,
Pre-existing structural control on the recent Holuhraun eruptions along the Bárðarbunga spreading center, Iceland
, Scientific Reports, ISSN: 2045-2322 -
Journal articleNorth T, Muxworthy A, Williams W, et al., 2024,
The effect of stress on paleomagnetic signals: a micromagnetic study of magnetite's single-vortex response
, Geophysical Research Letters, Vol: 51, ISSN: 0094-8276In this study we use micromagnetic modeling to show that the magnetizations of magnetically single-vortex particles rotate toward the stress axis on the application of a differential compression stress. This is the exact opposite response to magnetically single-domain particles, which previously provided the theoretical underpinning of the effect of stress on the magnetic signals of rocks. We show that the magnetization directions of single-vortex and equant single-domain particles are altered by much lower stresses than previously predicted, c.f., 100 versus 1,000 MPa; where a change in magnetization is defined as a rotation of >3° after the removal of stress. The magnetization intensity of assemblages also drops by ∼20%–30% on the application and removal of stress of ∼100 MPa. Given that single-vortex particles are now thought to dominate the magnetization of most rocks, future studies should account for paleomagnetic directional uncertainties and potential underestimation of the ancient magnetic field intensity.
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Journal articleNagy L, Tauxe L, Williams W, et al., 2023,
Chasing tails: Insights from micromagnetic modeling for thermomagnetic recording in non-uniform magnetic structures
, Geophysical Research Letters, Vol: 49, ISSN: 0094-8276Paleointensities are key to understanding the formation and evolution of Earth and are determined from rocks which record magnetic fields upon cooling; however, experimental protocols for estimating paleointensities frequently fail. The primary reason is that laboratory protocols assume that rocks are dominated by uniformly magnetized, single-domain grains, instead of much more common non-uniformly magnetized grains. Our model for larger grains shows a multiplicity of stable domain states; with preferred states changing as a function of temperature. We show that domain state distribution depends on the thermal history of the sample—in nature and the laboratory. From numerical thermomagnetic modeling, we show that particles with non-uniform domain states will theoretically fail standard experimental paleointensity protocols, preventing us from determining reliable ancient geomagnetic field intensities. We propose that recognizing this type of behavior, and the resulting bias, will yield more reliable paleointensity records, and a better understanding of the Earth.
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Journal articleTurney J, Weiss D, Muxworthy AR, et al., 2023,
Greigite formation in aqueous solutions: critical constraints into the role of iron and sulphur ratios, pH and Eh, and temperature using reaction pathway modelling
, Chemical Geology, Vol: 635, Pages: 1-16, ISSN: 0009-2541Greigite forms as an intermediate phase along the pyrite reaction pathway. Despite being considered metastable, it is observed in numerous shallow natural systems, suggesting it could be a unique proxy for diagenetic and environmental conditions. We use thermodynamic reaction pathway modelling in PHREEQC software, to understand the role of iron and sulphur ratios, pH and Eh, and temperature on the formation and retention of greigite in aqueous solutions. With newly available experimental thermodynamic properties, this work identifies the chemical boundary conditions for greigite formation in aqueous solutions. Greigite precipitation is likely favourable in anoxic and alkaline aqueous solutions at or below 25 °C. Our numerical experiments show that greigite is closer to saturation in iron-rich solutions with minor sulphur input. Greigite precipitation in strongly alkaline solutions suggest polysulfides and ferric iron-bearing minerals may be favourable reactants for its formation. Greigite precipitates at iron and sulphur concentrations that are over two orders of magnitude greater than iron sulphide-hosted natural porewaters. This disparity between model and field observations suggest microenvironments within bulk solutions may be important for greigite formation and retention. These constraints suggest greigite is more likely to form alongside pyrite in shallow, non-steady state aqueous solutions.
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Journal articleNoble JPP, Bending SJ, Muxworthy AR, et al., 2023,
Simplified Model for Minor and Major Loop Magnetic Hysteresis and its Application for Inference of Temperature in Induction Heated Particle Beds
, Journal of Physics D: Applied Physics, ISSN: 0022-3727 -
Journal articleDi Chiara A, Muxworthy AR, Trindade RIF, et al., 2023,
Mesoproterozoic geomagnetic field strength from Nova Guarita mafic dykes (Amazon Craton)
, Studia Geophysica et Geodaetica: a journal of geophysics, geodesy, meteorology and climatology, ISSN: 0039-3169 -
Journal articleLin F, Qi L, Zhang N, et al., 2023,
An ongoing lithospheric dripping process beneath northeast China and its impact on intraplate volcanism
, GEOLOGY, Vol: 52, Pages: 435-440, ISSN: 0091-7613 -
Journal articleMuxworthy AR, Turney J, Qi L, et al., 2023,
Interpreting high-temperature magnetic susceptibility data of natural systems
, Frontiers in Earth Science, Vol: 11, ISSN: 2296-6463High-temperature susceptibility (HT-χ) data are routinely measured in Earth, planetary, and environmental sciences to rapidly identify the magnetic mineralogy of natural systems. The interpretation of such data can be complicated. Whilst some minerals are relatively unaltered by heating and are easy to identify through their Curie or Néel temperature, other common magnetic phases, e.g., iron sulphides, are very unstable to heating. This makes HT-χ interpretation challenging, especially in multi-mineralogical samples. Here, we report a review of the HT-χ data measured primarily at Imperial College London of common magnetic minerals found in natural samples. We show examples of “near pure” natural samples, in addition to examples of interpretation of multi-phase HT-χ data. We hope that this paper will act be the first reference paper for HT-χ data interpretation.
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Journal articleSteele SC, Fu R, Volk MWR, et al., 2023,
Paleomagnetic evidence for a long-lived, potentially reversing martian dynamo at ~3.9 Ga
, Science Advances, Vol: 9, Pages: 1-13, ISSN: 2375-2548The 4.1-billion-year-old meteorite Allan Hills 84001 (ALH 84001) may preserve a magnetic record of the extinct martian dynamo. However, previous paleomagnetic studies have reported heterogeneous, nonunidirectional magnetization in the meteorite at submillimeter scales, calling into question whether it records a dynamo field. We use the quantum diamond microscope to analyze igneous Fe-sulfides in ALH 84001 that may carry remanence as old as 4.1 billion years (Ga). We find that individual, 100-μm-scale ferromagnetic mineral assemblages are strongly magnetized in two nearly antipodal directions. This suggests that the meteorite recorded strong fields following impact heating at 4.1 to 3.95 Ga, after which at least one further impact heterogeneously remagnetized the meteorite in a nearly antipodal local field. These observations are most simply explained by a reversing martian dynamo that was active until 3.9 Ga, thereby implying a late cessation for the martian dynamo and potentially documenting reversing behavior in a nonterrestrial planetary dynamo.
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