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Journal articleQi L, Muxworthy AR, Collier J, et al., 2025,
Magnetization of ultramafic rocks in the Troodos ophiolite: implications for ridge axis serpentinization and ophiolite emplacement
, Journal of Geophysical Research: Solid Earth, Vol: 130, ISSN: 2169-9313Ultramafic rocks exposed in ophiolites are almost always serpentinized, but it is unclear whether the serpentinization occurs during lithospheric formation or subsequent ophiolite emplacement. The Troodos ophiolite offers an opportunity to discriminate between different serpentinization processes, incorporating rock magnetism, paleomagnetism and forward modeling of field magnetic data. Our results revealed distinct magnetic property zones: weakly magnetic mantle Artemis and Olympus zones, and a highly magnetic lower crust Cumulate zone. The Artemis and Olympus samples have magnetite concentrations <1%, magnetic susceptibility <0.01 SI and natural remanent magnetization (NRM) <4 A/m, consistent with low-temperature serpentinization related to subduction or meteoric water. In contrast, the Cumulate zone rocks have magnetite content up to 8%, magnetic susceptibility up to 0.1 SI and NRM up to 12 A/m, interpreted as high-temperature serpentinite near a spreading ridge. This ridge-related serpentinization is supported by the paleomagnetic results. The Cumulate zone has a mean direction of declination = 280°, inclination = 69°, α95 = 16°, comparable to the direction of the lower crust gabbro, which suggests serpentinization-associated chemical remagnetization during Cretaceous oceanic crust formation. Existing geological, gravity and seismic studies indicate a Pliocene subduction-related serpentinization event which led to the diapir uplift and surface relief of the Artemis and Olympus zones. Ongoing meteoric water-related serpentinization following the exposure of ultramafic rocks has caused surface remagnetization of the Artemis and Olympus zones in the current field.
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Journal articleQI L, Allerton S, Muxworthy A, et al., 2025,
Remagnetization of serpentinite during deformation: evidence from a fossil oceanic transform fault zone of the Troodos ophiolite
, Journal of Geophysical Research: Solid Earth, Vol: 130, ISSN: 2169-9313Serpentinization and associated chemical remagnetization of ultramafic rocks are common in tectonically active oceanic zones such as transform zones; however, it remains unclear how chemical remagnetization occurs during the deformation of serpentinite. This study aims to discuss this magnetization process with evidence from a serpentinite shear zone within the fossil transform fault of the Troodos ophiolite. We examine how serpentinite microstructures, serpentine polytypes, iron behaviors, rock magnetic properties and paleomagnetic directions evolve with increasing shearing deformation—a process that provides pathways for serpentinization fluid circulation. As serpentinite deformation increases from massive-fractured serpentinite adjacent to the shear zone to scaly and phyllonitic serpentinites within the shear zone, rock microstructure changes from unoriented mesh textures to oriented ribbon and fibrous structures. Meanwhile, the dominant serpentine mineral shifts from lizardite to chrysotile, indicating dynamic recrystallization during increasing deformation, likely resulting from elevated water/rock ratios driven by hydrothermal circulation. Rock magnetic results suggest that highly deformed scaly and phyllonitic serpentinites contain coarser magnetite grains and higher magnetite concentration compared to the less deformed massive-fractured serpentinites. These coarser magnetite grains are also attributed to higher water/rock ratios within the shear zone. More magnetite forms due to the iron released from the replacement of iron-rich lizardite by iron-poor chrysotile. The formation of magnetite records remagnetization, which helps reconstruct the deformation history of tectonically active zones. For example, paleomagnetic directions of the differentially deformed serpentinites in Troodos ophiolite indicate clockwise block rotations of up to 90°, providing evidence for dextral slip along a fossil transform fault.
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Journal articleDonardelli Bellon U, Williams W, Muxworthy AR, et al., 2025,
Efficiency of thermoremanent magnetization acquisition in vortex-state particle assemblies
, Geophysical Research Letters, ISSN: 0094-8276 -
Journal articleGergov H, Muxworthy AR, Williams W, et al., 2025,
Magnetic recording fidelity of basalts through 3D nanotomography
, Geochemistry, Geophysics, Geosystems, Vol: 26, ISSN: 1525-2027Volcanic basalt samples originating from two historic eruptions, that is, the 1991 C.E. Hekla,Iceland, and 1944 C.E. Vesuvius, Italy, have been studied to determine the 3D tomographic and spatialdistributions of their constituent (titano)magnetite minerals using SEM‐FIB slice‐and‐view. Determining themorphology is key to quantifying the magnetic recording fidelity of a rock, as grain morphology is a primarycontrol of the magnetic (domain) state of a grain, which in turn determines magnetic recording fidelity. Smallergrains are magnetically uniform and are termed single domain (SD). A surface morphology resolution of ∼2 nmwas achieved and the smallest grains that were resolved with ∼21 nm in diameter; a total of 971 particles wereanalyzed. We determined a median equivalent‐volume spherical diameter of 70 nm for the Hekla sample, and135 nm for the Vesuvius sample. The particles had nearest‐neighbor distances of 184 and 355 nm, indicate themajority of grains were free from magnetostatic interactions. In both samples there was a roughly even splitbetween oblate and prolate grains. This number of oblate grains is much higher than traditionally assumed, andwill have implications for many paleomagnetic methods which assume prolate grains, for example, anisotropyof magnetic susceptibility analysis. Numerical micromagnetic analysis of the grain‐morphologies, predict that∼64% of the Hekla grains have SD ground‐states (∼6% by volume), but only ∼26% of the Vesuvius grains haveSD ground‐states (∼1% by volume). Both samples are predicted to be excellent paleomagnetic recorders, withmedian relaxation times far larger than the length of the Universe
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Journal articlePei Z, Williams W, Nagy L, et al., 2025,
FORCINN: First‐Order Reversal Curve Inversion of Magnetite Using Neural Networks
, Geophysical Research Letters, Vol: 52, ISSN: 0094-8276<jats:title>Abstract</jats:title><jats:p>First‐order reversal curve (FORC) diagrams are a standard rock magnetic tool for analyzing bulk magnetic hysteresis behaviors, which are used to estimate the magnetic mineralogies and magnetic domain states of grains within natural materials. However, the interpretation of FORC distributions is challenging due to complex domain‐state responses, which introduce well‐documented uncertainties and subjectivity. Here, we propose a neural network algorithm (FORCINN) to invert the size and aspect ratio distribution from measured FORC data. We trained and tested the FORCINN model using a data set of synthetic numerical FORCs for single magnetite grains with various grain‐sizes (45–400 nm) and aspect ratios (oblate and prolate grains). In addition to successfully testing against synthetic data sets, FORCINN was found to provide good estimates of the grain‐size distributions for basalt samples and identify sample size differences in marine sediments.</jats:p>
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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 articleAbdulkarim MA, Fraser A, Neumaier M, et al., 2024,
Unravelling the history of a complex hydrocarbon play through petroleum systems modelling – Catcher Area Development (CAD), UK Central North Sea region
, Marine and Petroleum Geology, Pages: 107053-107053, ISSN: 0264-8172 -
Journal articlePaterson GA, Moreno R, Muxworthy AR, et al., 2024,
Magnetic Hysteresis Properties of Magnetite: Trends with Particle Size and Shape
, G3: Geochemistry, Geophysics, Geosystems: an electronic journal of the earth sciences, ISSN: 1525-2027 -
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 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 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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