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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: 0094-8276
In 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.
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: 0094-8276
Paleointensities 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.
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: 0009-2541
Greigite 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.
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 articleMuxworthy AR, Turney J, Qi L, et al., 2023,
High-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.
Journal articleSteele SC, Fu R, Volk MWR, et al., 2023,
The 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.
Journal articlePerkins JR, Fraser AJ, Muxworthy AR, et al., 2023,
Journal articleBaker EB, Muxworthy A, 2023,
Using Preisach theory to evaluate chemical remanent magnetization and its behavior during Thellier-Thellier-Coe paleointensity experiments, Journal of Geophysical Research: Solid Earth, ISSN: 2169-9356
Journal articleHu J, Zhang Y, Jia D, et al., 2023,
Combining paleomagnetic and Re-Os isotope data to date hydrocarbon generation and accumulation processes, Journal of Geophysical Research: Solid Earth, ISSN: 2169-9356
Journal articleNorth TL, Collins G, Davison T, et al., 2023,
Impact-generated shock waves can change the physical properties of meteorites and their constituent minerals. Accounting for these effects is key to recovering information about the early solar system from meteorite observations. ALH 84001 is a rare ancient sample from the Martian crust, providing a unique window into the thermal and metamorphic evolution of Mars. A well-studied meteorite, past geochemical and petrologic investigations have attempted to deduce its thermal and impact history with some contradictory results. By simulating the passage of a planar shock wave through a synthetic analog for samples of ALH 84001 using the iSALE-2D shock physics code we have determined the meteorite’s likely thermodynamic and physical response during an impact. Our simulations show that heterogeneous shear heating, induced by the planar shock wave, can produce strong thermal gradients on the sub-millimeter ‘mesoscale’ throughout the meteorite, even in relatively weak shock waves (5 GPa). We are able to place new constraints on deformation events experienced by the meteorite during its time on the parent body, including the maximum pressure ALH 84001 has experienced since it acquired its remanent magnetization and its subsequent ejection from Mars.
Journal articleElmore RD, Muxworthy A, Heij G, et al., 2022,
Journal articleMoreno R, Williams W, Muxworthy A, et al., 2022,
The meaning of maxima and minima in first order reversal curves: determining the interaction between species in a sample, Journal of Magnetism and Magnetic Materials, Vol: 564, ISSN: 0304-8853
First-order reversal curves (FORCs) are a characterization technique for magnetic materials used in a wide range of research fields. Since their first application in the Earth Sciences two decades ago, their importance in science has been continuously growing and new experimental techniques have been subsequently designed based on the original idea of FORCs. Nonetheless, very recent experimental works on very well designed and simple magnetic structures demonstrate that even for the most simple cases the interpretation of FORC data lacks understanding. In this work, we address this problem analytically, explaining the meaning of maxima, minima and noisy tails and set a strategy to extract the interaction field between magnetic structures. The origin of this interaction field is often the magnetostatic energy, however, we propose that this strategy could be applied for estimating exchange interactions too.
Journal articleMuxworthy A, Lam C, Green D, et al., 2022,
Iron-bearing particulate matter produced by vehicle emissions is known to be toxic. To better quantify potential health risks, we have conducted the first magnetic study of a time-series of London's inhalable particulate matter (<10 μm, PM10), captured by three monitoring stations in central London (Marylebone Road, Earl's Court Road and Oxford Street) through 2010 and 2012. We conducted room-temperature analysis on all the samples, and a limited number of samples were analysed at both high and low temperatures. The high-temperature measurements identified magnetite as the dominant magnetic phase. The low-temperature measurements revealed high numbers of nanoparticles, which, assuming magnetite, are in the grain-size range 1–4 nm. It is estimated that as much as ∼40% of the total magnetic signal at 10 K is from particles <4 nm, that are magnetically ‘invisible’ at room-temperature and are being routinely under-estimated in room temperature-based magnetic studies. From the low-temperature measurements, the total concentration of magnetite was estimated at ∼7.5%, significantly higher than previously reported. The room-temperature magnetic data were compared with other pollution data, e.g., NOX and PM10, and meteorological data. Mass-dependent terms like the saturation magnetisation were found to display a strong correlation with NOX and PM10, indicating a common source for these pollutants, i.e., vehicle emissions. Magnetic coercivity measurements, which are independent of abundance, and provide information on grain-size, were consistent across all three sampling localities, again suggesting a major dominant source. Relatively small variations in coercivity were correlated with meteorological events, e.g., temperature and precipitation, suggesting preferential removal of larger airborne grains, i.e., >50 nm.
Journal articleAbdulkarim M, Muxworthy AR, Fraser A, 2022,
High temperature susceptibility measurements: A potential tool for the identification of oil water transition zone (OWTZ) in petroleum reservoirs, Frontiers in Earth Science, Vol: 10, ISSN: 2296-6463
Determining the position and thickness of the oil water transition zone (OWTZ) in hydrocarbon reservoirs is important to reserveestimation and production planning. This paper describes a magnetic method of identifying this zone. High temperaturesusceptibility (HT‐χ) measurements on core samples from Paleogene reservoirs of the UK Central North Sea revealed distinctsignatures around the oil water interface. Rapid increases in susceptibilities at temperatures < 250 °C were observed forsamples around the oil water interface unlike the main oil leg where alteration involving increase in susceptibility occurred atsignificantly slower rates and higher temperatures. The HT‐χ data together with Mössbauer measurements revealed that thevariation in alteration characteristics is due to the increasing concentration of hexagonal pyrrhotite and/or lepidocrocite aroundthe oil water interface. Hexagonal pyrrhotite was identified in reservoirs existing at temperatures of < 80 °C, whilelepidocrocite dominated the signature around the contact of deeper reservoirs. These observations suggest that the precipitationof hexagonal pyrrhotite is related to OWTZ centred biogenic activities i.e., biodegradation. The dominance of lepidocrocite indeeper diagenetic settings has been related to hydrolysis of hydrocarbon at the oil water interface, together with cessation ofbiogenic activities.
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