Imperial College London

Professor Adrian Muxworthy

Faculty of EngineeringDepartment of Earth Science & Engineering

Professor of Earth and Planetary Magnetism
 
 
 
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Contact

 

+44 (0)20 7594 6442adrian.muxworthy

 
 
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Location

 

4.48Royal School of MinesSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

336 results found

Nagy L, Tauxe L, Williams W, Muxworthy Aet 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 article

Turney J, Weiss D, Muxworthy AR, Fraser Aet 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 article

Noble JPP, Bending SJ, Muxworthy AR, Hill AKet 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 article

Di Chiara A, Muxworthy AR, Trindade RIF, Bispo-Santos F, Baker EBet 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 article

Muxworthy AR, Turney J, Qi L, Baker EB, Perkins J, Abdulkarim Met al., 2023, Interpreting high-temperature magnetic susceptibility data of natural systems, Frontiers in Earth Science, Vol: 11, ISSN: 2296-6463

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 article

Steele SC, Fu R, Volk MWR, North TL, Muxworthy A, Collins GS, Davison T, Brenner ARet al., 2023, Paleomagnetic evidence for a long-lived, potentially reversing martian dynamo at ~3.9 Ga, Science Advances, Vol: 9, Pages: 1-13, ISSN: 2375-2548

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 article

Perkins JR, Fraser AJ, Muxworthy AR, Neumaier M, Schenk Oet al., 2023, Basin and petroleum systems modelling to characterise multi-source hydrocarbon generation: A case study on the inner Moray Firth, UK North Sea, Marine and Petroleum Geology, Vol: 151, Pages: 106180-106180, ISSN: 0264-8172

Journal article

Baker 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 article

Hu J, Zhang Y, Jia D, Muxworthy A, Selby D, Li Y, Brzozowski MJ, Wei G, Yin Het 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 article

North TL, Collins G, Davison T, Muxworthy A, Steele S, Fu Ret al., 2023, The heterogeneous response of Martian meteorite Allan Hills 84001 to planar shock, Icarus, Vol: 390, ISSN: 0019-1035

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 article

Elmore RD, Muxworthy A, Heij G, Elliot WCet al., 2023, Remagnetization and Diagenesis, Frontiers in Earth Science, ISSN: 2296-6463

Journal article

Moreno R, Williams W, Muxworthy A, Paterson GA, Heslop Det 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 article

Muxworthy A, Lam C, Green D, Cowan A, Maher B, Gonet Tet al., 2022, Magnetic characterisation of London’s airborne nanoparticulate matter, Atmospheric Environment, Vol: 287, Pages: 1-8, ISSN: 1352-2310

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 article

Abdulkarim 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.

Journal article

North TL, Muxworthy AR, Collins GS, Davison TMet al., 2022, THERMOREMANENT MAGNETISATION RECORDED DURING IMPACT-INDUCED COMPACTION EXPERIMENTS ON SYNTHETIC CHONDRITIC METEORITES, LSPC, Publisher: WILEY, ISSN: 1086-9379

Conference paper

Abdulkarim MA, Muxworthy A, Fraser A, Sims M, Cowan Aet al., 2022, Effect of hydrocarbon presence and properties on the magnetic signature of the reservoir sediments of the Catcher Area Development (CAD) region, UK North Sea, Frontiers in Earth Science, Vol: 10, Pages: 1-20, ISSN: 2296-6463

This paper presents a detailed study investigating the effect of hydrocarbon presence on magnetic mineral diagenesis in sediments from the Catcher Area Development (CAD) region, UK North Sea, between 1,000 and 1,500 m (True Vertical Depth Sub-Sea). Magnetic analysis of core samples from hydrocarbon fields of the region and nearby dry-well sandstones (background) was carried out to determine if their signatures can serve as a proxy for understanding petroleum reservoir systems. From the background samples, nanometric and micron-sized magnetite, hematite and titano-iron oxides, were identified. Hydrocarbon presence in the reservoir sediments was found to diminish the iron-oxide signature and favour the precipitation of hexagonal pyrrhotite, siderite and potentially vivianite, lepidocrocite, greigite and paramagnetic iron sulphides. Hexagonal pyrrhotite was found at the oil-water transition zones. This relationship is possibly related to biodegradation at this interface. Siderite was found in increased abundance at shallower depths within the reservoir, which we attribute to hydrocarbon vertical migration and biodegradation. The interbedded shales also experienced significant magnetic mineral diagenesis that depended on its proximity to the hydrocarbon plume. These findings suggest that mineral magnetism can be applied to the identification of oil-water transition zones, reserve estimation, production planning and the determination of hydrocarbon migration pathways. It also suggests that mineral magnetic methods can be used to estimate the timing of hydrocarbon migration.

Journal article

Roberts AP, Heslop D, Zhao X, Oda H, Egli R, Harrison RJ, Hu P, Muxworthy AR, Sato Tet al., 2022, Unlocking information about fine magnetic particle assemblages from first-order reversal curve diagrams: Recent advances, EARTH-SCIENCE REVIEWS, Vol: 227, ISSN: 0012-8252

Journal article

Abdulkarim M, Muxworthy A, Fraser A, Neumaier M, Hu P, Cowan Aet al., 2022, Siderite occurrence in petroleum systems and its potential as a hydrocarbon-migration proxy: a case study of the Catcher Area Development and the Bittern area, UK North Sea, Journal of Petroleum Science and Engineering, ISSN: 0920-4105

Journal article

Roberts AP, Zhao X, Hu P, Abrajevitch A, Chen Y-H, Harrison RJ, Heslop D, Jiang Z, Li J, Liu Q, Muxworthy A, Oda H, O'Neill H, Pillans BJ, Sato Tet al., 2021, Magnetic domain state and anisotropy in hematite (alpha-Fe2O3) from first-order reversal curve diagrams, Journal of Geophysical Research. Solid Earth, ISSN: 2169-9356

Journal article

Muxworthy A, Baker E, 2021, ThellierCoolPy: A cooling-rate correction tool for paleointensity data, G3: Geochemistry, Geophysics, Geosystems: an electronic journal of the earth sciences, Vol: 22, Pages: 1-8, ISSN: 1525-2027

We report a new approach of implementing cooling-rate corrections in absolute ancient magnetic field intensity (paleointensity) studies. Nearly all methods of determining paleointensity estimates rely on rocks having recorded a thermoremanent magnetization (TRM), on cooling from above the rock’s constituent minerals’ Curie temperature. Typically paleointensity estimates are made by comparing natural TRM, with a TRM induced in the laboratory; however, TRM intensity has long been reported to be dependent on cooling rate. Natural cooling rates are impractical in laboratories. We have developed a new cooling-rate correction method and corresponding software (ThellierCoolPy), that directly corrects the unprocessed paleointensity data, using first-order reversal curve data collected on a sister sample. This site tailored cooling-rate correction has a unique correction for each temperature step within the paleointensity data set. This new method differs from previous approaches which apply a blanket cooling-rate correction independent of the material properties of the sample. Paleointensity data from historical lavas from Parícutin, Mexico, are used to demonstrate the new software. For this data set, it is shown that cooling time of 1 million years yields a reduction of the paleointensity of ∼7%. The software is available for download.

Journal article

Noble JPP, Bending SJ, Sartbaeva A, Muxworthy AR, Hill AKet al., 2021, A Novel In Situ High-Temperature Magnetometry Method for Radiofrequency Heating Applications, ADVANCED ENERGY MATERIALS, Vol: 12, ISSN: 1614-6832

Journal article

Badejo SA, Muxworthy A, Fraser A, Stevenson G, Zhao X, Jackson Met al., 2021, Identification of magnetic enhancement at hydrocarbon/water contacts, American Association of Petroleum Geologists (AAPG) Bulletin, Vol: 105, Pages: 1973-1991, ISSN: 0149-1423

Identifying the depths of the hydrocarbon-fluid contacts in a reservoir is important for determining hydrocarbon reserves and production planning. Using core samples from the Tay sandstone reservoir in the Central North Sea, we show that thereis a magnetic enhancement at the hydrocarbon-fluid contacts, that is detectable both through magnetic susceptibility measurements and magnetic hysteresis measurements. We observed this magnetic enhancement at both gas-oil and oil-water contacts, that have been independently identified using non-magnetic methods; we did not consider gas-water contacts in this study. We demonstrate that this magnetic enhancement is due to the precipitation of new nanometric iron oxide (magnetite) and iron sulphide (greigite)phases. The magnetic enhancement may be caused by diagenetic changes or preferential biodegradation at the top of the oil column during early filling and at the oil water contact. Our findings have the potential to be used to identify paleo-hydrocarbon-fluid contact in both structurally modified fields and failed wells. The technique can also be used to infer the fill history of a basin and calibrate petroleum systems models. Magnetic susceptibility measurements have the advantage that they can easily and quickly be measured in the field on whole core-material.

Journal article

Badejo SA, Muxworthy AR, Fraser A, Neumaier M, Perkins JR, Stevenson GR, Davey Ret al., 2021, Using magnetic techniques to calibrate hydrocarbon migration in petroleum systems modelling: A Case Study from the Lower Tertiary, UK Central North Sea, Geophysical Journal International, Vol: 227, Pages: 617-631, ISSN: 0956-540X

Magnetic minerals form or alter in the presence of hydrocarbons, making them a potential magnetic proxy for identifying hydrocarbon migration pathways. In this paper, we test this idea by magnetically measuring core samples from the Tay Fan in the Western Central Graben in the Central North Sea. In a companion paper, 3-D petroleum systems modelling has been carried out to forward model migration pathways within the Tay Fan. Rock magnetic experiments identified a range of magnetite, maghemite, iron sulphides, siderite, goethite and titanohematite, some of which are part of the background signal, and some due to the presence of hydrocarbons. Typical concentrations of the magnetic minerals were ∼10–200 ppm. Importantly, we have identified an increasing presence of authigenic iron sulphides (likely pyrite and greigite) along the identified lateral hydrocarbon migration pathway (east to west). This is likely caused by biodegradation resulting in the precipitation of iron sulphides, however, though less likely, it could alternatively be caused by mature oil generation, which subsequently travelled with the migrating oil to the traps in the west. These observations suggest mineral magnetic techniques could be a rapid alternative method for identifying the severity of biodegradation or oil maturity in core sample, which can then be used to calibrate petroleum systems models.

Journal article

Badejo S, Fraser A, Neumaier M, Muxworthy A, Perkins Jet al., 2021, 3D Petroleum Systems Modelling as an exploration tool in mature basins: A study from the Central North Sea, UK., Marine and Petroleum Geology, ISSN: 0264-8172

Journal article

Hu P, Oda H, Zhao X, Harrison R, Heslop D, Sato T, Muxworthy AR, Roberts APet al., 2021, Assessment of magnetic techniques for understanding complex mixtures of magnetite and hematite: the Inuyama red chert, Journal of Geophysics Research - Solid Earth

Journal article

Abdulkarim M, Muxworthy A, Fraser A, Neumaier Met al., 2021, PRECIPITATION OF SIDERITE IN HYDROCARBON ENVIRONMENT, Pages: 2417-2421

Migration of hydrocarbons in the subsurface has been shown to create an environment that promotes the precipitation and/or alteration of magnetic minerals. For example, iron oxides and iron sulphides have been shown to precipitate due to the reducing conditions created by hydrocarbon migration. Siderite, a paramagnetic mineral with Neel temperature of 37K has been variously identified in hydrocarbon environment and has also been suggested to be an authigenic product of hydrocarbon migration. However, it is commonly found in sedimentary settings. Here we show via experimental studies that siderite is precipitated due to hydrocarbon migrations and suggested the mechanism responsible for this process. Magnetic minerals precipitation along migration pathways suggests the creation of a magnetic fingerprint that if thoroughly understood can be applied to oil and gas exploration.

Conference paper

Chang L, Hong H, Bai F, Wang S, Pei Z, Paterson GA, Heslop D, Roberts AP, Huang B, Tauxe L, Muxworthy Aet al., 2020, Detrital remanent magnetization of single-crystal silicates with magnetic inclusions: constraints from deposition experiments, Geophysical Journal International, ISSN: 0956-540X

Journal article

Zhang Y, Muxworthy A, Jia D, Zhang Y, Chen Z, Wang M, Zhigang Let al., 2020, Fluid migration and widespread remagnetization in the Dabashan fold and thrust belt, China, Journal of Geophysical Research. Solid Earth, Vol: 125, ISSN: 2169-9356

To better understand the fluid migration in orogenic zones and associated chemical remagnetization, we have conducted a detailed magnetic, petrographic, and strontium isotope study in an important orogenic belt of China, the Jurassic Dabashan fold and thrust belt. This belt formed by the continued collision of the North and South China blocks after the Late Triassic closure of the Paleo‐Tethys Ocean. Samples were collected in a variety of rock units of Ediacaran to Permian age, in both the thrust and the fold belts. Paleomagnetic analysis indicates that all the samples were remagnetized and carry a Middle‐Late Jurassic paleo‐direction. Rock magnetic data and scanning electron microscopy observations found that the proposed remagnetization is carried by framboidal magnetite, which likely formed by the replacement of pyrite. The pervasive nature of the chemical remagnetization in these units and belts and its temporal and spatial association with the orogeny suggest that it resulted from the alteration of orogeny‐induced fluids. Sr‐isotopic analysis of the units that are thought to be remagnetized suggests that the sediments in the thrust belt were altered by externally derived evolved fluids, whereas the Permian samples in the fold belt were altered by internal pore fluid mixing during the orogeny. Together with the lithological and structural features, we conclude that the external orogenic fluids migrated preferentially along thrust faults and unconformities but were blocked by layers of low‐permeability gypsum. Our results help to constrain the origin of widespread remagnetization in South China.

Journal article

Heslop D, Roberts AP, Oda H, Zhao X, Harrison RJ, Muxworthy AR, Hu P, Sato Tet al., 2020, An automatic model selection‐based machine learning framework to estimate FORC distributions, Journal of Geophysical Research: Solid Earth, Vol: 125, Pages: 1-16, ISSN: 2169-9313

First‐order reversal curve (FORC) distributions are a powerful diagnostic tool for characterizing and quantifying magnetization processes in fine magnetic particle systems. Estimation of FORC distributions requires the computation of the second‐order mixed derivative of noisy magnetic hysteresis data. This operation amplifies measurement noise, and for weakly magnetic systems, it can compromise estimation of a FORC distribution. Previous processing schemes, which are based typically on local polynomial regression, have been developed to smooth FORC data to suppress detrimental noise. Importantly, the smoothed FORC distribution needs to be consistent with the measurement data from which it was estimated. This can be a challenging task even for expert users, who must adjust subjectively parameters that define the form and extent of smoothing until a “satisfactory” FORC distribution is obtained. For nonexpert users, estimation of FORC distributions using inappropriate smoothing parameters can produce distorted results corrupted by processing artifacts, which can lead to spurious inferences concerning the magnetic system under investigation. We have developed a statistical machine learning framework based on a probabilistic model comparison to guide the estimation of FORC distributions. An intuitive approach is presented that reveals regions of a FORC distribution that may have been smoothed inappropriately. An associated metric can also be used to compare data preparation and local regression schemes to assess their suitability for processing a given FORC data set. Ultimately, our approach selects FORC smoothing parameters in a probabilistic fashion, which automates the derivative estimation process regardless of user expertise.

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

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