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  • Journal article
    Wilkinson JJ, Vowles K, Muxworthy A, Mac Niocaill Cet al., 2017,

    Regional remagnetization of Irish Carboniferous carbonates dates Variscan orogenesis, not Zn-Pb mineralization

    , Geology, Vol: 45, Pages: 747-750, ISSN: 1943-2682

    Paleomagnetic methods have been used in economic geology to date mineralization in sediment-hosted ore deposits and thereby help to develop ore deposit models and understand the geodynamic settings in which mineralization can occur. However, paleomagnetic ages are sometimes inconsistent with other geochronological techniques and with geological observations. Here we test the veracity of paleomagnetic ages for sediment-hosted ores through a study of the Irish Midlands ore field. We find that unaltered rocks distal to mineralization that are of equivalent age to the ore host sequence have comparable characteristic remanent magnetic directions to those previously derived from the ores. This indicates that remagnetization of the rocks was probably independent of the ore-forming process. Comparison with the apparent polar wander path for Europe suggests an age of ca. 310 Ma for this event, consistent with the timing of the Variscan orogeny. Fold test results support this, indicating the signal was acquired after tilting and/or folding of the host rocks. Petrology and magnetic data suggest that nanometric magnetite particles are the remanence carrier. Based on independent geochronological and geological constraints, we conclude that mineralization formed in Ireland in the early Carboniferous coincident with basin development and that paleomagnetic dates were reset during the later orogenic overprint. Caution is therefore warranted in the interpretation of paleomagnetic dates for ore systems, and geodynamic models for mineral systems based on these may be erroneous.

  • Journal article
    Hofman J, Maher BA, Muxworthy AR, Wuyts K, Castanheiro A, Samson Ret al., 2017,

    Biomagnetic monitoring of atmospheric pollution: a review of magnetic signatures from biological sensors

    , Environmental Science & Technology, Vol: 51, Pages: 6648-6664, ISSN: 0013-936X

    Biomagnetic monitoring of atmospheric pollution is a growing application in the field of environmental magnetism. Particulate matter (PM) in atmospheric pollution contains readily measurable concentrations of magnetic minerals. Biological surfaces, exposed to atmospheric pollution, accumulate magnetic particles over time, providing a record of location-specific, time-integrated air quality information. This review summarizes current knowledge of biological material (“sensors”) used for biomagnetic monitoring purposes. Our work addresses the following: the range of magnetic properties reported for lichens, mosses, leaves, bark, trunk wood, insects, crustaceans, mammal and human tissues; their associations with atmospheric pollutant species (PM, NOx, trace elements, PAHs); the pros and cons of biomagnetic monitoring of atmospheric pollution; current challenges for large-scale implementation of biomagnetic monitoring; and future perspectives. A summary table is presented, with the aim of aiding researchers and policy makers in selecting the most suitable biological sensor for their intended biomagnetic monitoring purpose.

  • Journal article
    Berndt T, Paterson GA, Cao C, Muxworthy ARet al., 2017,

    Experimental test of the heating and cooling rate effect on blocking temperatures

    , Geophysical Journal International, Vol: 210, Pages: 255-269, ISSN: 1365-246X

    The cooling rates at which rocks acquire thermoremanent magnetizations (TRMs), affect their unblocking temperatures in thermal demagnetization experiments; similarly the heating rates at which the thermal demagnetization experiments are done also affect the unblocking temperature. We have tested the effects of variable cooling and heating rates on the unblocking temperatures of two natural non-interacting, magnetically uniform (single-domain, SD) (titano)magnetite samples and a synthetic SD magnetoferritin sample. While previous studies have only considered unblocking temperatures for stepwise thermal demagnetization data (i.e. the room-temperature magnetization after incremental heating), in this work we derive an expression for continuous thermal demagnetization of both TRMs and viscous remanent magnetizations (VRMs) and relate the heating rate to an effective equivalent hold time of a stepwise thermal demagnetization experiment. Through our analysis we reach four main conclusions: First, the theoretical expressions for the heating/cooling rate effect do not accurately predict experimentally observed blocking temperatures. Empirically, the relation can be modified incorporating a factor that amplifies both the temperature and the heating rate dependence of the heating/cooling rate effect. Using these correction factors, Pullaiah nomograms can accurately predict blocking temperatures of both TRMs and VRMs for continuous heating/cooling. Second, demagnetization temperatures are approximately predicted by published ‘Pullaiah nomograms’, but blocking occurs gradually over temperature intervals of 5–40 K. Third, the theoretically predicted temperatures correspond to ∼54–82 per cent blocking, depending on the sample. Fourth, the blocking temperatures can be used to obtain estimates of the atomic attempt time τ0, which were found to be 3 × 10−10 s for large grained (titano)magnetite, 1 × 10−13&t

  • Journal article
    Berndt T, Ramalho RS, Valdez-Grijalva MA, Muxworthy ARet al., 2017,

    Paleomagnetic field reconstruction from mixtures of titanomagnetites

    , Earth and Planetary Science Letters, Vol: 465, Pages: 70-81, ISSN: 0012-821X

    Stepwise thermal demagnetization and alternating field (AF) demagnetization are commonly used in paleomagnetic studies to isolate remanent magnetic components of different origins. The magnetically hardest, i.e. highest unblocking temperature/peak field component is often interpreted as the primary magnetization and magnetically softer components as subsequent remagnetizations due to geological events posterior to the formation of the rock, such as reheating or formation of new magnetic minerals. The correct interpretation of the sequence of the geological events such as tectonic rotations from paleomagnetic data often relies on correctly attributing the observed magnetic directions to the remanence carriers and acquisition mechanisms. Using a numerical model to simulate remanence acquisition and stepwise thermal and AF demagnetization experiments, we show that the presence of mixtures of different magnetic minerals, such as magnetite and titanomagnetites of varying titanium-content can have very significant effects on Zijderveld plots. In thermal demagnetization experiments a spurious third component at intermediate temperatures or a continuous curvature may arise from an overlap of the primary remanence with a subsequent thermal or viscous remagnetization carried by small-grained iron-rich magnetite and large-grained titanium-rich titanomagnetite. AF demagnetization plots of magnetic mixtures are even more complex: primary and secondary remanences carried by different minerals may appear as either three or four components in Zijderveld plots. During alternating field demagnetization the highest coercivity component is not necessarily equivalent to the primary remanence and does not necessarily correspond to the highest temperature component in an analogous thermal demagnetization experiment, i.e., the primary remanence direction cannot be recovered. The effects are shown to be due to the different responsiveness of magnetite and titanomagnetites towards viscous or

  • Journal article
    Berndt T, Muxworthy AR, 2017,

    Dating Icelandic glacial floods using a new viscous remanent magnetization protocol

    , Geology, Vol: 45, Pages: 339-342, ISSN: 1943-2682

    A new protocol using the viscous remanent magnetization (VRM) of boulders to date cataclysmic geological events such as tsunamis, glacial floods, and landslides is presented and its performance is assessed against two jökulhlaups (glacial floods) of known age in Iceland. High-intensity jökulhlaups have the ability to break off large boulders from bedrock and emplace and rotate them. These rocks originally carried a remanent magnetization parallel to the geomagnetic field during their formation. After being rotated by the flood, they acquire a VRM parallel with Earth's magnetic field. In continuous thermal demagnetization experiments the unblocking temperature of the VRM can be determined, and subsequent rock magnetic VRM acquisition experiments can be used to establish a relationship between the unblocking temperature and the acquisition time, from which the time since the flood can be determined. The protocol was tested on 44 boulders from 2 historical jökulhlaups in Iceland and found to yield good order-of-magnitude estimates: 72 yr (confidence limits 11–360 yr) versus known 155 yr at the Sólheimajökull jökulhlaup and 290 yr (confidence limits 80–2300 yr) versus known 288 yr for the Kotarjökull jökulhlaup. The method can therefore be a valuable tool for future dating of cataclysmic events.

  • Conference paper
    Shah J, Muxworthy AR, Almeida TP, Kovacs A, Russell SS, Genge M, Dunin-Borkowski Ret al., 2017,

    Dusty olivine: our oldest record of rock magnetism? (invited)

    , Magnetic Interactions, Edinburgh
  • Book chapter
    Maidment SCR, Balikova D, Muxworthy AR, 2017,

    Magnetostratigraphy of the Upper Jurassic Morrison Formation at Dinosaur National Monument, Utah, and Prospects for Using Magnetostratigraphy as a Correlative Tool in the Morrison Formation

  • Conference paper
    Williams W, Nagy L, Fabian K, Ó Conbhuí P, Muxworthy AR, Almeida Tet al., 2016,

    The Hunting of the ‘Psark' - 40 Years On (invited)

    , AGU Fall 2016
  • Conference paper
    Nagy L, Williams W, Fabian K, Muxworthy AR, Almeida TP, Ó Conbhuí Pet al., 2016,

    Thermomagnetic Stability in Pseudo-Single Domain Grains

    , AGU Fall 2016
  • Conference paper
    Muxworthy AR, 2016,

    Recovering ancient magnetic field intensities from rocks and meteorites using FORC Measurements (invited)

    , 2nd FORC Workshop
  • Conference paper
    Maidment S, Muxworthy AR, 2016,

    A chronostratigraphic framework for the Morrison Formation and the latitudinal biodiversity gradient in Morrison dinosaurs (poster)

    , 76th Annual Meeting Society of Vertebrate Paleontology
  • Journal article
    Døssing A, Muxworthy AR, Supakulopas R, Riishuus MS, Mac Niocaill Cet al., 2016,

    High northern geomagnetic field behavior and new constraints on the Gilsá event: Paleomagnetic and 40Ar/39Ar results of ∼0.5–3.1Ma basalts from Jökuldalur, Iceland

    , Earth and Planetary Science Letters, Vol: 456, Pages: 98-111, ISSN: 0012-821X

    Recent paleomagnetic results of extrusive rocks from high southern latitudes (>60°S) and high northern latitudes (>60°N) have been suggested to reflect a hemispheric asymmetry of the geomagnetic field on time-scales of 105 to 106 yrs, with higher and more stable fields in the north. This interpretation, however, is based on only a few modern-standard paleodirectional data sets and on high northern stable field paleointensity data of rocks that are mainly younger than 100 kyr. The sparsity of modern-standard data questions the validity (and age range) of this potential geomagnetic asymmetry. In 2013 and 2014, we sampled basaltic lava flows in Jökuldalur, north-eastern Iceland, to obtain high-standard paleodirectional and paleointensity data at relatively high-northern latitudes (65.2°N). On average, we sampled >15 cores per site at 51 sites of predominantly Matuyama age. Complete demagnetization was carried out on all samples using AF or thermal demagnetization. We present 45 distinct paleomagnetic directions based on overall N>10 ChRMs per site and α95<3.5°. We obtain a mean direction of D=355.7°, I=76.3°, and α95=3.2 for N=45 sites that is not significantly different from a GAD field. The resulting 45 VGPs distribute around the North Pole, and the global mean paleomagnetic pole (View the MathML source, View the MathML source) is coincident with the North Pole within the α 95 confidence limit. We calculate a VGP dispersion View the MathML source and an average inclination anomaly View the MathML source for our 38 Matuyama age data. The dispersion SB overall supports the interpretation of a dependence of SB on latitude during the Matuyama, while the negligible ΔI suggests little deviation from a GAD field. Based on relatively strict cut-off criteria we also present six new field strength estimates from the time interval ∼1.2–1.83 Ma, thus filling a large data gap of the high-northern sta

  • Conference paper
    Penny C, Muxworthy AR, Fabian K, 2016,

    The Curie temperature of magnetite nanoparticles (poster)

    , EMRS Fall 2016
  • Conference paper
    Muxworthy AR, Almeida TP, Williams W, Dunin-Borkowski Ret al., 2016,

    Imaging magnetic stability of PSD magnetite

    , 15th Castle Meeting
  • Conference paper
    Shah J, Bates H, Muxworthy AR, Russell SS, Genge MJet al., 2016,

    A micro-CT conglomerate test (poster)

    , 15th Castle Meeting

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