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Journal articleEvans ME, Muxworthy AR, 2018,
A re-appraisal of the proposed rapid Matuyama–Brunhes geomagnetic reversal in the Sulmona Basin, Italy
, Geophysical Journal International, Vol: 213, Pages: 1744-1750, ISSN: 0956-540XAn extremely sharp magnetic reversal observed in lacustrine sediments in central Italy has been interpreted as a record of the Matuyama-Brunhes geomagnetic polarity reversal that may represent less than a decade (Sagnotti et al., 2014. 2016). Here, we report new results from the same Sulmona Basin outcrop that question this interpretation. In particular, we find evidence of reversed (Matuyama) directions well above the proposed Matuyama-Brunhes Boundary (MBB). Coercivity spectra of anhysteretic remanent magnetization (ARM) imply a 3-component magnetic mineralogy: low-, intermediate-, and high-coercivity. The low-coercivity component is found in all but one of the samples and carries a strong modern overprint seen throughout the section. The high-coercivity component is dominated by volcanic material which is prone to remagnetization. Since it is much more magnetic than the surrounding lacustrine sediments, it may influence the remanence signal even when present at very low concentrations. The intermediate-coercivity component is the main carrier of any true primary remanence, but whether or not this can be isolated depends on the blocking-temperature and coercivity spectra of individual samples, and on the demagnetization method used. The complexity of the magnetization, the reversed zones above the proposed MBB, and the normal zones that Sagnotti and 2 colleagues found below it, lead to the conclusion that this section does not carry a reliable high-resolution record of the geomagnetic field. Thus, we feel that inferences about the stratigraphic position and duration of the MBB are premature.
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Journal articleRoberts AP, Almeida TP, Church NS, et al., 2017,
Resolving the origin of pseudo-single domain magnetic behavior
, Journal of Geophysical Research, Vol: 122, Pages: 9534-9558, ISSN: 0148-0227The term “pseudo-single domain” (PSD) has been used to describe the transitional state in rockmagnetism that spans the particle size range between the single domain (SD) and multidomain (MD) states.The particle size range for the stable SD state in the most commonly occurring terrestrial magneticmineral, magnetite, is so narrow (~20–75 nm) that it is widely considered that much of the paleomagneticrecord of interest is carried by PSD rather than stable SD particles. The PSD concept has, thus, become thedominant explanation for the magnetization associated with a major fraction of particles that recordpaleomagnetic signals throughout geological time. In this paper, we argue that in contrast to the SD and MDstates, the term PSD does not describe the relevant physical processes, which have been documentedextensively using three-dimensional micromagnetic modeling and by parallel research in material scienceand solid-state physics. We also argue that features attributed to PSD behavior can be explained bynucleation of a single magnetic vortex immediately above the maximum stable SD transition size. Withincreasing particle size, multiple vortices, antivortices, and domain walls can nucleate, which producevariable cancellation of magnetic moments and a gradual transition into the MD state. Thus, while the termPSD describes a well-known transitional state, it fails to describe adequately the physics of the relevantprocesses. We recommend that use of this term should be discontinued in favor of “vortex state,” whichspans a range of behaviors associated with magnetic vortices.
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Journal articleValdez-Grijalva MA, Nagy L, Muxworthy AR, et al., 2017,
The magnetic structure and palaeomagnetic recording fidelity of sub-micron greigite (Fe3S4)
, Earth and Planetary Science Letters, Vol: 483, Pages: 76-89, ISSN: 0012-821XWe present the results of a finite-element micromagnetic model of View the MathML source to View the MathML source greigite (Fe3S4) grains with a variety of equant morphologies. This grain size range covers the magnetic single-domain (SD) to pseudo single-domain (PSD) transition, and possibly also the PSD to multi-domain (MD) transition. The SD–PSD threshold d0 is determined to be View the MathML source depending on grain shape. The nudged elastic-band method was used to determine the room temperature energy barriers between stable states and thus the blocking volumes. It is found that, in the absence of interparticle magnetostatic interactions, the magnetisation of equant SD greigite is not stable on a geological scale and only PSD grains View the MathML source can be expected to carry a stable magnetisation over billion-year timescales, i.e., all non-interacting SD particles are essentially superparamagnetic. We further identify a mechanism for the PSD to multi-domain (MD) transition, which is of a continuous nature from PSD nucleation up to View the MathML source, when structures typical of MD behaviour like closure domains begin to form.
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Journal articlePaterson GA, Muxworthy AR, Yamamoto Y, et al., 2017,
Bulk magnetic domain stability controls paleointensity fidelity
, Proceedings of the National Academy of Sciences, Vol: 114, Pages: 13120-13125, ISSN: 0027-8424Nonideal, nonsingle-domain magnetic grains are ubiquitous in rocks; however, they can have a detrimental impact on the fidelity of paleomagnetic records—in particular the determination of ancient magnetic field strength (paleointensity), a key means of understanding the evolution of the earliest geodynamo and the formation of the solar system. As a consequence, great effort has been expended to link rock magnetic behavior to paleointensity results, but with little quantitative success. Using the most comprehensive rock magnetic and paleointensity data compilations, we quantify a stability trend in hysteresis data that characterizes the bulk domain stability (BDS) of the magnetic carriers in a paleomagnetic specimen. This trend is evident in both geological and archeological materials that are typically used to obtain paleointensity data and is therefore pervasive throughout most paleomagnetic studies. Comparing this trend to paleointensity data from both laboratory and historical experiments reveals a quantitative relationship between BDS and paleointensity behavior. Specimens that have lower BDS values display higher curvature on the paleointensity analysis plot, which leads to more inaccurate results. In-field quantification of BDS therefore reflects low-field bulk remanence stability. Rapid hysteresis measurements can be used to provide a powerful quantitative method for preselecting paleointensity specimens and postanalyzing previous studies, further improving our ability to select high-fidelity recordings of ancient magnetic fields. BDS analyses will enhance our ability to understand the evolution of the geodynamo and can help in understanding many fundamental Earth and planetary science questions that remain shrouded in controversy.
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Journal articleAlmeida TP, Muxworthy AR, Kovacs A, et al., 2017,
Observation of thermally-induced magnetic relaxation in a magnetite grain using off-axis electron holography
, Journal of Physics : Conference Series, Vol: 902, ISSN: 1742-6588A synthetic basalt comprising magnetic Fe3O4 grains (~ 50 nm to ~ 500 nm in diameter) is investigated using a range of complementary nano-characterisation techniques. Off-axis electron holography combined with in situ heating allowed for the visualisation of the thermally-induced magnetic relaxation of an Fe3O4 grain (~ 300 nm) from an irregular domain state into a vortex state at 550˚C, just below its Curie temperature, with the magnetic intensity of the vortex increasing on cooling.
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Journal articleMuxworthy AR, 2017,
Considerations for latitudinal time-averaged-field palaeointensity analysis of the last five million years
, Frontiers in Earth Science, ISSN: 2296-6463 -
Journal articleDi Chiara A, Muxworthy AR, Trindade RIF, et al., 2017,
Paleoproterozoic Geomagnetic Field Strength From the Avanavero Mafic Sills, Amazonian Craton, Brazil
, Geochemistry, Geophysics, Geosystems, Vol: 18, Pages: 3891-3903, ISSN: 1525-2027A recent hypothesis has suggested that Earth's inner core nucleated during the Mesoproterozoic, as evidenced by a rapid increase in the paleointensity (ancient geomagnetic field intensity) record; however, paleointensity data during the Paleoproterozoic and Mesoproterozoic period are limited. To address this problem, we have determined paleointensity from samples from three Paleoproterozoic Avanavero mafic sills (Amazonian Craton, Brazil): Cotingo, 1782 Ma, Puiuà 1788, and Pedra Preta, 1795 Ma. We adopted a multi-protocol approach for paleointensity estimates combining Thellier-type IZZI and LTD-IZZI methods, and the non-heating Preisach protocol. We obtained an average VDM value of 1.3 ± 0.7 × 1022Am2 (Cotingo) of 2.0 ± 0.4 × 1022Am2 (Puiuà) and 6 ± 4 × 1022Am2 (Pedra Preta); it is argued that the Cotingo estimate is the most robust. Our results are the first data from the upper Paleoproterozoic for South America and are comparable to data available from other regions and similar periods. The new data do not invalidate the hypothesis of that Earth's inner core nucleated during the Mesoproterozoic.
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Journal articleNagy L, Williams W, Muxworthy AR, et al., 2017,
Stability of equidimensional pseudo-single domain magnetite over billion year time-scales
, Proceedings of the National Academy of Sciences of the United States of America, Vol: 114, Pages: 10356-10360, ISSN: 1091-6490Interpretations of paleomagnetic observations assume that naturally occurring magnetic particles can retain their primary magnetic recording over billions of years. The ability to retain a magnetic recording is inferred from laboratory measurements, where heating causes demagnetization on the order of seconds. The theoretical basis for this inference comes from previous models that assume only the existence of small, uniformly magnetized particles, whereas the carriers of paleomagnetic signals in rocks are usually larger, nonuniformly magnetized particles, for which there is no empirically complete, thermally activated model. This study has developed a thermally activated numerical micromagnetic model that can quantitatively determine the energy barriers between stable states in nonuniform magnetic particles on geological timescales. We examine in detail the thermal stability characteristics of equidimensional cuboctahedral magnetite and find that, contrary to previously published theories, such nonuniformly magnetized particles provide greater magnetic stability than their uniformly magnetized counterparts. Hence, nonuniformly magnetized grains, which are commonly the main remanence carrier in meteorites and rocks, can record and retain high-fidelity magnetic recordings over billions of years.
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Journal articleShah J, Bates H, Muxworthy AR, et al., 2017,
Long-lived magnetism in chondrite parent bodies
, Earth and Planetary Science Letters, Vol: 475, Pages: 106-118, ISSN: 1385-013XWe present evidence for both early- and late-stage magnetic activity on the CV and L/LL parent bodies respectively from chondrules in Vigarano and Bjurböle. Using micro-CT scans to re-orientate chondrules to their in-situ positions, we present a new micron-scale protocol for the paleomagnetic conglomerate test. The paleomagnetic conglomerate test determines at 95% confidence, whether clasts within a conglomerate were magnetized before or after agglomeration, i.e., for a chondritic meteorite whether the chondrules carry a pre- or post-accretionary remanent magnetization. We found both meteorites passed the conglomerate test, i.e., the chondrules had randomly orientated magnetizations. Vigarano's heterogeneous magnetization is likely of shock origin, due to the 10 to 20 GPa impacts that brecciated its precursor material on the parent body and transported it to re-accrete as the Vigarano breccia. The magnetization was likely acquired during the break-up of the original body, indicating a CV parent body dynamo was active ∼9 Ma after Solar System formation. Bjurböle's magnetization is due to tetrataenite, which transformed from taenite as the parent body cooled to below 320 °C, when an ambient magnetic field imparted a remanence. We argue either the high intrinsic anisotropy of tetrataenite or brecciation on the parent body manifests as a randomly orientated distribution, and a L/LL parent body dynamo must have been active at least 80 to 140 Ma after peak metamorphism. Primitive chondrites did not originate from entirely primitive, never molten and/or differentiated parent bodies. Primitive chondrite parent bodies consisted of a differentiated interior sustaining a long-lived magnetic dynamo, encrusted by a layer of incrementally accreted primitive meteoritic material. The different ages of carbonaceous and ordinary chondrite parent bodies might indicate a general difference between carbonaceous and ordinary chondrite parent bodies, and/or format
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Journal articleMuxworthy AR, Bland PA, Davison TM, et al., 2017,
Evidence for an impact-induced magnetic fabric in Allende, and exogenous alternatives to the core dynamo theory for Allende magnetization
, Meteoritics & Planetary Science, Vol: 52, Pages: 2132-2146, ISSN: 1086-9379We conducted a paleomagnetic study of the matrix of Allende CV3 chondritic meteorite, isolating the matrix’s primary remanent magnetization, measuring its magnetic fabric and estimating the ancient magnetic field intensity. A strong planar magnetic fabric was identified; the remanent magnetization of the matrix was aligned within this plane, suggesting a mechanism relating the magnetic fabric and remanence. The intensity of the matrix’s remanent magnetization was found to be consistent and low (~6 μT). The primary magnetic mineral was found to be pyrrhotite. Given the thermal history of Allende, we conclude that the remanent magnetization formed during or after an impact event. Recent mesoscale impact mode ling, where chondrules and matrix are resolved, has shown that low-velocity collisions can generate significant matrix temperatures, as pore-space compaction attenuates shock energy and dramatically increases the amount of heating. Non-porous chondrules are unaffected, and act as heat-sinks, so matrix temperature excursions are brief. We extend this work to model Allende, and show that a 1km/s planar impact generates bulk porosity, matrix porosity, and fabric in our target that match the observed values. Bimodal mixtures of a highly porous matrix and nominally zero-porosity chondrules, make chondrites uniquely capable of recording transient or unstable fields. Targets that have uniform porosity, e.g., terrestrial impact craters, will not record transient or unstable fields. Rather than a core dynamo, it is therefore possible that the origin of the magnetic field in Allende was the impact itself, or a nebula field recorded during transient impact heating.
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Journal articleWilkinson JJ, Vowles K, Muxworthy A, et al., 2017,
Regional remagnetization of Irish Carboniferous carbonates dates Variscan orogenesis, not Zn-Pb mineralization
, Geology, Vol: 45, Pages: 747-750, ISSN: 1943-2682Paleomagnetic 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.
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Journal articleHofman J, Maher BA, Muxworthy AR, et al., 2017,
Biomagnetic monitoring of atmospheric pollution: a review of magnetic signatures from biological sensors
, Environmental Science & Technology, Vol: 51, Pages: 6648-6664, ISSN: 0013-936XBiomagnetic 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.
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Journal articleBerndt T, Paterson GA, Cao C, et al., 2017,
Experimental test of the heating and cooling rate effect on blocking temperatures
, Geophysical Journal International, Vol: 210, Pages: 255-269, ISSN: 1365-246XThe 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
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Journal articleBerndt T, Ramalho RS, Valdez-Grijalva MA, et al., 2017,
Paleomagnetic field reconstruction from mixtures of titanomagnetites
, Earth and Planetary Science Letters, Vol: 465, Pages: 70-81, ISSN: 0012-821XStepwise 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
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Journal articleBerndt T, Muxworthy AR, 2017,
Dating Icelandic glacial floods using a new viscous remanent magnetization protocol
, Geology, Vol: 45, Pages: 339-342, ISSN: 1943-2682A 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.
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