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Background: Studies investigating changes in blood metal ion levels during the second decade of the implant lifetime in MoM hip resurfacing patients are scarce. Methods: Patients implanted with either Birmingham Hip Resurfacing (BHR) or Articular Surface Replacement (ASR) hip resurfacings with >10 years follow-up and repeated blood metal ion measurements were identified at 2 large specialist European arthroplasty centres. After excluding patients with initial metal ion levels >7 ppb, the proportion of patients with an increase in blood metal ion levels above previously validated implant-specific thresholds (cobalt 2.15 ppb for unilateral implants, cobalt or chromium 5.5 ppb for bilateral) was assessed. Results: We included 2743 blood metal ion measurements from 457 BHR patients (555 hips) and 216 ASR patients (263 hips). Of patients with initial metal ion levels below implant specific thresholds, increases in cobalt or chromium level, respectively, to above these thresholds during the second decade were seen as follows: unilateral BHR (cobalt = 15.6%), unilateral ASR (cobalt = 13.8%), bilateral BHR (cobalt = 8.2%, chromium = 11.8%), bilateral ASR (cobalt = 8.5%, chromium = 4.3%). Measurement-to-measurement changes exceeding +2.15 ppb or +5.5 ppb were, however, uncommon during the second decade. Subgroup results with small diameter (Conclusions: We recommend less frequent blood metal ion measurements are needed (every 3 to 5 years) for hip resurfacing patients if initial values were below 7ppb.

SUMMARY Thermoremanent magnetization (TRM), the primary magnetic memory of igneous rocks, depends for its stability through geologic time on mineral carriers with high coercivities and high unblocking temperatures. The palaeomagnetic record of past magnetic field directions and intensities is the key to unraveling Earth's tectonic history. Yet we still do not fully understand how the familiar mineral magnetite, in the micrometer grain size range typically responsible for stable TRM, acquires and holds its signal. Direct indicators of magnetite remanence-carrying capacity and coercivity at high temperature T are saturation remanence relative to saturation magnetization Mrs/Ms and coercive force Hc. This study is the first to measure the variation of these hysteresis properties for magnetite, from room temperature to the Curie point, across the entire size range from 25 nm to 135 µm, covering superparamagnetic, single-domain, vortex, pseudo-single-domain and multidomain magnetic behaviour. The paper focuses on: (1) Hc(T) and Mrs(T) observations and their reproducibility; (2) mathematical relationships of Hc(T) and Mrs(T) to Ms(T), used in modelling TRM and for unbiased comparisons of thermal variations; (3) the shapes of magnetite grains and the number of domains they contain, revealed by demagnetizing factors N = Hc/Mrs and (4) the grain size dependences of Hc and Mrs at ordinary and elevated T, delineating domain structure changes and mechanisms of coercivity.

SUMMARY The magnetic properties of iron-bearing minerals at above-ambient temperatures control their magnetic expression at depth in the Earth and other planets, as well as the permanent memory they retain as thermoremanence or thermochemical remanence when brought to the surface and cooled. This paper reports magnetic hysteresis parameters measured at temperatures up to the Curie point TC for natural pyrrhotite and hematite and for suites of sized magnetites, both natural and synthesized. Domain structure changes can be inferred from the ratio of saturation remanence Mrs to saturation magnetization Ms. In almost all magnetites and pyrrhotites studied, Mrs decreases more rapidly with increasing measurement temperature T than Ms, indicating thermal unblocking or vortex development in single-domain grains and addition or remobilization of domain walls at high T in multidomain grains. During cooling of a rock, iron minerals might then denucleate domains or vortices. Coercive force Hc, a measure of stability against changing magnetic fields, also decreases with increasing measurement T, usually at a rate similar to that of Mrs, but often retains a finite value near the Curie point.

We report saturation magnetization, Ms, saturation remanence, Mrs, coercive force, Hc, and remanence coercivity, Hcr, as a function of grain size, d, and temperature, T, for 0.6–135 μm magnetites. Five annealed and four unannealed samples were measured at 5–10 K intervals from 300 to 20 K. Mrs and Hc increase by factors of 1.5–4 in cooling through the Verwey transition (TV ≈ 120 K) and by smaller amounts around 50 K. Hysteresis properties change continuously over ≈20 K below TV or for annealed 0.6, 3, and 6 μm grains, within ≈10 K below TV. Hc(d) changes for annealed magnetites from ∼d–0.5 at 300 K to ∼d–0.6–d–0.7 at 120–130 K to ∼d–0.3 at 80–100 K. Day plots of Mrs(T)/Ms(T) versus Hcr(T)/Hc(T) indicate major domain structure changes with T, e.g., 6 μm grains change from large pseudo-single-domain (PSD) at 300 K to multidomain (MD) just above TV and return to PSD below TV, evolving to higher Mrs and Hc down to 20 K. Hysteresis loops change from normal at 300 K to slightly constricted near TV to severely constricted below 50 K. We interpret these results in the light of electron microscopic observations by Kasama et al. (2010 , 2012) . Hardening of magnetic hysteresis below TV and the evolution from MD to PSD, and even to single-domain in the finest grains, results from subdivision of grains by monoclinic twinning, reduced magnetic domain sizes in monoclinic magnetite, and confinement of magnetic domains within twin domains. Constricted hysteresis loops indicate coexisting magnetically hard and soft phases, initially growing monoclinic regions and residual cubic magnetite.

Perching in unmanned aerial vehicles is appealing for reconnaissance, monitoring, communications, and charging. This paper focuses on modeling, simulation, and control of bioinspired perching in unmanned aerial vehicles on cylindrical objects, which will be used for future planning and control research. A modular approach is taken where the quadrotor, legs, feet, and toes are modeled separately and then integrated to form a complete simulation system. New models of these components consider kinematics and dynamics of each element and their coupling through tendons that provide actuation. The integrated model is assembled to simulate a physical prototype and then validated based upon physical experiments to provide calibration. Simulation results evaluate the validated model performing perching with different gripper-perch alignments. The simulation environment developed in this research provides a foundation to research control approaches for use with the discussed passive perching mechanism. The simulation was validated to capture the dynamics of the real perching mechanism. This platform will be used in future work to develop a control approach that will be implemented in a quadrotor system to land and take-off from a perch in a reliable manner.

We investigated whether blood metal ions could effectively identify bilateral metal-on-metal total hip arthroplasty (THA) patients at risk of adverse reactions to metal debris (ARMD). Whole blood metal ions were sampled in 50 patients with bilateral 36mm Corail-Pinnacle THAs. Patients were divided into ARMD (n=10) and non-ARMD groups (n=40), with optimal ion thresholds for identifying ARMD determined using receiver operating characteristic analysis. Maximum cobalt or chromium produced the highest area under the curve (71.8%). The optimal ion threshold for distinguishing between patients with and without ARMD was 4.0μg/l (90.0%=sensitivity, 65.0%=specificity, 39.1%=positive predictive value, 96.3%=negative predictive value). Fixed regulatory authority thresholds missed more patients with ARMD (10%-12% missed) compared to our threshold (2% missed). Bilateral THA patients with blood metal ions below our threshold were at low-risk of ARMD. Compared to currently recommended fixed authority thresholds, our threshold appears preferable for managing patients with these particular implants.

In room-temperature hysteresis, 14 submicron hematites (0.12-0.45 µm) had large coercive forces Hc (150-350 mT), while 22 natural 1-5.5 mm hematite crystals had Hc = 0.8-23 mT (basal-plane measurements). Single-domain (SD) and multidomain (MD) hematites owe their high Hc mainly to magnetoelastic anisotropy, caused in fine particles by internal strains and in large crystals by defects like dislocations, with a smaller contribution by triaxial magnetocrystalline anisotropy. A strong correlation between Hc and the defect moment Md measured below hematite's Morin transition also favors magnetoelastic control. Saturation remanence/saturation magnetization ratios Mrs/Ms and coercivity ratios Hcr/Hc (Hcr is remanent coercive force) are distinctive: Mrs/Ms = 0.5-0.9, Hcr/Hc = 1.02-1.17 for MD hematites; Mrs/Ms = 0.5-0.7, Hcr/Hc = 1.45-1.62 for SD hematites. In high-temperature (20-690°C) hysteresis, Hc(T) ~ Ms(T) to a power 1.8-2.4 above 385°C. Magnetoelastic wall pinning by crystal defects is thus more likely than control by domain nucleation which depends on magnetocrystalline anisotropy. Our results compare well with existing Hc vs. crystal size d data. A suggested peak in Hc around 15 µm and a proposed slope change around 100 µm are both questionable. Using only near-saturation data, Hc varies continuously as d−0.61 from ≈0.1 µm to 2 mm. The SD threshold size d0 may be >15 µm but there is no strong evidence that d0 ≈100 µm. Direct domain observations are needed to settle the question. Augmented data sets for Hc and Mrs vs. d show that SD hematite is increasingly affected by thermal fluctuations below ≈0.3 µm and generally confirm a superparamagnetic threshold size ds of 0.025-0.03 µm.

John Tuzo Wilson coined the term “plate” in plate tectonics. He is famous for inventing transform boundaries, hot spot tracks, and the Wilson cycle of ocean birth, growth, and decline. Less well remembered is his work in the 1950s on tectonic and radiometric age provinces of the Canadian Shield, as part of which he fathered U/Pb geochronology in Canada. This work gave strong support to the notion of continental growth through accretion of successively younger terranes onto an ancient cratonic core. The present paper reviews how paleomagnetism can trace the motions of continents to test Wilson’s ideas. Continental accretion often involves deep burial of one of the colliding elements through subduction or crustal underplating; such was the case with the Grenville orogen and its subprovinces in their Proterozoic accretion onto the Laurentian craton. The resulting heating and metamorphism erases most pre-collisional magnetic information but adds something new: the possibility of following the post-metamorphic uplift and cooling history, in time and space. The time element is provided by a new form of isotopic geochronology, thermochronometry, which provides dates for specific minerals together with the temperatures at which they became closed to isotopic migration. U/Pb dating of sphene is one method used; another is the 40Ar/39Ar variant of K/Ar dating applied to hornblende, micas, and feldspars, which have a wide range of Ar closure temperatures. The two specific Grenville studies described deal with parallel uplift histories determined by 40Ar/39Ar dating and by magnetics for the accreted terranes of the Central Metasedimentary Belt in Ontario and with the paleomagnetic detection of the post-1240 Ma closing of a small ocean between the Elsevir terrane and Laurentia during the Grenvillian orogeny.

Flying robots capable of perch-and-stare are desirable for reconnaissance missions. Inspired by an adaptation that enables songbirds to sleep in trees without active muscle control, the research presented herein details the design for a passive mechanism that enables a rotorcraft to perch reminiscent of a bird perching on a tree branch. Perching is accomplished through the integration of a compliant, underactuated gripping foot and a collapsing leg mechanism that converts rotorcraft weight into tendon tension in order to passively actuate the foot. Analysis of mechanism behavior is presented, and stability tests were performed to characterize the ability of the system to reject disturbances. The results indicate that it is possible to passively perch a rotorcraft on multiple surfaces and support reasonable environmental disturbances. The analysis in this paper can enable passive perching design optimization in vertical take-off and landing systems.

We investigated whether blood metal ions could effectively identify patients with metal-on-metal hip implants with two common designs (Birmingham Hip Resurfacing [BHR] and Corail-Pinnacle) who were at risk of adverse reactions to metal debris.This single-center, prospective study involved 598 patients with unilateral hip implants (309 patients with the BHR implant and 289 patients with the Corail-Pinnacle implant) undergoing whole blood metal ion sampling at a mean time of 6.9 years. Patients were classified into two groups, one that had adverse reactions to metal debris (those who had to undergo revision for adverse reactions to metal debris or those with adverse reactions to metal debris on imaging; n = 46) and one that did not (n = 552). Three metal ion parameters (cobalt, chromium, and cobalt-chromium ratio) were compared between groups. Optimal metal ion thresholds for identifying patients with adverse reactions to metal debris were determined using receiver operating characteristic analysis.All ion parameters were significantly higher (p0.0001) in the patients who had adverse reactions to metal debris compared with those who did not. Cobalt maximized the area under the curve for patients with the BHR implant (90.5%) and those with the Corail-Pinnacle implant (79.6%). For patients with the BHR implant, the area under the curve for cobalt was significantly greater than that for the cobalt-chromium ratio (p = 0.0005), but it was not significantly greater than that for chromium (p = 0.8483). For the patients with the Corail-Pinnacle implant, the area under the curve for cobalt was significantly greater than that for chromium (p = 0.0004), but it was similar to that for the cobalt-chromium ratio (p = 0.8139). Optimal blood metal ion thresholds for identifying adverse reactions to metal debris varied between the two different implants. When using cobalt, the optimal threshold for identifying adverse reactions to metal debris was 2.15 μg/L for the BHR group and 3.57 μg/L for the Corail-Pinnacle group. These thresholds had good sensitivities (88.5% for the BHR group and 80.0% for the Corail-Pinnacle group) and specificities (84.5% for the BHR group and 76.2% for the Corail-Pinnacle group), high negative predictive values (98.8% for the BHR group and 98.1% for the Corail-Pinnacle group), and low positive predictive values (34.3% for the BHR group and 20.0% for the Corail-Pinnacle group). The authority thresholds proposed by the United States (3 μg/L and 10 μg/L) and the United Kingdom (7 μg/L) missed more patients with adverse reactions to metal debris at 2.0% to 4.7% (twelve to twenty-eight patients) compared with our implant-specific thresholds at 1.2% (seven patients missed).Patients who underwent metal-on-metal hip arthroplasty performed with unilateral BHR or Corail-Pinnacle implants and who had blood metal ions below our implant-specific thresholds were at low risk of adverse reactions to metal debris. These thresholds could be used to rationalize follow-up resources in asymptomatic patients. Analysis of cobalt alone is acceptable. Implant-specific thresholds were more effective than currently recommended fixed authority thresholds for identifying patients at risk of adverse reactions to metal debris requiring further investigation.Diagnostic Level II. See Instructions for Authors for a complete description of levels of evidence.

Charting the movement of tectonic plates or the evolution of a dynamo ultimately relies on the behavior of often imperfect mineral grains in Earth’s magnetic field.

Emile and Odette Thellier produced the first reliable determinations of paleointensity following an experimental protocol used earlier by Johann Koenigsberger. Although Koenigsberger did groundbreaking work on thermoremanent magnetization (TRM), it was the Thelliers who formulated the fundamental idea of partial TRMs as building blocks for TRM. In his 1938 doctoral thesis and a series of short notes, Emile Thellier minutely examined the data on TRM and partial TRM, ultimately establishing for bricks and other baked clays his laws of pTRM reciprocity, independence and additivity. In 1946 he speculated that blocking represents “…immobilization of elementary magnetic moments below a temperature Θ … Θ will vary at each point in the body, perhaps with the dimensions and the shape of the crystalline grains … One can thus explain thermoremanence by the progressive fixing, in the course of cooling, of moments which find themselves held fast when they pass through their individual temperature Θ.” Thellier thus established the physical basis of TRM blocking and recognized the essential role of grain size and shape. In 1949 Louis Neel quantified these concepts in terms of the properties of single-domain grains. Today the Thellier–Thellier method remains the benchmark of reliable paleointensity data. The challenge has been the non-ideality of real geological and archeological materials: TRM carriers larger than single-domain size and physicochemical alteration during heating. The Thelliers avoided these problems by using bricks and pottery previously fired under conditions similar to those in laboratory heatings, eschewing volcanic and other rocks. But despite their problems, we have to deal with the material nature provides. This paper provides insights into the physics underlying the Thellier–Thellier method and check procedures that detect non-ideal behavior, as well as reviewing recent advances in paleointensity methodology.

SUMMARY The sources of long wavelength magnetic anomalies (LWA) in the crust are poorly understood. We have measured remanent and induced magnetizations of 210 samples of anorthosite, tonalite and mafic gneiss from the Kapuskasing uplift, an exposed partial crustal crosssection in northwestern Ontario, Canada. Anorthosites have generally high Q ratios of remanent/induced magnetization, in the range 0.3–60, and their natural remanent magnetization (NRM) is resistant to both thermal and alternating-field (AF) demagnetization. However, anorthositeNRMs(0.001–0.3Am −1 )aretooweaktoexplainLWAamplitudes.Maficgneisses also have relatively high Q ratios, peaking in the range 1–10, and the NRM is resistant to thermal and AF demagnetization. NRM and induced magnetization (IM) intensities are in the ranges 0.01–2 and 0.01–0.6 A m −1 , respectively. Tonalites have a bimodal distribution of magnetization. The more strongly magnetic group has both NRM and IM intensities in the range 0.1–5 A m −1 and wide-ranging Q values, from 0.1 to 10 approximately. Some tonalites could be an LWA source, although the long-term stability of their NRMs at high temperature in the crust is questionable because unblocking temperatures are broadly distributed from 100 to 600 ◦ C. In general, Q values measured at surface temperatures overestimate remanence at depth. In the deep crust, IM remains more or less constant but remanence decreases both reversibly and irreversibly, leading to Q ratios of

SUMMARY Continuous thermal demagnetization, in which measurements of magnetization are made at high temperatureT during heating, is considerably faster than the conventional palaeomagnetic method of stepwise demagnetization, in which measurements are made at room temperature T 0 in a series of cooling-reheating cycles. In the case of single-domain (SD) grains, the two methods give equivalent results after the continuous measurements are converted to equivalent room-temperature values by correcting for the reversible decrease of spontaneous magnetization M S between T 0 and T. To test for equivalence of the two methods in larger pseudo-SD and multidomain grains, three different samples containing magnetite of different grain sizes and origins were heated in zero magnetic field and measurements taken either continuously at T during heating or at T 0 after a set of cooling steps from T. Two samples contained 100–125 μm (mean 110 μm) and 125–150 μm (mean 135 μm) sieve fractions from a crushed natural crystal of magnetite, while the third sample is a natural diabase core sample containing coarse magnetite in the dark minerals and groundmass and fine magnetite in the plagioclase. Two different vibrating-sample magnetometers were used for continuous demagnetization and a mini-furnace and SQUID magnetometer were used for stepwise measurements. Both total thermoremanent magnetization (TRM) and partial TRMs with non-overlapping blocking temperatures (T C, T I) and (T I, T 0) were demagnetized. The Thellier laws of partial TRMs are approximately although not exactly obeyed. In M S(T)-corrected continuous data, there is little overlap of unblocking temperatures: pTRM (T C, T I) demagnetizes almost entirely above T I and pTRM (T I, T 0) almost entirely below T I, demonstrating reciprocity and independence. The stepwise measurements decrease more rapidly in intensity with increasing T than the corrected continuous results, some of which increase slightly with heating up to T ≈ T I. Some additional decay of magnetization must occur during cooling from T, where the continuous measurement is made, to T 0, where the stepwise result is measured. There are no high-temperature measurements of subsidiary cooling-heating cycles to confirm this deduction, but continuously recorded heating-cooling cycles below room temperature in inverse thermal demagnetization (or low-temperature demagnetization) show both reversible and irreversible features, depending on the remanence tested. The most important conclusion of this study is that M S(T)-corrected continuous demagnetization results do not exactly reproduce measured stepwise demagnetization results except for very fine grains, of SD size or close to it. Continuous thermal demagnetization cannot be used in general as a time-saving alternative to stepwise demagnetization if exact equivalence is required.

A paleomagnetic traverse across the Central Metasedimentary Belt of the Precambrian Grenville Province in southern Ontario, linked to an earlier study of the adjoining Central Gneiss Belt, sampled 54 sites in the Frontenac, Sharbot Lake, Mazinaw, Elsevir and Bancroft terranes. 40 Ar/ 39 Ar thermochronometry indicates differential unroofing of these terranes, with slow cooling (1.0–1.5 °C/Ma) along parallel temperature–time paths. Presently exposed rocks cooled through ≈ 500 °C around 1100 Ma (Frontenac), 1010 Ma (Elsevir), 960 Ma (Central Metasedimentary Belt boundary zone) and 930 Ma (Mazinaw) [Cosca, M.A., Sutter, J.F., Essene, E.J., 1991. Cooling and inferred uplift/erosion history of the Grenville orogen , Ontario: constraints from 40 Ar/ 39 Ar thermochronology, Tectonics 10 (1991) 959–977]. 1100–930 Ma was a time of rapid paleolatitude shift but the various terranes do not have distinct paleomagnetic signatures. Frontenac, Sharbot Lake and Mazinaw terranes have similar reverse-polarity paleopoles in the central part of the Grenville polar wander track. Frontenac and Sharbot Lake normal poles are typical ≈ 1000 Ma Grenville A poles, while Mazinaw normal poles seem to be somewhat older. Frontenac poles bear no resemblance to 1100 Ma Keweenawan poles. The Elsevir pole and some Frontenac poles resemble Grenville B poles (600–800 Ma). Although 40 Ar/ 39 Ar hornblende ages were reset thermally between 1100 and 930 Ma during differential uplift of terranes, the terranes were not remagnetized at these same times. Frontenac, Sharbot Lake and Mazinaw terranes (like much of the boundary zone and the Central Gneiss Belt) seemingly were all remagnetized between ≈ 1000 and ≈ 900 Ma, perhaps by a widespread thermochemical event. A 600–800 Ma event affected Elsevir and part of the Frontenac, at the western and eastern extremes of the traverse, but not areas between. The lack of correlation between 40 Ar/ 39 Ar ages and paleomagnetic results for the Frontenac, Sharbot Lake, Mazinaw and Elsevir casts doubt on the thermochronometric age calibration of the Grenville apparent polar wander path, which is based on the assumption that the magnetic and isotopic systems respond in similar ways to slow cooling during uplift so that their blocking temperatures can be directly compared. The most discordant results – those of the Frontenac – could be reconciled if the Frontenac terrane acquired its A magnetization around 1100 Ma before it docked with Laurentia. This is geologically plausible but the match between the Frontenac poles and 900–1000 Ma Grenvillian poles seems too close to be coincidental.

This chapter reviews current knowledge of the permanent and induced magnetizations of terrestrial and extraterrestrial rocks and minerals. The emphasis is on recent developments and on aspects of rock magnetism that are most useful in magnetic anomaly interpretation and paleomagnetism. Beginning with atomic-scale magnetism (diamagnetism, paramagnetism, ferromagnetism, antiferromagnetism, and ferrimagnetism) and magnetic anisotropies (magnetocrystalline and magnetoelastic), we show how nanometer-scale magnetic domains develop in response to self-demagnetization. The very different responses of magnetic crystals subdivided into domains (multidomain or MD grains) and smaller crystals that resist such subdivision (single-domain or SD grains) to magnetic fields, temperature, and time are all important in paleomagnetism. SD grains are capable of preserving a record of the ancient magnetic field of the Earth and other planets that is stable throughout geologic time, while MD grains preserve a much inferior record or no record at all. The grain size that separates SD from MD behavior varies from mineral to mineral, being smallest in strongly magnetic minerals like iron and magnetite and largest in weakly magnetic minerals like hematite. Magnetic stability, both short term and long term, is much affected by temperature, and many important phenomena, for example, blocking of stable remanent magnetization and the Hopkinson peak in induced magnetization, occur just below the Curie temperature, which also varies from one mineral to another. We show that fundamental principles underlie standard paleomagnetic ‘cleaning’ techniques and laboratory stability tests, as well as the best methods of determining paleomagnetic field direction and intensity. Next, we review the latest developments in knowledge about the basic processes by which igneous, sedimentary, and metamorphic rocks are magnetized and partially remagnetized in nature. These include thermoremanent, viscous, thermoviscous, chemical, and depositional/postdepositional magnetizations. Finally, we summarize our current knowledge and major unsolved problems for the future.

We report Mossbauer spectra, magnetic hysteresis, acquisition and demagnetization of isothermal remanent magnetization (IRM), and low-temperature IRM warming curves of biotite crystals, as well as a summary of domain observations on magnetic extracts. The biotites are from 1.0 to 1.2 Ga gneisses and 1.85 Ga plutons of the Grenville, Churchill and Bear Provinces (Canada). Most have paramagnetic susceptibilities in the range (0.45–1.2) × 10− 3 SI, similar to reported values for iron-rich biotites. Magnetite, identified by its 120 K Verwey transition, is an inclusion in all the biotites. Sizes and domain states of inclusions correlate with magnetite content as measured by saturation magnetization Ms. The higher Ms is, the lower are the domain state indicators Mrs / Ms (Mrs is saturation IRM) and coercive force Hc. Added to a base population (0.003–0.1%) of pseudo-single-domain (PSD) grains with Mrs / Ms from 0.1 to 0.3 and Hc from 7 to 20 mT is ≤ 1% (in two exceptional cases, 7% and 14%) of coarse-grained multidomain (MD) magnetite with much softer hysteresis properties. The underlying PSD magnetite fraction in biotites potentially retains useful paleomagnetic remanence. In the more magnetic micas, this stable fraction may be swamped by MD magnetite with less stable behavior.

First-order reversal curve (FORC) and remanence-based Preisach diagrams are alternative ways of determining the Preisach distribution of a sample, which incorporates information about the coercivity spectrum and the distribution of interactions and self-demagnetizing fields. We compare results of the two methods for well-characterized synthetic and natural samples containing single-domain (SD) and pseudo-SD (PSD) magnetite, maghemite, titanomagnetite and titanomaghemite. The greater time requirements of remanence as opposed to in-field measurements limited our Preisach diagrams to a few hundred points, compared to several thousand points for the corresponding FORC diagrams. Only minimal smoothing could be applied in order to limit the regions near the axes of the diagrams in which function values must be extrapolated. In spite of these restrictions, we find excellent agreement between the essential features of the distributions determined by the two methods. The main features, the location and spreading of the distribution peak, are very consistent. However, the low-coercivity part of the Preisach distribution is sometimes poorly resolved or not imaged at all for remanence-only measurements. Features in this region can be diagnostic of PSD and multidomain (MD) grains. The essential agreement between our FORC and Preisach diagrams in the region where they overlap justifies using the much faster FORC routine instead of traditional remanence-based Preisach methods to determine the Preisach distribution of palaeomagnetic samples without strong interactions. We propose a symmetric FORC protocol that would permit separation of the irreversible and reversible parts of the Preisach distribution. The irreversible part is what is determined by remanence-only methods and what is desired for characterization of the remanence behaviour of palaeomagnetic samples. The reversible part is most significant in detecting MD behaviour and screening out samples containing large PSD and MD grains.

SUMMARY We measured palaeodirections and palaeointensities by the Thellier method on 93 samples from three of the Emperor seamounts: 20 from Detroit seamount (81 Ma), 48 from Nintoku seamount (56 Ma) and 25 from Koko seamount (48 Ma). Reliable palaeodirections obtained from three lava flows on Nintoku seamount give an average palaeolatitude of 32.7 ◦ , which is different from the present-day latitude of Hawaii and supports the hypothesis of a moving hotspot. According to the selection criteria traditionally used in palaeointensity determination, 17 samples give a reliable result. The samples show a very wide variety in unblocking temperatures, revealing an important variation in titanium content and the oxidation state of titanomagnetites. In order to assess the reliability of the palaeofield recording in the accepted samples, we carried out measurements of saturation isothermal remanent magnetization at low temperature and thermomagnetic curves. We found Curie temperatures varying from 250 to 580 ◦ C, not only between seamounts but even within one lava flow. Thermomagnetic curves enabled us to identify titanomaghemite in several lava flows. After rejecting the results from samples showing evidence of maghemitization, only four samples, all from Nintoku seamount, can be considered as truly reliable. The palaeointensity values range between 34.2 and 36.9 µT. The low virtual axial dipole moment (VADM) values calculated from the palaeofield values are consistent with the most reliable VADM estimates in this time range and they are very close to the average VADM in the 0.3‐300 Ma time interval.

We report low-temperature remanence and memory of octahedral crystals of titanomagnetite from Mt. Haruna, Japan and Mt. Pinatubo, Philippines. The crystals have Curie temperatures of 460–490°C, indicating a low Ti content (0.11≤x≤0.16). Saturation isothermal remanence (SIRM) produced at 20 K decreased rapidly in warming through the isotropic temperature, 42 K≤Ti≤55 K, where the first magnetocrystalline anisotropy constant K1 changes sign and momentarily vanishes. 96–99% of the original SIRM was demagnetized at Ti and none was recovered in recooling from 300 K to 20 K. SIRM produced at 300 K decreased continuously in the course of zero-field cooling, leveling out around Ti. On rewarming to 300 K, 45–60% of the initial SIRM was recovered. This memory or recovered remanence is very large compared to that of magnetite and must be due to domain walls that are more strongly pinned, probably magnetostrictively by crystal defects, which are common in natural crystals. The present low-temperature experiments provide a quick, non-destructive method of identifying iron-rich titanomagnetites in soils, sediments and rocks.

Background: Two-stage exchange arthroplasty remains the standard treatment of infection at the site of a total knee arthroplasty. The clinical and functional outcomes associated with the use of an articulating antibiotic spacer for two-stage revision for infection are not well established. We conducted a retrospective study to evaluate the outcomes associated with the use of the PROSTALAC articulating spacer between the first and second stages. Methods: Fifty-eight patients underwent two-stage revision total knee arthroplasty for infection between January 1997 and December 1999. Of these, fifty-four were alive at the time of follow-up and forty-seven were available for inclusion in the present retrospective study. In all patients, a prosthesis of antibiotic-loaded acrylic cement (the PROSTALAC system) was implanted during the first stage after debridement. The amount of osteolysis that occurred between the stages and the range of motion of the knee joint were measured. After two years of follow-up, outcomes were assessed with use of the WOMAC, Oxford-12, and SF-12 instruments as well as a satisfaction questionnaire. Results: At a minimum of two years (average, forty-one months) after revision arthroplasty, two patients (4%) had had a recurrence of infection. The amount of bone loss was unchanged between stages, and the range of movement of the knee improved from 78.2° before the first stage to 87.1° at two years. The average normalized WOMAC function and pain scores were 68.9 and 77.1, respectively; the average Oxford-12 score was 67.3; the average SF-12 mental and physical scores were 53.7 and 41.2, respectively; and the average satisfaction score was 71.7. Conclusion: A revision operation for infection at the site of a total knee replacement with use of an articulating spacer was associated with reasonable function and satisfaction scores. These findings may be related to the articulating features of the PROSTALAC system, which permits full active movement of the knee in the early postoperative period. Level of Evidence: Therapeutic study, Level IV (case series [no, or historical, control group]). See Instructions to Authors for a complete description of levels of evidence.

First-order reversal curve (FORC) diagrams have been experimentally shown to be a better way of discriminating domain states in a sample compared to the straightforward use of major hysteresis loops. In order to better understand the fundamental behavior of assemblages of single-domain (SD) grains, we used a micromagnetic model witha conjugate gradient algorith m to calculate FORC diagrams for isolated grains of magnetite as well as for arrays of grains. In the case of individual elongated grains, we found that the FORC diagram consists of a single peak centered on the coercive force Hc if the grain is SD. For a pseudo-single-domain (PSD) grain with vortex structure, we observe multiple peaks on the FORC diagram. The modeling of arrays of elongated SD particles reveals two distinct types of patterns depending on the spacing between particles. In a 2U2U2 array of particles, a secondary branchon the reversal curves appears if the spacing between particles is less than about twice the particle length. This feature translates into the appearance of one negative and three positive peaks on the FORC diagram. In the case of a 3U3U3 array of particles, we again observe several secondary branches when the spacing between grains is less than about twice the particle length, leading to the appearance of multiple peaks on the FORC diagram. Splitting of the central peak on the Hu axis when particles interact could explain the vertical spread of FORC distributions of natural interacting samples as an effect of superposition of multiple peaks caused by the random orientation and distributions of particle spacing and switching fields of a large number of grains. The presence of symmetric peaks on a FORC diagram can be an indicator of the presence of either small PSD grains or magnetic interactions in an ensemble of grains.

Louis Neel’s theories of thermoremanent magnetization (TRM) underlie all of rock magnetism. Neel’s relaxation time equation for thermal activation of single-domain (SD) moments has been verified over geological time and temperature scales by laboratory thermal demagnetization of TRM overprints acquired in nature. For multidomain (MD) grains, disagreement with Neel’s predictions is explained by re-equilibration of domain walls by the changing internal demagnetizing field during heating. Much effort has been devoted to testing the laws of additivity, reciprocity, and independence of partial TRMs, which partition the blocking temperature range and appear in nature as successive overprints of the original TRM. For SD grains, the laws are explained by Neel’s theories and are verified experimentally. For MD grains, additivity holds but partial TRMs do not demagnetize over exactly the original blocking temperature interval (nonreciprocity) and are not entirely independent of one another when acquired in different directions. The current frontier in rock magnetism is to overcome this nonideal partial TRM behavior in order to extract precise and trustworthy records of ancient Earth’s magnetic field directions and intensities.

Low-temperature demagnetization (LTD) promotes single-domain (SD)-like alternating field (AF) and thermal demagnetization properties of anhysteretic remanent magnetization (ARM) and thermoremanent magnetization (TRM), by eliminating low-coercivity or low-unblocking-temperature fractions of remanence. We have investigated the effectiveness of LTD in improving the independence of partial ARMs (pARMs). Independence of pARMs and of partial TRMs (pTRMs) is crucial because multivectorial directional analyses and paleointensity determinations are otherwise compromised. In normalized vector plots of AF demagnetization of orthogonal pARMs, LTD-treated samples showed universal improvement in the degree of independence. The greatest improvement was for synthetic SD and pseudo-single-domain magnetites and natural lake sediments. On the other hand, independence barely improved for synthetic multidomain magnetite, granites, and gabbros. In general, LTD provides a better resolution of superimposed vectors in multicomponent remanence. The direction of the original ARM (corresponding to primary NRM in nature) is perfectly recovered. However, the pARM (the overprint) is not perfectly resolved, especially for overprinting coercivities

The independence of partial anhysteretic remanent magnetizations (pARMs) was tested by alternating field demagnetizing orthogonal pARMs whose blocking field intervals do not overlap with each other. Experimentally the two pARMs demagnetized independently for single-domain grains but for multidomain and pseudo-single-domain grains, a superimposed remanence significantly affected the demagnetization of the pre-existing remanence. Experimentally and in numerical simulations, when a total anhysteretic remanent magnetization (simulating primary remanence in a field H a ) was orthogonally overprinted by pARM (simulating remagnetization in H b ), estimation of the intensity of H b was relatively successful but the intensity of H a was always underestimated. Conversely, the direction of H a was preserved but the direction of H b was spurious. These striking results are caused by the violation of the pARM independence law, implying that the conventional sample rejection criteria in pseudo-Thellier paleointensity determination require modification.

We test the possibility of using the pseudo-Thellier method as a means of determining absolute paleointensity. Thellier analysis of anhysteretic remanent magnetization (ARM) and pseudo-Thellier analysis of thermoremanent magnetization (TRM) have been carried out on a large collection of sized synthetic magnetites and natural rocks. In all samples, the intensity of TRM is larger than that of ARM and the ratio R (=TRM/ARM) is strongly grain size dependent. The best-fit slope ( b TA ) from pseudo-Thellier analysis of TRM shows a linear correlation with R . The ratio b TA / R yielded approximately correct paleointensities, although uncertainties are larger than in typical Thellier-type determinations. For single-domain and multidomain magnetites, alternating field and thermal stabilities of ARM and TRM are fairly similar. However, for ∼0.24 μm magnetite, ARM is both much less intense and less resistant to thermal demagnetization than TRM, reflecting different domain states for the two remanences and resulting in severely non-linear Arai plots for Thellier analysis of ARM.

We report geomagnetic field intensities for the A (∼1000 Ma) and B (∼850 Ma) natural remanent magnetizations of the Cordova Gabbro of southern Ontario. We interpret A and B to be secondary thermal overprints produced during regional uplift and cooling following the Grenvillian Orogeny. Multivectorial A+B paleointensity data were analyzed using pairs of Arai diagrams for the separated A and B vectors. From 18 reliable A results and 15 reliable B results, we find paleofield values of 18.1±2.1 μT and 7.1±1.5 μT, respectively. The corresponding virtual axial dipole moments (VADMs) are (3.12±0.36) and (1.82±0.38)×1022 A m2, about 70% and 40% of the average Phanerozoic dipole moment based on data from 0.3–300 Ma. The Cordova data and other Precambrian paleointensities are generally within a range of approximately 0.5–1.5 times the Phanerozoic mean VADM. Three sets of paleofield values as a function of paleolatitude, including the Cordova data, support a dipole configuration for the Earth’s field in 25–150 Ma time windows around 2700 Ma, 1100 Ma, and 850 Ma.

The recently developed first-order reversal curve (FORC) technique for rapidly examining magnetic domain state has great potential for paleomagnetic and environmental magnetic investigations. However, there are still some gaps in the basic understanding of FORC diagrams, in particular the behavior of pseudo-single-domain (PSD) grains and the contribution of magnetostatic interactions. In this paper we address some of these problems. We report the first FORC diagrams measurements on narrowly sized and well-characterized synthetic PSD through multidomain (MD) magnetite samples. The FORC diagrams evolve with grain size from single-domain (SD)-like to MD-like through the PSD grain size range. Since each sample contains grains of essentially a single size, individual PSD grains evidently contain contributions from both SD-like and MD-like magnetic moments, in proportions that vary with grain size; the evolving FORC diagrams cannot be due to physical mixtures of SD and MD grains of widely different sizes. The FORC diagrams were all asymmetric. Small PSD samples have FORC diagrams with a distinctive closed-contour structure. The distributions of the larger MD grains display no peak, and lie closer to the interaction-field axis. To assess the effect of magnetostatic interactions, we measured FORC diagrams between room temperature and the Curie temperature. On heating the FORC distributions contract without changing shape until ∼500°C. Above this temperature the diagrams become more MD-like, and in addition become more symmetric. The temperature dependence of the interaction-field parameter is proportional to that of the saturation magnetization, in accordance with Neel’s interpretation of the Preisach diagram. The decrease in asymmetry with heating suggests that the origin of the asymmetry lies in magnetostatic interactions. The magnetic hysteresis parameters as a function of temperature were determined from the FORC curves. As the grain size decreased the normalized coercive force was found to decrease more rapidly with temperature.

Submicron magnetite crystals with mean sizes of 0.037, 0.10 and 0.22 μm undergo major changes in hysteresis properties and domain states in crossing the Verwey transition ( T V ≈120 K). The 0.037 μm crystals are single-domain (SD) both in the cubic phase at room temperature T 0 and in the monoclinic phase below T V . The 0.10 and 0.22 μm crystals have a mixture of SD and two-domain (2D) states at room temperature T 0 , but mainly SD structures below T V , in agreement with micromagnetic calculations. Coercive force H c increases on cooling through T V , by a factor 3–5 in the submicron magnetites and 40 in a 1.3 mm single crystal, because of the high crystalline anisotropy and magnetostriction of monoclinic magnetite. As a result, domain walls and SD moments are so effectively pinned below T V that all remanence variations in warming or cooling are reversible. However, between ≈100 K and T 0 , remanence behavior is variable. Saturation remanence (SIRM) produced in monoclinic magnetite at 5 K drops by 70–100% in warming across T V , with minor recovery in cooling back through T V (ultimate levels at 5 K of 23–37% for the submicron crystals and 3% for the 1.3 mm crystal). In contrast, SIRM produced in the cubic phase at 300 K decreases 5–35% (submicron) or >95% (1.3 mm) during cooling from 300 to 120 K due to continuous re-equilibration of domain walls, but there is little further change in cooling through T V itself. However, the submicron magnetites lose a further 5–15% of their remanence when reheated through T V . These irreversible changes in cycling across T V , and the amounts of the changes, have potential value in determining submicron magnetite grain sizes. The irreversibility is mainly caused by 2D→SD transformations on cooling through T V , which preserve or enhance remanence, while SD→2D transformations on warming through T V cause remanence to demagnetize.

Paleointensity determination by the Coe-modified Thellier method was carried out on the ≈1100 Ma Tudor Gabbro of southern Ontario. The Tudor has a bivectorial natural remanent magnetization (NRM). The C NRM is a Grenvillian uplift thermoviscous remagnetization which overprints the A NRM, the surviving primary thermoremanent magnetization (TRM) of the Tudor intrusion. A sharp junction at ≈530°C between the A and C vectors in laboratory thermal demagnetization matches the peak metamorphic reheating temperature of 480–500°C in nature when the difference in time scales is taken into account. The unblocking temperature distributions of the A NRM and of laboratory TRM also match. A is carried by single-domain or small pseudo-single-domain grains, probably magnetite rods exsolved in plagioclase or pyroxene at temperatures above 580°C. Arai plots of NRM versus partial TRM are linear from 520°C or 530°C to 580°C for 45 specimens of 19 samples from 9 sites. The 45 reliable paleointensity values from these specimens give a virtual axial dipole moment (VADM) of 4.6±0.8 × 1022 A m2 around 1100 Ma. This is about one half of the modern field intensity but is similar to the mean Cretaceous-Cenozoic intensity and to most Archean and Proterozoic VADMs. Keweenawan rocks, whose ages are within ≈20 Myr of the Tudor Gabbro, have a VADM of 11.4×1022 A m2. The difference between the Tudor and Keweenawan VADMs is similar to differences recorded over similar time intervals in the Cretaceous and seems to represent normal variation in geomagnetic field strength throughout Earth's history.

Partial thermoremanent magnetization (pTRM) was imparted over a narrow temperature interval, T = 370–350 ◦ C, to a suite of crushed and annealed natural magnetite samples, ranging in grain size from ≈1 to 125–150m and in domain state from small pseudo-single-domain (PSD) to multidomain (MD). In this way, effectively a single blocking temperature, TB ,o f pTRM was activated. Stepwise thermal demagnetization did not erase the pTRMs sharply at TB, as for single-domain (SD) grains. Demagnetization began well below 350 ◦ C and continued above 370 ◦ C, with a median unblocking temperature, TUB, close to 360 ◦ C. The largest grains deviated most from SD behavior. Their pTRM demagnetized over the entire interval from room temperature to the Curie point, in accordance with predictions for MD grains. In terms of the unblocking temperature distribution f(TUB) or slope dM/dT of the thermal demagnetization curve, SD grains have a sharp spectrum, T UB = T B ;M D grains were observed to have a broad, roughly symmetrical spectrum centered on TB; and intermediate size grains in the PSD range had non-Gaussian spectra, combining a central peak near TB with broad tails above and below TB. In this respect, PSD grains display a superposition of SD and MD remanences, not a blend between the two. Practical implications of these observations are that Thellier’s law of reciprocity ( T UB = T B) will be increasingly violated as grain size increases in the PSD range. The low-TUB part of f(TUB) produces anomalously large demagnetization of NRM in low-temperature heating steps of Thellier-type paleointensity determinations and a sagging shape of the Arai plot. The high- TUB part of f(TUB) results in undemagnetized remanence at and above TB in thermal demagnetization. Among pre-treatments designed to make remanence more SD-like in subsequent thermal cleaning, alternating field (AF) pre-cleaning sharpened f(TUB) more effectively than low-temperature demagnetization for the 20m sample. © 2001 Elsevier Science B.V. All rights reserved.

Thermoremanent magnetization (TRM) in hematite is larger than TRM in magnetite for grain sizes ≥ 10 μm. We show that hematite's weak spontaneous magnetization M s causes its strong TRM, since the self-demagnetizing field H d opposing large domain wall displacements is proportional to M s . In hematite, H d is comparable to the Earth's magnetic field but in magnetite, H d is 1000 times larger. As a result, Earth's field TRM of MD hematite (0.3 Am 2 /kg) outweighs TRM and induced magnetization of MD magnetite ( 0.01-0.02 Am 2 /kg) and rivals TRM of single-domain and PSD magnetite as a source of magnetic anomalies on Earth and perhaps on Mars.

An archeomagnetic study was carried out on 24 pottery fragments from Grand Banks, southern Ontario. 14 C dating on maize found in the same pit as the potsherds gives an age in the range A.D. 990^1160 (modal age A.D. 1030). Hysteresis measurements indicate that the natural remanent magnetization is carried by pseudosingle-domain magnetite grains. Paleointensity experiments were done on 63 samples using a Thellier-type double-heating method. Half of the samples were heated in helium and the other half in air. Hysteresis measurements on samples after heating to various temperatures reveal only slight mineralogical changes during heating steps. Thirty reliable results give a paleofield intensity of 42.0 ˛ 7.4 WT, and a virtual axial dipole moment of 7.0 ˛ 1.3U10 22 Am 2 . This result is in agreement with the low virtual axial dipole moment values given by studies on material from Ontario compared to the values from the southwestern USA in the interval A.D. 900^1400. fl 2001 Elsevier Science B.V. All rights reserved.

An archeomagnetic study was carried out on potsherds samples from sites in Ontario with ages ranging from A.D. 90 to A.D. 1640 as determined by 14C dating. Thellier double-heating paleointensity experiments were performed in air on 65 specimens of 52 samples from seven sample sets. Reliable paleointensity estimates were obtained for 49 specimens. Alternating field and thermal demagnetization, temperature dependence of weak-field susceptibility, and hysteresis measurements indicate that magnetite of pseudo-single-domain grain size is the carrier of natural remanent magnetization. The paleointensity results follow a half-cycle sine curve, with a steady decrease from 54.0±5.9 μT to 37.6±5.7 μT between A.D. 90 and A.D. 885 and a monotonic increase from 52.0±6.1 μT to 59.4±1.7 μT between A.D. 1200 and A.D. 1900. The paleointensities determined yield virtual axial dipole moments (VADMs) of the Earth's magnetic field that agree well with those from other parts of North America, except between A.D. 900 and A.D. 1400, when they are systematically lower. This discrepancy is probably caused by a substantial non-dipole field in southwestern North America from the tenth to the fifteenth century, since secular variation studies using potsherds from Arizona and lake sediments from Minnesota show different inclination variations during that period.

Thermal demagnetization of viscous remanence (VRM) and partial thermoremanence (pTRM) of 20- and 135-µm natural magnetites reveals a broad spectrum f(TUB) of unblocking temperatures TUB, both > and < TB, the blocking temperature. In contrast, 0.04-µm single-domain (SD) grains demagnetize sharply at TUB ≈ TB. High- and low-TUB tails of f(TUB) were wider for 135-µm multidomain (MD) grains than for 20-µm PSD grains. 10-mT alternating-field demagnetization rendered the VRM of the 20-µm magnetite more SD-like in subsequent thermal cleaning, selectively erasing both low- and high-TUB tails of f(TUB). Primary and secondary remanences are then cleanly separated, and non-linear Thellier paleointensity determination becomes linear. Our results agree well with (TB, TUB) data for 105-year thermoviscous overprints in the Milton Monzonite over the range of log(time) common to the two studies. Because (TUB)av ≈ TB for PSD and MD as well as SD grains, the Pullaiah et al. paleothermometry method will work if (TUB)av values are used instead of (TUB)max values.

The dehydration of goethite has been studied by low-temperature induced magnetization (LTIM) and X-ray diffraction on well-characterized acicular crystals. Fresh samples were heated in air to temperatures between 155°C and 610°C. Goethite and hematite were the magnetically dominant phases after all runs except 500°C and 610°C, for which only hematite was found. However, partially dehydrated goethites after the 238–402°C runs had broad peaks or inflections in the LTIM curves around 120 K, suggesting the formation of an intermediate spinel phase. These samples were next given a saturation remanence in a field of 2 T at 10 K and the remanence was measured continuously during zero-field warming to 300 K. There was a decrease in remanence at the Verwey transition (120 K), diagnostic of magnetite. The possible formation of a small amount of magnetite is of serious concern in studies of goethite-bearing sediments and rocks. Chemical remanent magnetization (CRM) of this strongly magnetic spinel phase could significantly modify the direction as well as the intensity of the original goethite CRM. As well, it would be a new source of paleomagnetic noise as far as primary remanence carried by other mineral phases is concerned.

Paleotemperature controls the maturation of coal and hydrocarbons in sedimentary basins and is also important in determining paleogeothermal gradient and hence tectonic style in exhumed metamorphic terrains. One method of estimating paleotemperature analyses the partial remagnetization of a rock due to heating in thick volcanic or sedimentary sequences, over subcrustal heat sources such as plumes, or at convergent plate margins. The overprinted natural remanent magnetization (NRM) of a rock records both the age and the paleotemperature of remagnetization, but a temperature correction from laboratory to geological time scales is required, using theoretical time–temperature relations. Time–temperature relations are well known for magnetite (Fe 3 O 4 ) but are reported here for the first time for pyrrhotite (Fe 7 S 8 ), another common NRM carrier. Data for each mineral separately yield independent estimates of paleotemperature if geologically reasonable estimates of heating time can be made. Paleotemperature can be estimated without geological input if data for both minerals are combined. Together with the age of remagnetization, determined from the paleomagnetic pole of the NRM overprint, these paleotemperature estimates can be used to infer the history of heating and uplift following burial. As a test case, we examine thermally acquired NRM overprints carried by pyrrhotite (Fe 7 S 8 ) and magnetite (Fe 3 O 4 ) in the Milton Monzonite of southeastern Australia. These overprints record a heating event about 100 Ma ago, probably thermal doming prior to rifting of the Tasman Sea, that upgraded coal rank in the Sydney Basin. Extrapolating from laboratory to geological times, using the new time–temperature contours for pyrrhotite, we estimate that the presently exposed Sydney Basin in the vicinity of the Milton Monzonite was remagnetized by heating to 165±30°C for ≈100 ka. Assuming a paleogeothermal gradient of 70°C/km appropriate for young or incipient rifts, the depth of burial at the time of remagnetization is estimated to have been 2.3±0.4 km. This figure is in excellent agreement with independent estimates based on reflectance data for the coal accessory mineral vitrinite.

We have measured saturation induced and remanent magnetizations and induced magnetization as a function of field at low temperatures, between 300 K and 10 K, on an oriented 1.5-mm single crystal of magnetite. The induced magnetization curves along the cubic [001], [1 1 0] , and [110] axes at 10 K have very different approaches to saturation. The crystal is easy to magnetize along [001] but difficult along [1 1 0] and [110], the hard directions of magnetization for monoclinic magnetite. The temperature dependence of saturation magnetization between the Verwey transition temperature, Tv = 119 K, and 10 K is also different along the three axes, indicating that below Tv the crystal has uniaxial symmetry. The room-temperature saturation remanence (SIRM) produced along [001] decreases continuously in the course of zero-field cooling, levelling out at the isotropic temperature, Ti = 130 K, where the first magnetocrystalline anisotropy constant becomes zero. At Ti, 86% of the initial SIRM was demagnetized. The domain wall pinning responsible for this soft remanence fraction must be magnetocrystalline controlled. The remaining 14% of the SIRM is temperature independent between Ti and Tv and must be magnetoelastically pinned. This surviving hard remanence is the core of the stable magnetic memory. The Verwey transition at 119 K, where the crystal structure changes from cubic to monoclinic, is marked by a discontinuous increase in remanence, indicating that the cubic [001] direction suddenly becomes an easy direction of magnetization. The formation of monoclinic twins may also affect the intensity of remanence below Tv. Reheating from 10 K retraces the cooling curve, with a decrease at Tv back to the original remanence level, which is maintained to 300 K. When SIRM is not along [001], the initial SIRM is larger but the reversible changes across the Verwey transition are much smaller. The SIRM produced at 20 K is an order of magnitude larger than the 300 K SIRM, but the only change during warming is a discontinuous and irreversible drop to zero at Tv.

A simple and elegant interpretation of thermoremanent magnetization (TRM) in uniformly magnetized single-domain (SD) grains was given by Neel 50 years ago, but the TRM acquisition processes in larger, nonuniformly magnetized grains are more varied and difficult to describe theoretically. SD TRM is a frozen high-temperature partition between two microstates: spins parallel or antiparallel to an applied magnetic field. Nonuniformly magnetized grains have a much greater choice of microstates (local energy minimum or LEM states), and partitioning among various LEM states continues to change during cooling. These changes may involve Barkhausen jumps of domain walls between positions of minimum local energy or nucleation of new domains and walls. Because of the lower remanence capacity of nonuniform microstates compared to the uniform SD state, TRM intensity decreases as grain size increases, although certain microstates, e.g., single-vortex states, seem to contribute little to TRM. Thermal demagnetization of TRM begins just above room temperature and continues to the Curie point, quite unlike the sharp “unblocking” of SD TRM. This continuous demagnetization, resulting from changes in microstates driven by the changing internal demagnetizing field during heating, profoundly affects the separation of different components of natural remanent magnetization and the determination of paleomagnetic field intensity.

Earth's magnetic field has protected our atmosphere from erosion by the solar wind ever since it started up. Silicate crystals from some of Earth's oldest rocks date that event to more than 3 billion years ago.

SUMMARY Micromagnetic modelling with a Monte Carlo method was performed to obtain 2-D micromagnetic structures of pseudo-single-domain (PSD) magnetite (0.2‐2.5 mm). Cubic model grains were subdivided into a 2-D array with a maximum of 100◊100 cells. In order to find an appropriate cell size, the variation of the self-consistency parameter (S) with cell size was examined. We regard solutions with S>0.95, which corresponds to cell sizes 0.03 mm, as representing realistic micromagnetic structures. For saturation remanence states, we found vortex structures for small PSD grains (0.25 mm) and closure domain structures for larger PSD grains (0.6‐2.5 mm). Between 0.25 and 0.6 mm, structures remained chaotic even after a very large number of Monte Carlo steps. In grains with a closure domain structure, domain walls were usually subdivided into several segments with opposite polarities of magnetization rotation.

We have made a regional paleomagnetic study of lithotectonic domains in the Central Gneiss Belt of the Grenville Province in Ontario along ten N-S and E-W traverses up to 200 km in length. Although originally intended to clarify the tectonic mechanism by which these exotic terranes were assembled and welded to the Archean Superior craton during the ∼1150-Ma Grenvillian orogeny, we actually learned much more about the timing of post-orogenic uplift of the various domains around 1000-900 Ma as they stabilized and became part of the Rodinia supercontinent. The normal (N) and reverse (R) natural remanent magnetizations (NRMs) of all domains, as well as those of reactivated regions flanking the Grenville Front (GF, the Superior-Grenville boundary) to the north and south, have paleomagnetic poles falling on the 980-920-Ma (40Ar/39Ar calibrated) portion of the Grenville apparent polar wander track for Laurentia. There is a general tendency for paleopoles to young with increasing distance of domains from the GF, implying that more southerly domains were uplifted and magnetized later, but two of the domains do not fit this pattern. Previously reported younging trends away from the GF, based on K/Ar thermochron maps and paleomagnetic ‘zone poles’, are untrustworthy because of hydrothermal alteration, which causes chemical remagnetization and anomalously old K/Ar ages near the GF. Another trend in our data is a regular increase in the R/N ratio with increasing distance south of the GF. In the reactivated zones flanking the GF, NRMs are overwhelmingly of N polarity, whereas well away from the GF, R/N is close to 50:50. Also, NRM intensities and susceptibility values increase 100-fold away from the GF, peaking ≈10 km south of the front, with a pulse-like pattern similar to that documented in anomalously high 40Ar/39Ar dates in the same region. Both the magnetic and Ar/Ar results are likely due to a ‘wave’ of hydrothermal alteration and remagnetization during which fluids were driven away from the GF. Since the remagnetized NRM near the GF has approximately the same direction as NRMs in domains away from the GF, hydrothermal activity along fault systems south of the GF must have continued until ∼1000 Ma, long after the collisional orogeny itself.

We have observed single-domain (SD) like behavior in a 3-mm natural single crystal of magnetite following low-temperature demagnetization (LTD), which consists of zero-field cycling through the Verwey transition to erase remanence carried by pinned domain walls. We compared stepwise alternating field (AF) and thermal demagnetization curves of 1-mT total thermoremanent magnetization (TRM), 1-mT partial TRM (pTRM) acquired between the Curie point (TC = 575°C) and 565°C, and saturation isothermal remanent magnetization (SIRM) measured with and without prior LTD. AF demagnetization curves of untreated TRM and SIRM decreased exponentially with increasing AF. SIRM was more resistant to demagnetization than was TRM, a multidomain (MD) result of the Lowrie-Fuller test. After LTD the TRM and SIRM memories had AF demagnetization curves with sigmoid shapes and initial plateaus below 8 mT in which little or no demagnetization occurred. Both features are reminiscent of SD behavior. During thermal demagnetization, untreated TRM and SIRM decreased almost linearly with increasing temperature up to ≈500°C. Such distributed unblocking temperatures TUB are expected for pinned walls in MD grains. The remaining 60% of TRM and 40% of SIRM were lost over a narrow temperature interval concentrated between 560°C and TC. The TRM memory after LTD was very stable against thermal demagnetization. There was no decrease in remanence below 550°C and very little change until 565°C, only 10°C below TC. This very high TUB fraction of TRM seems to have SD-like character. SIRM has similar behavior. In the case of pTRM, both the untreated remanence and the memory after LTD have almost entirely high TUBs, identical to the range of pTRM blocking temperatures TB, 565°C–TC. TUB = TB is a basic property of SD partial TRM. The pTRM memory fraction is also larger than that of total TRM or SIRM. These observations suggest that pTRM(TC, 565°C) isolates an SD-like fraction of remanence similar to that constituting TRM and SIRM memory. The high AF coercivities of the TRM and SIRM memories indicate that the source of SD-like behavior may be unusually strong domain wall pinning by crystal defects formed at monoclinic twin boundaries as a result of crystal distortion from cubic to monoclinic structure in passing through the Verwey transition.

The Early Triassic (∼245 Ma) Milton Monzonite of the Sydney Basin, Australia, has four distinct components of natural remanent magnetization (NRM) with only slightly overlapping ranges of unblocking temperatures. The low-temperature (LT) component, the first to be thermally demagnetized, is thought to be a Late Cretaceous (≈100 Ma) thermoviscous overprint acquired in slow cooling during uplift. The high-temperature (HT) component, the second to be demagnetized, is probably the primary thermoremanent magnetization (TRM) of the Milton intrusion but could possibly be a Jurassic overprint. LT and HT are usually carried by magnetite and occasionally by pyrrhotite. Samples from nine sites have a further NRM component which unblocks at higher temperatures than HT but below the magnetite Curie temperature of 580°C. This component is argued to be a chemical remanent magnetization (CRM) because of its discrete range of high unblocking temperatures, above those of the thermal components HT and LT, and is called CRM1. CRM1 has almost the same direction as LT and is likely carried by authigenic magnetite produced during uplift ∼100 Ma. Samples from five sites have a fourth NRM component, with a direction resembling that of HT but carried by hematite. This fourth component could be a primary TRM but is more likely a CRM and is therefore called CRM2. The HT-CRM2 mean direction is D=50°, I=75.5°, defining a paleopole at 16°S, 172°E. The HT-CRM2 paleopole falls near 150 Ma on the Australian apparent polar wander path but is a considerable distance from paleopoles of Permian and Early Triassic age. There is no known tectonic or other remagnetizing event in the Sydney Basin around 150 Ma. For this reason, we propose that the HT-CRM2 paleopole defines a new Triassic segment of the Australian polar wander path. The LT-CRM1 mean direction is D=348°, I=−79°, with a paleopole falling at 56°S, 1580°E, near 100 Ma on the polar wander path. This age is consistent with uplift and cooling related to initial rifting of the Tasman Sea.