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The discrepancy between the magnetic hysteresis properties of magnetite crystals that are precipitated from solution (0.3 micrometer) and of crushed sifted grains (0.3 micrometer) is not an inherent property of magnetite but is caused by the highly stressed state of crushed material and by adhering finer fragments. The size trends of magnetic properties exhibited by submicrometer-size precipitated grains continue in the size range from 1 micrometer to 1 millimeter in a set of hydrothermally recrystallized magnetite crystals. Coercive forces of these narrowly sized crystals follow a power law over a wide size range (0.1 micrometer to 1 millimeter) as predicted by theory. Dislocation etch pits show similar dislocation densities for hydrothermally grown (3 x 10(10) meter (-2)) and natural (1 x 10(10) meter(-2)) magnetite crystals. Hysteresis parameters of hydrothermally grown crystals are similar to those of natural crystals but are about one-fifth of those for crushed grains.

Maximum values for the single-domain threshold d(0) and superpara-magnetic threshold d(8) in pure magnetite are found to be 570+/-50 and 350+/-50 angstroms, respectively. Particles larger than do but smaller than about 0.25 micron have size-dependent saturation remanences and coercive forces like those of multidomain particles, but intense and stable thermoremanent magnetization like that of single-domain particles. The presence of magnetite grains in this size range could account for the essentially single-domain character of stable natural remanence in many volcanic and intrusive rocks.

We report the temperature dependence of in-phase and quadrature susceptibilities, k′ and k″, between 20 K and 300 K for a stoichiometric natural single crystal of magnetite. Measurements were made for amplitudes of the AC driving field ranging from H = 30 A/m to 2 kA/m and frequencies ranging from f = 40 Hz to 4 kHz. In cubic magnetite above the Verwey transition, TV = 120 K, k′ is limited by self-demagnetization and does not vary greatly with T, H or f. As the crystal cools through TV and transforms to monoclinic structure, k′ decreases by about a factor 2, with a further more gradual decrease of 10-20% in cooling from 40 to 20 K. Saturation remanence also drops sharply at T V but shows no further change in cooling below 40 K. Thus it appears that domain walls remain pinned throughout the 20-40 K range but small segments undergo reversible oscillations in an AC field, the amplitude of oscillation decreasing steadily with cooling below 40 K. In this same range, k″ reaches a peak, while the temperature at which k′ decreases most rapidly changes with frequency. Both observations indicate that domain wall oscillations lag appreciably behind the driving field at very low temperature. Both k′ and k″ increase markedly with increasing AC field amplitude below TV. The field dependence is particularly strong below 40 K. Analysis of the k′(f) data between 20 and 40 K based on an Arrhenius thermal activation equation gives a pre-exponential frequency factor f o ≈ 2.5 × 108 s-1 and an activation energy ΔE = 0.035 eV. The ΔE is appropriate for electron hopping but f o suggests an indirect mechanism for wall mobility related to changes in electron ordering within walls.

The Shenango and Nemegosenda alkaline complexes of the southern Kapuskasing Structural Zone (KSZ), an uplifted crustal block in northern Ontario, preserve primary thermoremanent magnetizations, with mean directions D=112°, I=−46° (k=13, α95=13°, N=9 sites) and D=304°, I=+51° ( k=31, α95=10°, N=7 sites) for Shenango and D=124°, I=−57° ( k=13, α9520°, N=4 sites) and D=312°, I=+54° (k=17, α95=13°, N=7 sites) for Nemegosenda. Their average paleomagnetic pole is 45°N, 189°E (dp=6°, dm=8°), falling on the Keweenawan track of the North American apparent polar wander path (APWP) between 1098–1107 Ma reverse-polarity (R) and 1087–1098 Ma normal-polarity (N) poles of Lake Superior lavas and dikes. Unlike the Keweenawan paleopoles, which have a reversal asymmetry of ≈30°, the Shenango and Nemegosenda R and N poles are almost 180° reversed, which was found also by Lewchuk and Symons (1990b). This symmetry of R and N poles casts doubt on the previously accepted predominance of asymmetric reversals of the Earth's field in Keweenawan time. The Archean Shawmere anorthosite is overprinted by a B magnetization with mean direction D=97°, I=−68° ( k=12, α95= 10°, N=18 sites). The Shawmere B paleopole at 38°N, 226°E (dp=13°, dm=l6°) falls between the carbonatite poles and ≈1850 Ma cratonic poles, and the B magnetization seems to be a composite of middle and late Proterozoic overprints. Sites in the western part of the Shawmere body close to the “carbonatite line” of faults linking the various alkaline complexes are severely altered, with an abundance of secondary magnetite and hematite. Virtual geomagnetic poles for these sites fall close to the Keweenawan track and have an average pole of 189°, 53°N (dp=15°, dm=20°) suggesting that the B NRM at these sites is an ≈1100 Ma thermochemical overprint caused by hydrothermal fluids at the time of carbonatite intrusion. More easterly sites are less altered. Their VGPs resemble ≈1850 Ma cratonic poles and average to 235°E, 30°N (dp=15°, dm=17°) These B natural remanent magnetizations are probably thermochemical remanent magnetizations impressed by middle Proterozoic hydrothermal activity associated with intrusion of early alkaline bodies, fault movement on the Ivanhoe Lake fault zone, or late-stage uplift of the KSZ.

We report paleomagnetic results from two traverses in the southern part of the Kapuskasing structural zone (KSZ) of north-central Ontario, which is hypothesized to be an oblique cross section through upper and middle Archean crust exposed by uplift and tilting. Anorthosites and granulite-grade gneisses were sampled to the east in the Shawmere traverse, while amphibolite-grade tonalitic gneisses were sampled to the west in the Racine traverse. Three distinct remanent magnetization (RM) components were isolated by alternating field and thermal demagnetization. The A RM has mean directions D = 50°, I = +62° (k = 138, α95 = 5°, N = 5 sites (23 samples)) for the Shawmere traverse and D = 23° I = +53° (k = 49, α95 = 7°, N = 8 sites (46 samples)) for the Racine traverse. Because of the high (>500°C) laboratory unblocking temperatures of the A RM in the Shawmere granulite-grade rocks, we believe this magnetization was acquired in early slow uplift, without tilting, from about 24-km to about 16-km depths, and has survived the later main KSZ uplift and tilting event. The Racine A RM has laboratory unblocking temperatures down to ≈400°C and was blocked prior to uplift from a shallower crustal level (about 8 km). With tilt corrections of 30° and 15°, respectively, about the 35° striking axis of the KSZ, the Shawmere and Racine A paleomagnetic poles fall on the Laurentian apparent polar wander path around 2.55 Ga, dating the time of initial uplift. Crustal tilting in the KSZ occurred after 2.55 Ga and was greater in the east than in the west. The B RM, found mainly in Shawmere anorthosites, has D = 115°, I = −68° (k = 17, α95 = 17°, N = 4 sites (14 samples)), with a paleopole at 144°W, 49°N, near 1.1 Ga Keweenawan reverse poles and the normal pole of the nearby 1.0 Ga Nemegosenda carbonatite. The B RM is probably a thermochemical overprint related to reactivation of the KSZ at the time of Keweenawan rifting and volcanism in the Lake Superior region. A novel C RM, with D = 64°, I = +2° (k = 18, α95 = 13°, N = 6 sites (10 samples)), is either an unresolved hybrid of the A and B RMs or an earliest Keweenawan (1.3 Ga) overprint.

We have carried out a laboratory simulation of the chemical remanent magnetization (CRM) of titanomaghemites produced by seafloor oxidation of titanomagnetites carrying a primary thermoremanent magnetization (TRM). The starting material was well-characterized synthetic monodomain titanomagnetite of composition Fe2.4Ti0.4Al0.2O4, dispersed in CaF2 and sealed in evacuated quartz capsules. First a TRM, JTRM, was induced in a field of either 50 or 100 μT Then the samples were removed from their capsules and oxidized by long heatings in air between 100 and 230°C, well below the Curie temperature of 270°C. Titanomaghemites with oxidation parameters of 0.32, 0.46, 0.62, 0.72, 0.91 and 0.97 resulted. For 0.15 ≤ z ≤, 0.72, single-phase spinels were produced. Faint hematite X-ray lines were observed only for the most oxidized samples. During oxidation, a field HCRM of either 50 or 100 μT was applied perpendicular to JTRM The resulting CRM, HCRM was parallel to JTRM and uninfluenced by HCRM throughout most of the single-phase oxidation range (0.15 ≤ z ≤ 0.62), and remained directionally stable during alternating field (AF) demagnetization to 100 mT. For z ≥ 0.72, JCRM was increasingly deflected toward HCRM For z = 0.97, the deflection amounted to 23.5°, but AF cleaning revealed that JCRM consists of a harder component (coercivities > 35 mT) with deflection 14.5° and a softer component with deflection 38°. We conclude that low and medium degrees of oxidation (z ≲ 0.7) of single-domain titanomagnetites should not change the original NRM directions of submarine basalts nor the pattern of magnetic stripes over the oceans.

This paper reports room temperature measurements of initial susceptibility χ0, second Rayleigh coefficient B, coercive force Hc, remanent coercive force HR, and the ratio Jrs/Js of near-saturation remanent and induced magnetizations for four samples containing single-crystal magnetite cubes with mean sizes of 0.037, 0.076, 0.10, and 0.22 μm. While χo and Hc have weak dependences on particle size d, Hc and Jrs vary strongly with size, as d−0.55 and d−0.75, respectively, over the 0.076–0.22 μm range. These hysteresis properties and their size dependences are incompatible with data for >l-μm crushed and annealed magnetite grains. Nearly perfect unstrained crystals tend to have Hc and Jrs values 2–5 times smaller than those of pulverized grains of nominally the same size. Magnitudes and size dependences of Hc and Jrs for >0.06-μm magnetites are approximately, although not perfectly, accounted for by (1) metastable single-domain (SD) particles, (2) rotatable SD-like domain wall (DW) moments, or (3) DW displacements impeded by crystal defects and limited by self-demagnetization. Only reversible DW displacements (or possibly domain rotations) opposed by self−demagnetization can explain the initial susceptibility data, however.

We report opaque mineralogical observations and magnetic properties of primary titanomagnetites in Tertiary submarine gabbros from DSDP, Legs 30 and 37 and in a late Archean, continental granitic pluton, the Shelley Lake granite. The titanomagnetites and silicates in all the submarine gabbros have been deuterically oxidized. There is no indication of subsequent low-temperature oxidation, although serpentization of olivines is pervasive in the deeper Leg 37 units. The Leg 30 samples, from a single thick sill, contain abundant coarse (≈100 μm) titanomagnetites with fully developed ilmenite exsolution lamellae. Curie temperatures are 515–550°C; there are no low Curie temperatures that would indicate surviving unoxidized titanomagnetite. The unserpentinized Leg 37 gabbros contain scarce opaques with pure magnetite Curie points that are barely resolvable microscopically; most occur as inclusions in pyroxene. In the Shelley Lake granite, on the other hand, many samples exhibit bimodal blocking-temperature spectra, with blocking temperature peaks at 250–300°C and 550–575°C. The low-blocking-temperature phase is unidentified. No pyrrhotite was seen in thin section. Optically homogeneous grains coexist with fully exsolved neighbours, but the electron microprobe indicates no titanium. The lamellae appear to be haematite, not ilmenite, and the primary composition of the opaques is pure magnetite. The oxidation state of the opaques is very inhomogeneous, even on a fine scale.

The average internal magnetic field Hi inside a multidomain (MD) grain is the sum of the applied field Ho and a demagnetizing field involving the demagnetizing factor N of the grain. It is shown that NMD governing MD wall displacements is not the same as NSD for single-domain (SD) rotations of the same grain when saturated, as determined by ferromagnetic resonance, for example. NMD can be determined from the measured hysteresis parameters XO (initial susceptibility), Hc (coercive force), Hcr (remanent coercive force), Jrs (saturation remanence), and JD (magnetization when HO = Hcr) by one of three formulas: (1) N = XO−1 − Xi−1, (2) N = (Hc/Jrs) − Xi−1 and (3) N = JD−1(Hcr − HC) − Xi−1. These formulas, which resemble equations derived recently by Smith and Merrill, reduce to conventional approximations for N as the intrinsic susceptibility Xi = dJ/dHi → ∞. In order to use the exact formulas, Xi must be estimated independently by assuming XiHc ≈ constant. Using hysteresis measurements for 2200-A cubes of magnetite, which should theoretically have a two-domain or three-domain structure, N is calculated to be about 2, in satisfactory agreement with predicted values of NMD and about one half NSD = 4π/3 for a cube.

Curie temperatures, hysteresis, alternating field properties and anhysteretic and ordinary susceptibilities have been used to characterize the titanomagnetites in a large collection of continental granites, diorites, syenites, anorthosites, gabbros, diabases and basalts. Low-Curie-point titanomagnetites or titanomaghemites were found only in basalts. In all shallow and deep-seated intrusive rocks, the predominant magnetic phase was nearly-titanium-free titanomagnetite with a Curie point of 520–580°C. Most felsic plutonic rocks owed their magnetic properties to coarse, discrete titanomagnetites with truly multidomain properties. Many mafic plutonic rocks (anorthosites, gabbros, norites) displayed bimodal magnetic properties, strong-field properties being due to the discrete titanomagnetites and weak-field properties being due to fine magnetite inclusions in deuterically altered silicates. The Lowrie-Fuller test and the anhysteretic induction curve were the most diagnostic tests of this bimodal behaviour. Grain-size variation within a single diabase dike or sill had a strong expression in all magnetic properties, except H R / H c and the Lowrie-Fuller test. On the other hand, the Lowrie-Fuller test was a sensitive indicator of changes in “effective” grain size in basalts due to the subdivision of grains by ilmenite lamellae.

The Earth Physics Branch of the Canadian Department of Energy, Mines, and Resources (formerly the Dominion Observatory) has been the primary government agency for geophysics in Canada since the turn of the century. It has operated the national observatories for seismology and geomagnetism and has established and maintained national networks, surveys, and standards in gravity, geodynamics, and geothermics. A reassessment of the role of federal government agencies now underway by the current Conservative administration in Canada is examining the question of whether the Earth Physics Branch should be combined with the Geological Survey of Canada in one agency that would carry the issue is expected within the next few out all federal geoscience activities. The opinion that such a merger would adversely affect the future of solid earth geophysics in Canada has been expressed by the Canadian Geophysical Union in a letter to the Minister of Energy, Mines, and Resources. Resolution of the issue is expected within the next few months.

Lunar materials exhibit two distinct types of viscous or time-dependent magnetic behavior. Igneous rocks and largely recrystallized breccias, whose magnetic properties are due to multidomain iron, typically have weak magnetic viscosity, but decay persists for very long times following even a brief exposure to a field. Lunar soils and low metamorphic grade breccias, which contain an important fraction of metallic iron of single-domain and superparamagnetic size, generally acquire an anomalously strong viscous remanence, whose decay time is about equal to the time of exposure to the field. Four theories of magnetic viscosity are reviewed in this paper in an attempt to interpret the very different viscous properties of these two types of rocks. The Richter (1937) and Neel (1949) theories are appropriate to soils and low-grade breccias, whereas igneous and recrystallized rocks are better described by the multidomain theories of Neel (1950) and Stacey (1963). Both the Stacey and Neel theories correctly predict logarithmic magnetic viscosity, in spite of the fact that the central role played by the internal demagnetizing field in multidomain grains is ignored in Neel's formulation. This apparent paradox has been resolved. Analysis of the particularly simple case of a two-domain particle shows that the distribution of asymmetrical nonidentical potential barriers required by Neel is automatically generated from the simpler distribution of symmetrical identical barriers proposed by Stacey through the action of the demagnetizing field. Experimental evidence on many facets of viscous magnetization, from terrestrial as well as lunar materials, is reviewed in detail before a final evaluation of the various theories is made. One interesting conclusion is that the magnetic viscosity of multidomain particles, although relatively weak, is still too strong to be explained by displacements of entire domain walls. Either displacements of small wall segments or rotation of pseudo-single-domain moments could account for the enhancement of magnetic viscosity in these particles.

The Neel theory of magnetization relaxation has been systematically and quantitatively tested for six samples containing synthetic and natural single domain grains. The results of a wide range of experiments involving time, temperature, and direct and alternating magnetic fields were compared with theoretical results predicted by using the Neel theory and the experimental grain size-coercivity distribution of each sample. Good agreement was found for many experiments involving small field remanences, where grain interaction effects are negligible. Most of the discordant results could be explained by using either the Preisach theory or an extended form of the Neel theory, which takes account of interactions. The two methods are shown to be equivalent, but the Neel theory is more useful in analyzing thermal processes that are intractable by the Preisach method. The similarity of the behavior of synthetic and natural samples suggests that monodomain material is important in many rocks, but experiments on a wide variety of rock types are needed for verification.