A theoretical model of single-domain (SD) grain sizes is applied to magnetite and titanomagnetite. In this model, transition to a two-domain configuration takes place at the SD threshold d0. This two-domain configuration is shown to be more applicable to fine-grained magnetites in igneous rocks than previous models involving transition to a circular spin configuration at d0. Calculations of the stable SD grain size range were accomplished by calculating the superparamagnetic threshold size ds by Neel's relaxation equation and calculating the SD threshold d0 at which SD to two-domain transition occurs. For cubic magnetite particles the SD range is extremely narrow and occurs at very small grain size. At room temperature, ds ≃ 0.05 μm, and d0 ≃ 0.076 μm. For cubic magnetite particles just above d0 a two-domain configuration is predicted in which a 180° domain wall occupies ∼60% of the particle volume. No SD range exists for cubic magnetites at T > 450°K. These results are in good agreement with experimental determinations of SD limits in equant magnetites and also agree with experimental observations of thermoremanent magnetization in submicron pseudo-single-domain (PSD) magnetites. The SD range increases rapidly with particle elongation. For a length : width ratio of 5 : 1, SD limits of ds ≃ 0.05 μm and d0 ≃ 1.4 μm are calculated. Both d0 and the SD range for titanomagnetites (Fe3−x Tix04) increase with Ti content. For cubic titanomagnetites of x = 0.6, ds ≃ 0.08 μm, and d0 ≃ 0.3 μm. Comparison of the calculated SD range with the available high-resolution grain size distributions of opaque grains in igneous rocks suggests that elongated SD grains or submicron PSD grains are the major carriers of stable natural remanence in igneous rocks.

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Theoretical single-domain grain size range in magnetite and titanomagnetite

It is demonstrated that the stability against af demagnetization of TRM carried by multidomain magnetite increases with increasing strength of the inducing field. This multidomain field-dependence trend of stability against af demagnetization is opposite to that of single-domain particles. A simple test to distinguish multidomain and single-domain carriers of remanence is designed that makes use of the distinctive af demagnetization characteristics. The test was applied to a variety of igneous rocks. None of these showed truly multidomain characteristics. Thus, although some TRM is undoubtedly multidomain in these rocks, the TRM is dominated by single-domain carriers. These results are consistent with the growing evidence of the importance of very fine grained magnetite as a carrier of NRM in many igneous rocks.

On the alternating field demagnetization characteristics of multidomain thermoremanent magnetization in magnetite

Particle size distributions have been obtained by grain counts on electron micrographs of four samples containing submicroscopic equant magnetite particles. Blocking temperature data indicate thermally unstable (superparamagnetic, SPM) magnetic behavior in one sample whose grains range from 100 to 650 A in size. The SPM is confirmed by a large increase of saturation remanence when the hysteresis of the sample is measured at 77°K. Quantitative estimates of the SPM, single-domain (SD), and multidomain (MD) magnetization fractions in this sample are made by comparing theoretical and observed values of the reduced saturation remanence at 77° and 300°K. The particle size distribution then yields a critical size ds = (290 − 360) ± 50 A for the onset of SPM in magnetite at room temperature, in close agreement with Neel's SD theory. The critical size d0 at which domain structure develops in equidimensional grains of magnetite is less well defined. It is certainly less than 480 ± 50 A and may even be less than ds; that is, a direct MD-SPM transition in magnetite is a distinct possibility. The extreme narrowness of the stable SD range in equidimensional magnetite particles suggests that either elongated SD grains or else small MD grains with pseudo-SD characteristics must be the major carriers of the high-stability natural remanence of igneous rocks.

Superparamagnetic and single-domain threshold sizes in magnetite

Interacting vs. non-interacting single domain behavior in natural and synthetic samples

[1] The basic features of the geomagnetic reversal chronology of the last 160 million years are well established. The relationship between this history and other features of the field, however, has been elusive. The determination of past field strength (paleointensity) is especially challenging. Commonly accepted results have come from analyses of bulk samples of lava. Historic lavas have been shown to faithfully record the past field strength when analyzed using the Thellier double-heating method. Data from older lavas, however, tend to show effects of in situ and laboratory-induced alteration. Here we review an alternative approach. Single plagioclase crystals can contain minute magnetic inclusions, 50–350 nm in size, that are potential high-fidelity field recorders. Thellier experiments using plagioclase feldspars from an historic lava on Hawaii provide a benchmark for the method. Rock magnetic data from older lavas indicate that the feldspars are less susceptible to experimental alteration than bulk samples. This resistance is likely related to the lack of clays. In addition, magnetic minerals are sheltered by the encasing silicate matrix from natural alteration that can otherwise transform the well-defined thermoremanent magnetization into an irresolute chemical remanent magnetization. If there is a relationship between geomagnetic reversal frequency and paleointensity, it should be best expressed during superchrons, intervals with few (or no) reversals. Thellier data sets based on single plagioclase crystals from lavas erupted during the Cretaceous Normal Polarity Superchron (∼83–120 million years ago) suggest a strong (>12 × 1022 Am2), stable field, consistent with an inverse relationship between reversal frequency and paleointensity. Superchrons may represent times when the pattern of core-mantle boundary heat flux allows the geodynamo to operate at peak efficiency, as suggested in some numerical models. Thellier data from single plagioclase crystals formed during times of moderate ( 10 reversals/million years) reversal occurrence suggest a weaker and more variable field. These paleointensity data, together with a consideration of paleomagnetic directions, suggest that geomagnetic reversals, field morphology, secular variation, and intensity are related. The linkages over tens of millions of years imply a lower mantle control on the geodynamo. On even longer timescales the magnetization held by plagioclase and other silicate crystals can be used to investigate the Proterozoic and Archean geomagnetic field during the onset of growth of the solid inner core. Data from plagioclase crystals separated from mafic dikes, together with directional data from whole rocks, indicate a dipole-dominated field similar to that of the modern, 2.5–2.7 billion years ago. Older Archean rocks are of great interest for paleomagnetic and paleointensity investigations because they may record a time when the compositionally driven convection of the modern dynamo may not have been operating and a solid inner core did not play its current role in controlling the geometry of outer core flow. Most rocks of this age have been affected by low-grade metamorphism; investigations using single silicate grains provide arguably our best hope of seeing through secondary geologic events and reading the early history of the geodynamo. Absolute paleointensity measurements of the oldest rocks on the planet will require the further development of methods to investigate silicate crystals with exsolved magnetic minerals that address the uncertainties posed by thermocrystallization remanent magnetization, anisotropy, and slow cooling. Fortunately, prior work in rock magnetism, together with advances in analytical equipment and techniques, provides a solid foundation from which these frontier issues can be approached.

https://www.eoas.ubc.ca/~mjelline/453website/eosc453/E_prints/newfer06/2005RG000189.pdf

The paleomagnetism of single silicate crystals: recording geomagnetic field strength during mixed polarity intervals, superchrons, and inner core growth

Measurement of natural, thermal, and saturation remanent magnetism and their stability with respect to alternating field and thermal demagnetization of concentrates of plagioclase, pyroxene, and oxide (80 percent ilmenite, 20 percent magnetite) separated from a sample of diabase from the Triassic Lambertville (N. J.) sill, reveal that the stable remanence of the whole rock is associated with the silicates and especially with the plagioclase. Measurement of Curie point and saturation moment of the feldspar indicates the presence of about 0.3 percent Fe ion in solid solution and 0.004 percent pure magnetite as a discrete phase. This magnetite cannot be identified microscopically, nor with an electron probe, and it is inferred to be present as submicroscopic, single domain particles.

R. B. Hargraves

Papers

Source of stable remanent magnetism in Lambertville diabase