GP31B-0797
Magnetic Susceptibility and Anisotropy of Hematite Single Crystals Determined by Increasing Low-fields and High-field Instruments
Hematite is one of the most important carriers of magnetic remanence in natural rocks. The magnetic properties of hematite bearing rocks have been intensively studied, particularly in the last decade, as instrumentation has become more sensitive, e.g., squid magnetometers and sophisticated vibrating sample magnetometers. The anisotropy of hematite and hematite-bearing rocks is more difficult to evaluate since its magnetic susceptibility is lower than other magnetic minerals such as magnetite. The magnetic anisotropy of hematite, however, is often responsible for inclination shallowing of the remanence in hematite-bearing rocks. It is important to estimate this degree of flattening, so that remanence shallowing can be compensated when making paleoreconstructions of terrains. . The anisotropy of magnetic susceptibility has been measured on a collection of hematite single crystals in increasing low fields between 2 and 450 A/m in high fields up to 1.5 T using magnetic torque . Magnetic torque at 77K has also been measured on the largest crystals of the collection. The magnetic susceptibility ellipsoid and magnetic structure of the crystals have been studied with emphasis on the magnetic properties within the basal plane. Additional rock magnetic characterisation has been obtained from IRM acquisition curves, thermomagnetic curves and magnetic in order to constrain the composition of the samples. Results reveal that the magnetic susceptibility variation for hematite is out of the Rayleigh region for applied fields in commercial instruments. At high fields, the magnetic structure within the basal plane is a combination of biaxial and triaxial structures. The triaxial structure has different intensities probably due to different accumulations of stress within the crystal lattice as it is derived by torque measurements at low temperature.
GP31B-0798
Magnetic Properties of the Precambrian Granitic Rocks in Minnesota
It has been known that granitic rocks have stable components of natural remanent magnetization (NRM) as well as unstable NRM. It is noted that remanent magnetization of plagioclase crystals in granitic and basaltic rocks can yield reliable paleomagnetic data (e.g. Wu et al., 1974; Geissman et al., 1988; Tarduno et al., 2001; Wakabayashi et al., 2006). The acquisition process of thermoremanent magnetization (TRM) of granitic rocks is not well-understood because the size of magnetic grains varies from less than a few μm to hundreds of μm and parts of them are included in each crystal of granitic rocks. Thus we have made rock-magnetic studies and microscopic observations on granitic rocks and their separated crystals. Samples used in this study are collected from multiple sites of the Sacred Heart Granite (2.6 Ga U-Pb zircon ages) and the St. Cloud Granite and Granodiorite (1.8 Ga U-Pb zircon age) in Minnesota. For most of the bulk samples from granitic rocks, the Verwey transition at 120 K is clearly recognized. Susceptibility- temperature (χ-T) curves show an abrupt drop at about 580°C. Hysteresis parameters of bulk samples are distributed along a mixing line between the multi-domain (MD) and pseudo-single-domain (PSD) areas on the Day plot. Saturation isothermal remanence (SIRM) cooling and warming curves indicate that low-temperature memories range in a few to several tens % of the initial SIRM. These results indicate the MD magnetite grains dominate the magnetic properties but more or less PSD (or single-domain (SD)) magnetite grains are present in the granitic rocks. The separated crystals of feldspar and quartz show the Verwey transition at 120 K and the Curie temperature of about 580°C. Hysteresis properties of them are similar to those of bulk samples. These suggest that the MD and PSD (or SD) magnetite are included in both feldspar and quartz, suggesting that those magnetite grains primarily formed during the initial formation of the granitic rocks. We will also present the results of microscopic observations and synchrotron radiation X-ray fluorescence analyses for detecting the shape and size of magnetite grains in the crystals.
GP31B-0799
Simulations of electron holographic observations of magnetic microstructure in exsolved titanomagnetites
Titanomagnetite inclusions in slowly-cooled rocks can contain exsolution microstructures that consist of closely-spaced ferrimagnetic magnetite (Fe3O4) prisms separated by paramagnetic ulvöspinel (Fe2TiO4) lamellae. Off-axis electron holography has recently been used to image the magnetic remanent states of such inclusions, and to show that the prisms are mostly magnetostatically-interacting single domains. The overall magnetic microstructure is found to depend sensitively on the shapes, spacings and orientations of the prisms, as well as on the shape of the inclusion and on its magnetic history. In order to understand the observed magnetic microstructures, we have carried out analytical simulations of linear arrays of uniformly-magnetized magnetite prisms by making use of known expressions for demagnetization factors. By systematically varying the size and spacing of the magnetite prisms in the simulations, it is possible to chart the magnetic microstructures of such mineral assemblages in the form of a "magnetic microstructure phase diagram". Whereas shape anisotropy suggests that the long dimension of each prism should be its easy axis, if more than one element is considered then interactions change the energy balance of the combined magnetic state. We consider linear arrays of identical prisms that are magnetized either in antiparallel directions perpendicular to the line joining them or parallel to each other and to the line joining them. For two 100-nm-wide prisms that are separated by 10 nm in a specimen of thickness 20 nm, the simulations suggest that the transition between the two states occurs at a prism length of 122 nm. A more general treatment that includes the influence of magnetocrystalline anisotropy on the energy balance will be presented, and the simulations will be compared with experimental observations.
GP31B-0800
Fe-Ti oxide inclusions in natural and synthetic (Fex, Mg1-x)2 SiO4 olivines
Olivine commonly hosts micro-inclusions of Fe-Ti oxides, which are generally thought to be exsolved during oxidation, under decreasing temperature and pressure. They consist mostly of ferrimagnetic and antiferromagnetic species characterized by high magnetic susceptibilities, spontaneous magnetizations and magnetic remanences (such as titanomagnetite (Fe2(Fex, Ti1-x)O4), although ilmenite (FeTiO3) has also been reported. They contribute significantly to the magnetic properties of the host grain and need to be isolated to measure the paramagnetic (or diamagnetic) properties of the silicate host only. Conversely, the magnetic remanence of ferromagnetic inclusions can yield valuable information regarding ancient planetary magnetic fields. The presence of such inclusions can be documented by electron microscopy techniques such as backscattered electron imagery. When they are large enough, their composition can be determined using electron microprobe analyses. Methods - The olivine samples of this study are both natural and synthetic and were studied under high field conditions using a Vibrating Sample Magnetometer (VSM) and a Magnetic Properties Measurement System (MPMS). At room temperature and low field conditions, the induced magnetization of a purely paramagnetic- diamagnetic assemblage (silicate host) should vary linearly with the applied field. Any departure from linearity indicates the presence of a ferromagnetic component (excluding contaminations). The saturation field (Hsat) above which ferromagnetic inclusions become saturated is a critical piece of information albeit difficult to quantify accurately. Also, the presence of small Fe-Ti oxide inclusions can be betrayed when the directional magnetic properties of the host silicate depart significantly from those predicted by the molecular field theory. Results - Most synthetic samples (synthesized near the QFM buffer) exhibit a mixture of paramagnetic and ferromagnetic behavior, with a ferromagnetic/bulk ratio comprised between 3 and 72%. Most samples exhibit properties consistent with SD to PSD titanomagnetite while a few display SP properties (no hysteresis). Natural samples were selected to be free of visible inclusions with optical microscopy (for the purpose of a study on silicates). About 30% of these samples host detectable amounts of ferromagnetic inclusions. The saturation field Hsat of synthetic samples can be as high as 1.15 Tesla which is higher than what would be expected for magnetite (Hsat (≤) 0.2). This could be explained by crystallographically oriented networks of SD magnetite exsolutions or by the presence of maghemite (oxidized magnetite).
GP31B-0801
Acquisition of thermal remanent magnetization by exsolved silicate-hosted titanomagnetite inclusions: Implications for determining paleodirection and paleointensity
Titanomagnetite inclusions exsolved in silicate minerals are a powerful tool for exploring the Earth's magnetic field behavior because they are protected from chemical alteration by their silicate hosts and are ubiquitous in Precambrian terranes composed of gabbros and anorthosites. Yet the extreme shape anisotropy of these needle-shaped inclusions creates the potential for remanence anisotropy and nonlinear remanence acquisition. To better determine the extent of these processes, we measured the thermal remanence magnetization (TRM) acquired by single-crystal samples of clinopyroxene (Messum Complex, Namibia) and plagioclase (Bushveld Complex, South Africa) at four field strengths (20,50,100, and 140 μT) in 6 orientations (positive and negative X, Y, and Z). Mineral alteration during thermal cycling was negligible and was monitored by measuring saturation isothermal remanent magnetization (SIRM) before and after each TRM step. The 6 three-dimensional magnetization vectors recorded at each field strength were used to calculate a least-squares, best-fit TRM tensor. In general, the clinopyroxene-hosted inclusions showed significantly higher anisotropy than those in plagioclase. Clinopyroxene grains displayed a strongly oblate anisotropy (2 < F < 128, 13 < P < 672), while plagioclase grains showed a comparatively more subdued prolate anisotropy (0.5 < F < 5, 1 < P < 11). The degree of anisotropy associated with each silicate phase is a consequence of the number of inclusion orientations in clinopyroxene (n = 2) and plagioclase (n = 5). The tensors' eigenvalues and eigenvectors for all samples were unaffected by variations in applied field strength. Samples acquired magnetization linearly up to fields of 100 μT, at which point samples approached saturation remanence. Additionally, we note that the slope of linear remanence acquisition was dependent on the crystallographic orientation of the single crystals. These results demonstrate that single crystals of clinopyroxene and plagioclase do exhibit strong remanence anisotropy. Despite this, their linear remanence acquisition up to 100 μT fields ensures that they will be useful recorders of paleointensity for most periods of geomagnetic behavior. Monte Carlo simulations using average anisotropy tensors allowed us to determine how many randomly oriented crystals are needed in a standard bulk sample to overcome the single-crystal anisotropy and accurately record the Earth's magnetic field direction. Thus, so long as a large enough population of grains is contained within a single bulk sample, their extreme remanence anisotropy can be overcome, and accurate paleodirections and paleointensities can be obtained.
GP31B-0802
Dislocations in single-domain grains: Linking stress fields to magnetic hysteresis properties
Among the single-crystal properties that can affect magnetic properties are dislocations - imperfections in the crystal structure that impose internal stresses on a crystal. Dislocations are known to pin domain walls in large ferromagnets and pinning may be the main source of magnetic hysteresis in such crystals. However, almost nothing is known about their effect on magnetic properties in submicron crystals, and there have been very few experimental investigations of dislocations in this size range. The source of domain wall pinning is a coupling between the stress field around a dislocation and the magnetization, giving rise to a local anisotropy. The stress is greatest close to the core of the dislocation, so there could be a large effect in small crystals. So far, there have been no published attempts to calculate this coupling in submicron crystals. The main impediment to such models is the boundary value problem for the stress. Numerical solutions have been difficult to obtain because of the singularity in the surface traction where a dislocation meets the surface. We show how a new method developed at Lawrence Livermore Laboratories solves the boundary value problem, allowing us to calculate the stress field around a dislocation in small octahedral particles. The effect on the magnetic anisotropy is also determined. This will be incorporated into a micromagnetic model, allowing us to determine the effect of dislocations on magnetic hysteresis.