GP31A-0785
Thermomagnetic Analysis of Surface Sediments From the Suburb of Beijing, and its Environmental Significance
High- and low-temperature thermomagnetic and Lowrie three-axis thermal demagnetization experiments have been conducted on surface sediments from the suburb of Beijing. The studied sedimentary sequence consists of pluvial and lacustrine silts and clay of the Holocene age. The Morin transitions at ~230 K, Verwey transitions at ~110 K, and the transition below 50 K indicated from the low-temperature magnetic experiments conducted on selected samples reveal the presence of hematite, magnetite and pyrrhotite, respectively. The Lowrie three-axis thermal demagnetization experiments also indicate the presence of hematite, magnetite and pyrrhotite. The soft, medium and hard coercivity components were all unblocked at 675° Celsius, the Neel point of hematite, which indicates that hematite contained in the section can acquire remanence in the wide range of field intensity from <50 mT to 2.4 T. The wide coercivity spectrum of hematite in the samples may indicate a wide size spectrum, which may include fine and coarse grains from different source. The magnetic susceptibility of samples at depths below 22 cm increased abruptly when heated to 240- 280° Celsius, which is attributed to the λ transition. The occurrence of λ transition on the χ-T curves reveals the existence of hexagonal pyrrhotite (Fe9S10). The parameters χ280-χ240 and (χ280-χ240)÷χ240 (χ280 and χ240 are the magnetic susceptibility values at 280° and 240° Celsius on the χ- T heating curves, respectively) can be used as crude measurements (representing the absolute and relative concentration, respectively) of the antiferromagnetic mineral hexagonal pyrrhotite at room temperature. Pyrrhotite doesn't appear at the top 20 cm but occurs abruptly below the depth of 22 cm at the section, which may suggest the (ever) existence of redox interface.
GP31A-0786
Hysteresis and Coercivity of Multidomain Hematite
In multidomain hematite, crystal defects lay a major role in hysteresis and coercivity by hindering the motion of domain walls. The kinds of defects that can pin walls are dislocations and growth and deformational twins and twin boundaries. Multidomain hysteresis is also affected by wall nucleation which generally occurs at irregular surfaces such as voids, cracks or at growth steps in the crystal surface. The temperature dependence of Hc is different for nucleation and for various types of defect pinning. The most rapid variation is Hc(T) varying as K(T)/Ms(T), where K is magnetocrystalline anisotropy constant, due to domain nucleation or magnetocrystalline controlled domain wall pinning. Pinning due to the stress field of dislocations or planar defects results in a slower temperature variation: Hc(T) varying as lambda(T)/Ms(T), where lambda is magnetostriction constant. Hc and Ms were measured as a function of temperature in mm-size single crystals of hematite using a PMC MicroVSM. The experimental Hc(T) data varied as the power 1.8-2.4 of Ms(T) between 400 and 625°C. Flanders and Schuele (1961) reported that K(T) varied as the 10th power of Ms in a large single crystal between 20 and 500°C. The magnetostriction constant of hematite has not been measured directly as a function of temperature. Nevertheless, it is reasonable to expect that the magnetostriction of hematite, as in other materials, should have a much weaker power-law dependence on Ms than does the magnetocrystalline anisotropy constant. In the present multidomain hematite crystals, the observed weaker power-law index of 1.8-2.4 indicates that the coercivity is mainly magnetoelastic in origin.
GP31A-0787
Stability Of Magnetic Remanence In Grains With Irregular Surfaces
Naturally occurring magnetic minerals, such as magnetite, often have morphologies that depart from their ideal crystalline shapes. A spectrum of irregular grain geometries is possible, from simple distorted surfaces of an octahedron to that of the extreme case of the "Christmas tree" patterns formed by constitutional supercooling of mixed metal oxides that can occur in submarine basalts. In this presentation we will examine the domain structures and stability of magnetic remanences that such irregular grain structures may have. We use a three-dimensional finite-element/boundary-element micromagnetic model, which provides the flexibility to generate the exact geometry required, unrestricted by the regular cell structure of finite difference models. Two sets of results will be presented. The first examines the effect of irregular surfaces on spherical grains of magnetite. The spherical grains are free from any form of shape or configurational anisotropy, so that the effects of the irregular surfaces are easily quantified. The surface irregularities are examined in terms of the amplitude inside or outside the mean surface radius, and their frequency distribution over the sphere. The second set attempts to reconstruct the dendritic geometries of iron oxides often observed in oceanic basalts that have been rapidly quenched.
GP31A-0788
Greigite magnetofossils of Pliocene age
Magnetotactic bacteria build tiny chains of magnetite and/or greigite that function as permanently-magnetised nano-magnets, which they utilise to navigate along the geomagnetic field lines to reach their preferred habitat. Magnetite magnetosomes are found in both ancient and recent sediments, but, to date, no fossil greigite magnetosomes have been reported, although magnetosomal greigite from recent environments is documented since 1990. We report the first evidence for fossil greigite magnetosomes in 5 million years old sedimentary rocks. Transmission electron microscopy, electron diffraction patterns and rock magnetic analysis revealed single domain, stoichiometric greigite particles, with a few crystallographic defects and shape consistent with crystals produced by magnetotactic bacteria. We conclude that these are magnetofossils that carry a primary remanent magnetisation, stable over geological times. The same rock samples also contain a second, distinctly different, population of larger (400-1,000 nm) octahedral crystals of authigenic greigite, formed deeper in the sediment. Close to polarity transitions, the greigite could be shown to have recorded the palaeomagnetic field after the main phase of compaction, with a delay on the order of several thousands of years. Our results indicate that fossil magnetosomal greigite carries a primary natural remanent magnetisation, stable over geological times. This opens new venues for obtaining reliable records of the ancient geomagnetic field from sedimentary records previously considered unsuitable.
GP31A-0789
The Effect of Nanoscale Intergrowths on Palaeomagnetic Interpretations
Magnetic minerals are abundant within our Earth's crust and can retain, through one of a number of processes, a remanent magnetisation induced by the Earth's magnetic field. Analyses of palaeomagnetic samples have been used for the past fifty years to improve our understanding of many of the Earth's major processes. Recent studies utilising newly developed imaging techniques, namely holographic transmission electron microscopy (HTEM), have for the first time allowed direct observations of the magnetic structure in palaeomagnetic samples on a nanoscale. It is commonly observed that igneous rocks contain closely packed magnetic lamellae with a non-magnetic matrix, a result of the chemical process of exsolution. However, the results of current micromagnetic models, generated to predict the magnetic structure within such samples, are not in agreement with these direct observations. The results do, however, show strong similarities to such observations, and the discrepancies indicate a lack of understanding of the magnetic interactions within such samples. A predictive micromagnetic model capable of examining multiphase materials has been produced. This model allows us to examine the stability of remanences within multiphase materials by utilising results from simulated hysteresis measurements. With such a model we can investigate the reliability of current palaeomagnetic samples; in particular the effect of differing magnetic ordering of individual phases where the magnetic ordering is related to the temperature of the phases. This is particularly important when considering the methods by which palaeomagnetic samples are cleaned to remove unwanted multi-domain effects.
GP31A-0790
Suitability of chondrules for studying the magnetic field of the early solar system: an examination of synthetically produced dusty olivine
Chondritic meteorites are rare, yet incredibly valuable windows into the geophysical and geochemical environment of the early solar system. Dusty olivine grains containing exsolved nanometer-scale iron-nickel alloy inclusions are present in many chondritic meteorites and their remanent magnetization may give insight into the strength of the solar dynamo at the time of chondrule formation. Laboratory methods for determining the paleointensity of these rare materials must be optimized prior to conducting experiments on actual meteorite samples. To this end, we have used high temperature recrystallization techniques to produce synthetic dusty olivine samples with textures remarkably similar to those observed in chondritic meteorites. The olivine grains used in these annealing experiments are from the 13 kya Haleyjabunga picritic basalt flow in Iceland and have compositions of Fo90, which closely resembles the olivine composition observed in chondritic meteorites. Samples were annealed at 1350°C either under vacuum in the presence of graphite or under controlled oxygen fugacity using pure CO gas. The laboratory-produced magnetic mineral assemblages in two sets of samples have been characterized using low and high temperature remanence and susceptibility measurements, hysteresis loops, FORC diagrams, and scanning electron microscopy. The room-temperature remanence properties of these materials have been explored using stepwise IRM and ARM acquisition and alternating field demagnetization. These synthesis techniques allow us to produce a wide rage of iron-nickel grain sizes with correspondingly large variations in coercivity (between 0 and 500 mT). High temperature measurements of saturation magnetization show that both samples reach their Curie temperatures at ~760°C, consistent with kamacite, a low-Ni high-Fe metal alloy. Multiple experiments have shown that care must be taken to rigorously control the atmosphere in which the samples are heated and cooled in order to avoid forming trace amounts of magnetite on the surface of the samples. Future research will explore the feasibility of using modified Thellier protocols to determine the paleointensity of laboratory-induced thermoremanent magnetizations.
GP31A-0791
Application of the Raman Spectroscopy to Identification of Titanomagnetites
The titanomagnetite-magnetite series serves as important magnetic carriers in paleomagnetic studies. Commonly Curie points are used to determine the composition of these magnetic phases. However, Curie points generally reflect bulk rock properties and do not provide insight for individual grains. Determination of individual Fe-Ti oxide grains can be done by petrography or with electron microprobe techniques. In contrast to these methods, which require special sample preparation, Raman spectroscopy can be done with minimal preparation. We have therefore investigated the Raman spectra for magnetite, TM20, TM40, and TM60, so that we can identify titanomagnetites with different Ti content in rocks. The samples were prepared following methods of Wanamaker and Moskovitz (1994). The Ti concentrations were verified by thermomagnetic analysis (Gilder and LeGoff, 2005). The Raman spectra were obtained with WITec Raman Confocal Microscope 200 using a green laser. Slight shifts and changes in relative intensities of the peaks at the characteristic wavelengths of the Raman spectra that correspond to different molecular vibrations were observed. These changes can serve to identify the composition of individual titanomagnetite grains. It also enables mapping of the variation of the composition within grains and the distribution of compositions of grains within a rock. We have applied the technique to synthetic Martian samples and found relatively uniform compositions between different grains. In contrast, variable oxide compositions are discerned using the Raman spectroscopy in natural basalts erupted from East Maui volcano.
GP31A-0792
Environmental Proxies: Characteristics of Al-Doped Ferrihydrite
Ferrihydrite is an important nanometer-sized mineral and is the precursor material for many other iron oxides during soil production. In addition, the stability of ferrihydrite and other iron oxide minerals can be profoundly affected by microbial respiration. Therefore, the presence and amount of iron oxides can potentially act as proxies of environmental change, including the presence of iron reducing bacteria. Natural ferrihydrite often exists with significant amounts of cation substitution (e.g., aluminum substituting for iron). However, little is known about their effects on the characteristic properties of the iron oxide. Due to the poorly crystalline nature and the nano-scale size of ferrihydrite particles, investigating their structure is difficult using standard characterization techniques. Alternatively, by employing low temperature magnetic measurements, variations in crystal structure and trends in particle-particle interactions can be determined. In addition, information from X-ray diffraction (XRD) and transmission electron microscopy (TEM) were also used to complement the magnetic data obtained. The observed trends in the magnetic parameters of Al-doped ferrihydrite allow us to determine magnetic size variation in the range of 3 to 7 nm, recognize lattice disorder created by Al adsorption and/or substitution, and distinguish between discrete and aggregated particles. Understanding these magnetic parameters allows for the development and calibration of a 'toolbox' of environmental proxies.
GP31A-0793
A new method of calculating magnetic hysteresis and thermal effects
All the easy theoretical problems in rock magnetism have been solved. Further advances involve models of hysteresis and thermal fluctuations in increasingly complex systems. To model the physics of such systems accurately, rock magnetists will need to solve two seemingly unrelated technical challenges. First, for models of thermal relaxation (with applications to magnetic viscosity, thermoremanent magnetization and so on), they will need to locate energy barriers in multidimensional parameter spaces - a difficult and computationally expensive problem. Second, they will need to improve calculations of magnetization curves near jumps, where the slope of the curve almost always approaches infinity. Micromagnetic models have trouble locating jumps accurately. A new algorithm is presented that tackles both of these problems while greatly reducing the time needed to calculate magnetization curves. The most novel aspect of this algorithm is the way it treats the magnetic field. Other methods solve for the magnetization at a given magnetic field, change the field, and then solve for the magnetization again while the field is kept fixed at the new value. The result is an equilibrium curve in a space of magnetization and field components. The new algorithm treats the field the same way as it treats the magnetization. It uses a tangent to the magnetization curve to estimate the next point on the curve and a corrector algorithm that refines the estimate. There is also an adaptive algorithm to determine an appropriate distance to step along the curve. This algorithm is much more robust near jumps. It can also calculate energy barriers because the top of an energy barrier (a saddle point) is on the same solution curve as the stable magnetization states. Previous methods could not follow the solution curve past the point of instability, but this method can because of the way it handles the field. This makes it possible to calculate energy barriers much faster and more accurately than any previous method. Applications to interacting fine particles will be presented.
GP31A-0794
Magnetic characterization of dispersed magnetite nanoparticles with variable particle size and surface functionalization
The enhanced properties of magnetic nanoparticles with respect to their bulk counterparts led to their application in environmental science, geology, biomagnetism and medicine. From a fundamental aspect, the investigation of these nanoparticles enables us to understand the effect of size and surface functionalization on magnetization. Magnetite nanoparticle is often the material of choice for such applications due to its superparamagnetic character and high magnetization. In this work, magnetite nanoparticles of controlled particle size, ranging from 6 to 20 nm in diameter, were synthesized by thermal decomposition of an iron precursor. The nanoparticles were functionalized with different coating agents bearing either carboxylate, catecholate, or phosphate anchoring groups. The magnetic behavior of the particles was studied by hysteresis loops, ac and dc low temperature measurements, Mössbauer spectroscopy and transmission electron microscopy (TEM). The ac and dc results indicated increasing blocking temperature with increasing particle size. The typical high magnetization for a ferrimagnetic material and Verwey Transition was observed. The data fitted to Mössbauer spectra at 4.2 K suggest the presence of a mixture of magnetite and maghemite. The effect of the different anchoring groups on the magnetization of the particles will also be presented.
GP31A-0795
Electron Holography of Oxy-Exsolution Microstructures in Synthetic Titanomagnetites
Intergrowths of titanomagnetite and titanohematite or ulvospinel resulting from oxy-exsolution or spinodal decomposition are regularly observed in paleomagnetic specimens. This microstructure is thought to be responsible for anomalous paleointensity measurements of historic lavas and experiments have shown that bulk magnetic properties are dramatically affected by the creation of these structures. However, previous studies have not been able to rigorously measure the effects of the geometry and composition of the magnetic phases. Electron holography, a TEM technique that allows the quantitative measurement of the magnetic flux at a nanometer scale, provides an opportunity to investigate how these variables influence magnetostatic interactions and inform our interpretation of magnetic data from rock samples showing these textures. The samples used in this study are synthetic polycrystalline spinels with compositions spanning the magnetite-ulvospinel solid solution, which were re-sintered under oxidizing conditions to create oxy-exsolution structures of paramagnetic titanohematite lamellae separating blocks of ferrimagnetic titanomagnetite. The composition of the titanomagnetite is governed by the starting material and oxygen fugacity during the re- sintering process; using the appropriate conditions we were able to exsolve samples with compositions approaching pure magnetite, and the size of the ferrimagnetic particles are pseudo-single-domain to multidomain. Electron holography was carried out on thinned samples to measure the magnetic flux between these interacting particles. Measurements of these magnetostatic interactions will inform micromagnetic models of interacting pseudo-single-domain particles, and future work will allow us to more carefully control the width of the paramagnetic lamellae and hence inter-particle spacing to accurately interpret the effect of these microstructures on the properties of natural paleomagnetic samples.
GP31A-0796
Oxidation Dynamics and their Effects on Texture and Magnetic Response in Remelted and Quenched Submarine Basaltic Glass
As part of an experimental and observational study of the magnetic response of submarine basaltic glass (SBG), we have examined -- using ion backscattering spectrometry (RBS), transmission and scanning electron microscopy (TEM; SEM), energy-dispersive X-ray spectrometry (EDS) and surface X-ray diffraction (XRD) -- the textures wrought by the controlled oxidation of glasses prepared by the controlled-environment remelting and quenching of natural SBG. Initial compositions with 9 < FeO*(wt%) < 12 were melted at 1700K with the oxygen fugacity buffered at fayalite-magnetite-quartz (FMQ); melts were cooled at rates in the range 0.08-2.30 K s-1 in air and, separately, in FMQ atmospheres; oxidation experiments involved reheating polished pieces of this reprocessed glass in dry air to temperatures of 425K, 875K or 1000K for times ranging from ~10 to >1000 h. (The glass transition temperature of the reprocessed glass is ~950K.) RBS demonstrates unequivocally that the dynamics of oxidation involves the outward motion of network- modifying cations. Highest temperature oxidation results in formation of nm-ƒİm-scale ferrites (close to pure magnetite) on the free surface, devoid of ionic Ti; lowest-temperature oxidation sees formation of a Na- enriched phase on the surface. The highest-T oxidation specimens have magnetizations elevated by one-to- two orders of magnitude relative to the as-quenched material. Specimens are being evaluated for internal structural/textural change accompanying the internal chemical changes associated with the oxidation dynamic.