GP21C-01 INVITED 08:00h
Magnetism and Microscopy: Applications to mineral magnetism at the nanometre scale
Several recent advances in electron microscopy are poised to revolutionise mineral magnetism over the next five to ten years. In this talk I review some of these advances and their application to the study of mineral magnetism at the nanometre scale. Arguably the most significant advance is the application of off-axis electron holography, a technique that yields a two-dimensional vector map of magnetic flux with nanometre resolution. The technique is capable of imaging the magnetization state within individual magnetic particles, as well as the magnetostatic interaction fields between neighbouring particles. Most imaging modes in a TEM suffer from the same drawback: the final recorded image is a spatial distribution of intensity; all information about the phase shift of the electron waves passing through the sample is lost. Electron holography provides an interference pattern from which the phase information can be recovered. Once corrections have been made for variations in sample thickness and mean inner potential, the gradient of the phase shift is proportional to the magnetic flux. Contour lines placed on a holographic image provide a quantitative image of the lines of magnetic flux with approaching nanometre spatial resolution. TEM offers two possibilities to yield compositional information with nanometre spatial resolution. Characteristic X-rays can be analyzed using conventional energy dispersive X-ray analysis (EDX) with a spatial resolution of around 10-20 nm. Alternatively, one can measure the intensity of electrons transmitted through the sample as a function of their energy loss. The resulting electron energy loss spectrum (EELS) contains a number of ionization edges. Using a post-column imaging filter it is possible to form an image of the sample using electrons that have suffered a specific energy loss. This technique can be used to produce quantitative elemental distribution maps with a spatial resolution approaching 1 nm or less. The shape of the Fe L$_{2-3}$ edge in an EELS spectrum is highly sensitive to the valence state of Fe, and can be used to determine the Fe$^{2+}$/Fe$^{3+}$ ratio to an accuracy approaching that of Mössbauer spectroscopy. Nanoscale microstructures in the ilmenite-hematite solid solution are thought to be responsible for lamellar magnetism in slowly-cooled metamorphic rocks. We present the results of preliminary experiments to test the lamellar magnetism hypothesis directly using off-axis electron holography. The magnetization of a hematite host containing fine scale ilmenite precipitates has been compared quantitatively with the magnetization of a region containing no exsolution lamellae. The magnetization is determined from the holographic phase shift accumulated across each region. Analysis yields estimates of 3.8-4.7 mT for the exsolved region and 0.4-0.7 mT for the precipitate-free region, corresponding to an enhancement in magnetization by a factor of $\sim$ 5. This is much lower than the theoretical maximum enhancement factor of $\sim$ 22, indicating that there is a $\sim$ 60:40 ratio of in-phase to out-of-phase lamellae. These results represent some of the smallest magnetic fields ever quantified using any technique at this spatial resolution, and further demonstrate why holography is at the forefront of attempts to understand mineral magnetism at the nanometre scale.
GP21C-02 INVITED 08:15h
Lamellar Magnetism: A New Magnetic Substructure?
Nearly 1 billion year old samples from Proterozoic terranes in Norway, Sweden and USA, contain finely exsolved members of the hematite ilmenite (Fe2O3 FeTiO3) series. Samples have strong and extremely stable remanent magnetization, suggesting an explanation for some magnetic anomalies in the deep Earth and on planetary bodies that no longer produce a magnetic field. Due to the high thermal stability and coercivity of these samples, understanding the nature of the magnetization may have commercial applications. Common to all samples are grains of ilmenite or hematite with multiple generations of exsolution lamellae. Observations of exsolution have been made at high resolution in transmission electron microscopy and, using images produced by electron energy loss spectroscopy showing exsolution ranging down to (1 to 2 nm) about that of one six-layer unit cell of a rhombohedral oxide. Images also show that the interfaces of the finest lamellae are coherent and have considerable lattice strain, a feature that may enhance coercivity and unblocking temperatures. Atomic simulations of the nanoscale exsolutions has led us to propose a new ferrimagnetic substructure created by ferrous ferric `contact layers' that reduce charge imbalance along lamellar contacts between antiferromagnetic hematite and paramagnetic ilmenite. Under perfect magnetic in phase-conditions, we estimate that such a lamellar magnetic material could have a saturation magnetization up to 150 kA/m, 70 times stronger than pure hematite, while retaining the high coercivity and thermal properties of single-domain hematite. Current research is focused on rock-magnetic experiments at low and high temperatures, observational and analytical TEM, effects of pressure and compositional variations on the Fe2O3 FeTiO3 phase diagram, with emphasis on the magnetic phases, crystal-chemical reconstructions, and additional atomic simulations of lamellar interfaces.
GP21C-03 08:30h
Shock-Induced Demagnetization of Pyrrhotite and Implications for the Martian Crust
Maps of the remanent magnetic field of Mars show demagnetized zones within and around giant impact basins. It is possible that vast regions of the Martian crust were demagnetized due to a phase transition of the magnetic carriers induced by a shock wave. This hypothesis is supported by the fact that around the Hellas and Argyre basins, the magnetized and unmagnetized zones are separated by a peak shock pressure contour line between 1 and 3 GPa. Static pressure experiments at room temperature have indicated that pyrrhotite (Fe$_{7}$S$_{8}$) undergoes a phase transition from ferrimagnetic to paramagnetic at $\sim$2.8 GPa, with rapid loss of magnetization above 1 GPa. Although pyrrhotite is not a major magnetic phase on Earth, it is a common carrier of magnetization in Martian meteorites. No previous experiments have demonstrated that shocks below 3 GPa can induce this phase change and demagnetize pyrrhotite. To investigate this possibility, we performed shock recovery experiments on pyrrhotites (Mrs/Ms$\sim$0.7) using a gas gun to simulate natural impact events. The experiments were preceded and followed by a suite of material and magnetic characterization measurements (including X-ray diffraction, magnetic hysteresis and low temperature magnetism) to assess the effects of the shock on the crystallographic and magnetic properties of the pyrrhotite. We will present results from experiments achieving shock pressures in the range of 1 to 4 GPa. These experiments serve as an analogue for the demagnetization of crustal rocks on Mars.
GP21C-04 08:45h
The Verwey Transition Under Nonhydrostatic Stress
Because the Verwey transition in magnetite is a coherent, reversible transformation that is characterized by a change in shape as well as volume, its thermodynamic phase boundary will be sensitive to nonhydrostatic stress. Synchrotron X-ray diffraction experiments by Wright [2002] imply that, as the low-temperature structure forms, the lattice extends 0.31 percent along a direction that departs by 3.7 degrees from the [111] cube diagonal and lies in the plane containing the monoclinic c-axis and bisecting the a- and b-axes. The lattice also contracts by about 0.15 percent perpendicular to that direction in the same plane and by about 0.12 percent perpendicular to that plane. Because these principal transformation strains involve both extension and contraction, the volumetric strain is considerably smaller than any of the principal strains in absolute value. Thus nonhydrostatic stress can have a larger effect on the Verwey transition temperature per GPa than hydrostatic pressure. Using Shepherd et al.'s [1991] entropy of transition of -5.91 J/mol K, thermodynamic theory predicts changes in the Vervey temperature of -23.3, 9.1, and 11.1 K/GPa for compression along each of the principal directions, respectively. This compares with -3.2 K/GPa for hydrostatic pressure. Internal nonhydrostatic stresses due to impurities and dislocations, as well as those generated by differential thermal contraction in igneous rocks, can be large, so their effect could be significant. Whether nonhydrostatic stress contributes significantly to the scatter in observed Verwey temperatures remains to be seen. One would think that as a crystal of cubic magnetite cools under nonhydrostatic stress, it would be transform to the most favorable twin orientation of the low-temperature monoclinic phase (the twin with the highest calculated Verwey temperature), unless physical constraints prevent it from doing so. In such case nonhydrostatic stress could only raise the Verwey temperature.
GP21C-05 09:00h
Low Temperature Demagnetization of Chemical Remanent Magnetization in Magnetite
Chemical remanent magnetization (CRM) has been studied for partially oxidized submicron synthetic (270 nm) and natural (500 nm) magnetites, before and after low-temperature demagnetization (LTD), which consists of zero field cycling through the Verwey transition. CRM's were produced at constant temperatures of 100, 150 and 200 C in controlled field. The intensity of CRM increased as the temperature of the run increased for both samples. AF demagnetization curves were soft before LTD, but became harder after LTD. The CRM memories have a much more SD-like shape to their AF decay curves than the original untreated remanences. Initial plateaus are well developed. The memory fraction of CRM's for the partially oxidized samples increased from 0.39 to 0.77 as the temperature of the run increased from 100 to 200 C. The increases reflect the fact that the remanences carried by SD moments are pinned by magnetoelastic anisotropy rather than crystalline anisotropy and thus survive LTD. This is because magnetic memory is controlled in part by the internal stresses developed at the magnetite-maghemite interface during oxidation. Low-temperature saturation remanent magnetization (SIRM) has also been measured for the partially oxidized magnetites. With increasing low-temperature oxidation, the Verwey transition becomes blurred, shifts to lower temperatures and eventually disappears. The SIRM memories for the partially oxidized samples increased with increasing oxidation to maghemite. LTD might provide a method of separating remanence of chemical origin from primary remanence.
GP21C-06 09:15h
Magnetic Measurements of Bacterial Activity in Mid Oceanic Ridge Basalt Samples.
Magnetic properties of the young oceanic crust gives insights on the 3D structure at ridges axis and on variations of the Earth's magnetic field. Accurate interpretations depend on our understanding of the magnetic stability of titanomagnetite crystals carrying the magnetic signal. The mean remanent intensity of young ocean basalts shows significant variations with age and it has been suggested that rapid chemical alteration of titanomagnetite could be responsible. Ubiquitous presence of bacteria in deep marine environment and their ability to interact with iron oxides suggest a possible important impact of these life forms on the stability of titanomagnetites. In vivo experiments have been conducted on more than 15 fresh lava macro samples with a batch of selected sulfate reducing bacteria of the desulfovibrio genus, relevant to the deep biosphere. Remanent magnetization of samples was measured at regular time interval, in addition to more complete magnetic and chemical characterization at beginning and end of experiments. After a few months results show a significant contribution of bacterial activity on magnetic properties of samples, in particular a well defined drop in remanence. Our results suggest a strong impact of bacteria on the alteration rate of iron oxides carrying the magnetic signal within the oceanic crust.
GP21C-07 09:30h
Multiple mechanisms of remagnetization involving sedimentary greigite (Fe3S4)
Sedimentary greigite (Fe3S4) is being increasingly implicated as the carrier of late diagenetic remagnetizations in fine-grained marine sediments. Scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectrometry is a powerful tool for discriminating between different iron sulphide phases. Determining authigenic growth relationships using SEM is an important supplemental tool to "remote sensing" using rock magnetic techniques. We have conducted detailed SEM observations and elemental microanalysis on polished sections from a wide range of sediments to identify greigite and to characterize the mode of occurrence and the relationships between the observed greigite and other authigenic and detrital mineral phases. This detailed observational work, in conjunction with recently published work, has enabled identification of several mechanisms for remagnetization involving greigite. (1) Neoformation of greigite on the surfaces of early diagenetic framboidal and nodular pyrite. (2) Greigite growth within cleavages of detrital sheet silicate grains. (3) Neoformed authigenic clays (smectite, illite) that have been corroded to provide iron for diagenetic neoformation of greigite. (4) Greigite nucleation on the surfaces of siderite, where the abundant reactive iron in the siderite has come into contact with sulfidic pore waters. (5) Oxidation of pyrite nodules to form gypsum, which has supplied sulphate for bacterial metabolism under later anoxic conditions, which, in the presence of dissolved iron, provided a site for nucleation of greigite. Many other mechanisms for greigite neoformation are conceivable. The key variables are appropriate redox conditions for generation of sulfide and availability of iron from the dissolution of a wide range of possible reactive detrital and authigenic iron-bearing minerals. Documentation of a wide range of mechanisms for neoformation of greigite provides compelling evidence that sediments containing greigite should be routinely suspected of remagnetization, which will complicate or compromise studies of environmental magnetism and geomagnetic field behavior.
GP21C-08 09:45h
Discrete element modeling of post depositional remanent magnetization acquisition: first results.
Marine and limnic sediments continuously record temporal changes in both the direction and intensity of the earth's magnetic field. Up to now, because of the underlying processes' complexity, there exists only empirical information and no physical theory describing the mode of post-depositional remanent magnetization (PDRM) acquisition. Using a discrete element model, we simulate the behaviour of sediment particle deposition and subsequent compaction from colloidal suspensions. A collection of two-dimensional models considering particle size distribution, particle shape, compaction, the external magnetic field, gravity, Brownian motion, van der Waals forces and bioturbation is used to investigate the theoretical background of PDRM acquisition. In particular, the controlling factors of particle alignment and fixation are studied. Employing Debye's theory of rotational Brownian motion, we will show that thermal fluctuations have a negligible effect upon the efficiency of the PDRM acquisition mechanism. In systems of deposited particles, the PDRM is found to lock gradually as compaction increases. The compaction required to fix the magnetic particles is primarily dependant on particle size distribution, shape and the external magnetic field. The packing structure of the sediment is strongly controlled by van der Waals forces and as such particle-particle electrostatic interactions play an important role in determining the position at which a particle is fixed and thus the PDRM intensity. Once the PDRM is formed, we investigate the effects of bioturbation. Bacterial scale reworking is accounted for by performing a two dimensional random walk through the sediment with a high-energy particle. The combination of different model results representing different stages of depositional processes will be demonstrated by a series of animations while numerical details of the model will be presented in an associated poster. Investigating the model outcome, we hope to obtain a detailed insight into the activation processes governing the dynamics of PDRM intensity.