Geomagnetism and Paleomagnetism [GP]

 CC:718A  Wednesday  1400h

Rock Magnetism: New Answers to Old Questions

Presiding:  O Ozdemir, University of Toronto; D J Dunlop, University of Toronto; W Williams, School of GeoSciences, University of Edinburgh


The J-Meter Coercivity Spectrometer - Hysteresis Loop, IRM Acquisition Spectrum and Viscosity Spectrum in 6 Minutes

* Enkin, R J (, Geological Survey of Canada, POB 6000, Sidney, BC V8L 4B2, Canada
Nourgaliev, D (, Geology, Kazan State University, Kremlyevskaya str. 18, Kazan, 420008, Russian Federation
Iassonov, P (, Geology, Kazan State University, Kremlyevskaya str. 18, Kazan, 420008, Russian Federation

The J-Meter Coercivity Spectrometer uses an innovative robust design for measuring a geological sample's magnetic hysteresis loop, IRM acquisition spectrum and viscosity spectrum in 6 minutes. With this tool, several labs around the world have been able measure large sample collections and develop useful magnetic proxies for a variety of paleoclimate, diagenesis and other studies. The main element of the J-meter is a pulse magnetometer, in which an electromotive force pulse is induced in an array of pick-up coils by the magnetic field of a sample moving at a high speed past the coils. The sample is placed near the rim of a 50 cm diameter plexiglas disk which rotates 18 times a second through the pole pieces of an electromagnet. Both the induced and remanent magnetization are measured during each rotation of the disk. Induced magnetization for hysteresis loops are measure with a set of pick-up coils mounted directly on the pole pieces, similar to the geometry used for a vibrating sample magnetometer. The magnetic remanence is measured with a second array of coils situated away from the electromagnet and surrounded by a three-layer mu-metal shield. The electromagnet field is ramped up to 500 mT, and the down to the opposite polarity (-500 mT). The J meter is called a coercivity spectrometer because it is particularly well suited to measuring the IRM acquisition curve with sufficient sensitivity and resolution to take the derivative which defines the coercivity spectrum. To finish each measurement, the magnetic field is cut to zero and the viscous demagnetization is monitored for 100s, mostly following a log(time) relationship but with perturbations determined by the grain size distribution of the finest grains. A suite of analysis programs have been developed to determine hysteresis parameters and S-ratios, and to characterize coercivity and viscosity spectra. We present a series of applications demonstrating the power of the J-Meter to trace sediment sources, paleoclimate variations and diagenetic alteration associate with bacterial activity.


Do Continuous and Stepwise Thermal Demagnetization Give Equivalent Results?

* Dunlop, D J (, University of Toronto, 3359 Mississauga Road North, Mississauga, ON L5L 1C6, Canada

Continuous thermal demagnetization with measurements made at high T during heating is much faster than conventional stepwise demagnetization, which requires a series of cooling-reheating cycles to room temperature To. For single-domain (SD) grains, the two methods give similar results after continuous measurements are converted to equivalent room-temperature values by correcting for the reversible decrease of spontaneous magnetization Ms between To and T. To test for equivalence of the two methods in larger PSD and multidomain (MD) grains, three samples containing magnetite of different grain sizes and origins were heated in zero magnetic field and measurements taken either continuously at T during heating or at To after a set of cooling steps from T. Two samples contained 110 and 135 um sieve fractions from a crushed magnetite crystal, while the third was a diabase containing coarse magnetite in dark ferromagnesian minerals and fine magnetite in plagioclase. Partial TRMs with non-overlapping blocking temperatures (Tc, Ti) and (Ti, To) were demagnetized, using vibrating-sample magnetometers for continuous measurements and a mini- furnace and SQUID magnetometer for stepwise data. Ms(T)-corrected continuous data show little overlap of unblocking temperatures: pTRM (Tc, Ti) demagnetizes almost entirely above Ti and pTRM (Ti, To) almost entirely below Ti, demonstrating reciprocity and independence. However, stepwise data decrease more rapidly with increasing T than the corrected continuous results, some of which increase slightly with heating up to T = Ti. Some additional decay of magnetization must occur during cooling from T, where the continuous measurement is made, to To, where the stepwise result is measured. The most important conclusion of this study is that Ms(T)-corrected continuous demagnetization results do not exactly reproduce measured stepwise demagnetization results except for very fine grains, of SD size or close to it. Continuous thermal demagnetization cannot be used as a time-saving alternative to stepwise demagnetization with larger PSD or MD grains if exact equivalence is required.


Properties of pTRM in Multidomain Grains and Their Implications for Palaeointensity Measurements

* Biggin, A J (, University of Liverpool, Geomagnetism Laboratory Dept. Earth and Ocean Sciences Oxford Street, Liverpool, L69 7ZE, United Kingdom
Michalk, D M (, GFZ Potsdam, Section 5.2 - Paleoclimate and Landscape Evolution, Potsdam, D-14473, Germany

As a consequence of their ubiquity in natural materials, much effort has been expended on trying to understand how 'multidomain' (sensu lato) grains behave in palaeointensity experiments. The known properties of multidomain thermoremanence (MD TRM) will be reviewed here and their implications for various types of palaeointensity experiments will be considered. The Dekkers-Boehnel and (quasi-) perpendicular palaeointensity methods tend to produce more accurate measurements from samples containing MD remanences than do Thellier-Thellier protocols. This is because they apply only a single type of thermal remagnetisation treatment and avoid the interleaving of demagnetisation and remagnetisation treatments which always produces non-ideal behaviour when MD grains are present in the sample. However, this benefit of using a single-heating technique does not apply if the TRM of the sample being measured carries a secondary (e.g. viscous) overprint. A kinematic model of MD TRM predicts that, if a substantial demagnetisation treatment is required to isolate the primary TRM of a sample, then even single-heating methods will produce non-ideal behaviour in the experiment. This effect probably explains why some recently made palaeointensity measurements performed using the Dekkers- Boehnel method on Mexican lavas appeared to produce over-high results. One way around this problem might be to perform the measurements of the remanence in the experiment at temperature instead of always cooling the sample to room temperature. This could enable the optimal experimental behaviour to be preserved in spite of a significant overprint but requires specialist equipment which is not available in all labs. In many palaeointensity experiments, it is simply not possible to avoid all the non-ideal effects associated with MD grains. Furthermore, there is the potential for sources of bias other than MD effects to impact on a palaeointensity experiment (thermochemical alteration being the most obvious) and the design of the experiment should take these into account also. Nonetheless, there are some steps that can be followed in any experiment in order to reduce the amount of bias that MD effects might have on the palaeointensity and these will be outlined.


N-type Self-Reversal in Oceanic Basalts Studied with XMCD

* Carvallo, C (, Institut de Minéralogie et de Physique des Milieux Condensés, Campus Boucicaut, 140 rue de Lourmel, Paris, 75015, France
Sainctavit, P (, Institut de Minéralogie et de Physique des Milieux Condensés, Campus Boucicaut, 140 rue de Lourmel, Paris, 75015, France
Guyodo, Y (, Institut de Minéralogie et de Physique des Milieux Condensés, Campus Boucicaut, 140 rue de Lourmel, Paris, 75015, France
Arrio, M (, Institut de Minéralogie et de Physique des Milieux Condensés, Campus Boucicaut, 140 rue de Lourmel, Paris, 75015, France
Smekhova, A (, European Synchrotron Radiation Facility, 6 rue J. Horowitz, BP220, Grenoble, 38043, France
Rogalev, A (, European Synchrotron Radiation Facility, 6 rue J. Horowitz, BP220, Grenoble, 38043, France
Wilhelm, F (, European Synchrotron Radiation Facility, 6 rue J. Horowitz, BP220, Grenoble, 38043, France

The question of magnetization self-reversal in natural rocks is at the center of the problem of the faithfulness of the Earth's magnetic field recording. Paleomagnetism rests on the fact that the thermoremanent magnetization (TRM) acquired by a rock when cooled through its Curie temperature in the Earth's magnetic field is oriented in a direction parallel to that of the field. However, several cases have been reported where the TRM in a rock orients antiparallel to the external field. One mechanism responsible for this behavior is Néel N-type magnetism. It occurs in single-phase ferromagnetic minerals where the sign of the total magnetization can change because of changes in the contributions of the two antiparallel sublattices with temperature or as a result of chemical transformations. Titanomaghemites showing N-type reversal were found in submarine basalts recovered during ODP Leg 192 (Doubrovine and Tarduno, EPSL, 2004; Carvallo et al., GJI, 2004). In order to better understand the mechanism of self-reversal, we carried out X-ray magnetic circular dichroism (XMCD) at Fe K-edge at room temperature and low-temperature. XMCD is an element-, site-, and symmetry-selective technique and allows to determine the site occupancies in several iron oxides and the magnetization associated with these ions. By comparison with XMCD spectra at Fe K-edge measured on pure magnetite and maghemite, we were able to separate the different ion contributions in the titanomaghemite sample. The N-type reversal was evidenced by the fact that the XMCD spectrum at 30 K is a mirror image of the XMCD spectrum at room temperature. This is the first time that self-reversal is shown on the atomic level.


Low Temperature Cycling of Partially Oxidized Submicron Magnetites

* Ozdemir, O (, Chemical and Physical Sciences, University of Toronto, 3359 Mississauga Road North, Mississauga, ON L5L 1C6, Canada

In nature, the oxidation of magnetite to maghemite is the most common oxide mineral alteration. Small deviations from stoichiometry, such as surface oxidation in magnetite, have a considerable effect on the Verwey transition. The present work studies the effect of partial oxidation on the Verwey transition temperature Tv of submicron magnetites with mean particle sizes of 40 nm to 210 nm. Samples were heated in air at 100, 150 and 200 C. Saturation isothermal remanent magnetization (SIRM) given to the oxidized nanoparticles by a 2.5 T field at 10 K decreased steadily during zero-field warming to 300 K, with little or no indication of the Verwey transition. After completing the thermal cycle by cooling in zero field to 10 K the stoichiometric magnetites, which had lost 70-90 percent of their SIRM in warming through Tv, recovered very little of their initial remanence. However, the partially oxidized magnetites, which had lost 30-60 percent of their SIRM in the warming half-cycle, recovered 50-90 percent of the initial remanence after cooling to 10 K. A complete set of zero-field cooling-warming cycles of SIRM produced at 300 K was also carried out. These curves have more structure in both cooling and warming and are more diagnostic of degree of oxidation than the usually measured warming curve of SIRM produced below Tv. The 300 K SIRM of stoichiometric magnetites decreases steadily with cooling to the isotropic point, with variable amounts of recovery in cooling through Tv. The oxidized magnetites behave quite differently: the SIRM at first increases in zero-field cooling from 300 K, then decreases as Tv is approached. The hump-like form of the zero-field warming curve above Tv is even more pronounced. With complete oxidation to maghemite and the disappearance of the Verwey transition, the 300 K SIRM increases monotonically throughout zero-field cooling from 300 to 10 K.


Micromagnetics of Irregularly Shaped Magnetite Nanocrystals

* Williams, W (, W. Williams, University of Edinburgh, Edinburgh, EH9 3JW, United Kingdom
Evans, M E (, M.E. Evans, University of Alberta, Edmonton, AB T6G 2G7, Canada

The grains responsible for palaeomagnetic signals in rocks often have non-ideal morphologies, whereas micromagnetic modelling usually considers simple shapes such as cubes and spheres. Using a three- dimensional finite-element/boundary-element (3D-FEBE) micromagnetic model, we investigate the magnetic stability and domain structure of magnetite grains having irregular shapes constructed by adding bumps and hollows to an initially spherical grain. The amplitude of the departures from the initial surface was varied from 0.1 to 0.9 of the radius, and their separation was varied from 5 to 90 degrees (in terms of the angle subtended at the grain centre). To span the size range of interest in palaeomagnetism, we studied grains with initial radii of 30, 90 and 120 nm. A total of 180 micromagnetic models were computed. Two competing effects emerge. In many cases, surface irregularities significantly decrease magnetic stability by acting as sites where domain reversals nucleate. However, increased stability is also found, probably caused by the overall shape anisotropy of grains having only a few large irregularities.


Plenty of Room at the Bottom: The Superparamagnetic Transition in Chains of Magnetite Crystals

* Newell, A (, North Carolina State University, MEAS, Box 8208, Raleigh, NC 27695-8208, United States

Among the few known fossil remnants of bacteria are chains of magnetite and greigite crystals left in sediments by magnetotactic bacteria. The information they provide about the abundance and nature of magnetotactic bacteria may provide useful information about environmental conditions back at least as far as the Cretaceous. Their usefulness depends on their readily identified features such as the size and shape of the crystals and their arrangement in chains. These features maximize the torque of the Earth's magnetic field on the bacteria by maximizing the magnetic remanence in the chains. The chains are in a single-domain (SD) state, with a uniform magnetization pointing along the chain axis. The single-domain state only occurs within a narrow range of crystal sizes. Smaller particles become superparamagnetic (SP), unable to hold any magnetic remanence, while larger particles are multidomain, having a smaller remanence per unit volume. Thus, one criterion for "magnetofossils" is that they fall within the single-domain size range. However, the SP size limit is only known for isolated crystals and many of the crystals in magnetotactic bacteria would be SP in isolation. They are SD only within chains, their magnetization stabilized by the magnetostatic interactions between crystals. The SP critical size is calculated for chains of magnetite crystals using a new algorithm that finds all the equilibrium magnetic states. From these the minima and saddle points in the energy surface are selected and energy gradients are followed from saddle points down to stable states. This network of connections determines the paths from positive to negative saturation and the decay rate for the magnetic moment of the chain. In turn, the decay rate determines the critical size. The transition paths depend on the ratio of the strength of the magnetostatic interactions to the internal magnetic anisotropy of the crystals. This ratio increases as the crystals get closer together or less elongated. In the limit of no interactions, there are 2N stable states (all the combinations of up and down for the single-crystal moments), and an equal number of transition paths. As the relative strength of the magnetostatic interactions increases, the number of stable states declines. The transition paths also decrease in number and involve coherent rotations of an increasing number of moments. When the crystals are touching and the length-to-width ratio is 1, there are only two stable states (positive and negative saturation) and two mirror-image paths between them. The reversal mode for chains of 4 or less cubes is the well-known fanning mode in which the single-crystal moments rotate alternately clockwise and anticlockwise at the same rate. In chains of 5 or more cubes there is a new reversal mode, the "two-domain fanning mode", in which half the moments rotate by nearly 180° by a fanning-like mechanism and then the other half rotate. As the number of crystals in the chain increases, the SP critical volume for magnetite approaches a limit that is nearly independent of the shape of the crystals. The cube root of this volume is about 10 nm. This is low enough to accommodate almost all the measured sizes of magnetite crystals produced by magnetotactic bacteria.∼ajnewell/AGU2009joint/magnetiteChains


Quantification of magnetofossils using first-order reversal curves (FORC)

Egli, R (, Ludwig-Maximilians University, Department of Earth and Environmental Sciences, Geophysics, Theresienstr. 41, Munich, 80333, Germany
* Chen, A P (, Ludwig-Maximilians University, Department of Earth and Environmental Sciences, Geophysics, Theresienstr. 41, Munich, 80333, Germany
Winklhofer, M (, Ludwig-Maximilians University, Department of Earth and Environmental Sciences, Geophysics, Theresienstr. 41, Munich, 80333, Germany
Kodama, K (, Lehigh University, Department of Earth and Environmental Sciences, 31 Williams Drive, Bethlehem, PA 18015, United States
Horng, C (, Academia Sinica of Taiwan, Institute of Earth Sciences, 128 Academia Road Sec. 2, Nankang, Tai 11529, Taiwan

Fossil remnants of magnetotactic bacteria in sediments can carry stable magnetic remanence and provide useful information about paleoenvironmental conditions. However, quantification of magnetofossil fraction in natural sediment remains a challenge. We here present a highly sensitive and accurate method to determine the fraction of intact magnetosome chains and of other well dispersed authigenic magnetic particles in their original sedimentary matrix, based on their diagnostic features in a first-order reversal curve (FORC) diagram. Furthermore, we are able to distinguish chains of magnetofossils from other dispersed authigentic particles. The method is successfully tested on a lake sediment sample known to contain abundant magnetofossils. Through this example we also demonstrate the importance in choosing adequate measuring parameters. The possibility of unmixing the contribution of non-interacting single-domain particles from other magnetic components and determining their switching field distribution opens new possibilities in the quantitative magnetic analysis of sediments. This is of particular interest for paleoenvironmental reconstructions, since the occurrence of magnetotactic bacteria and the preservation of magnetofossils are highly responsive to changes in the redox environment.