Advances in Basic and Applied Rock Magnetism
Presiding: A Smirnov, University of Rochester; A J Newell, North Carolina State University
GP12A-01 INVITED 10:30h
On the Conditions for Self-reversal of Remanent Magnetization in Titanomaghemite: Constraints From DSDP Site 307 Basalts
The possibility of self-reversed chemical remanent magnetization (CRM) carried by titanomaghemite in altered submarine basalts has been discussed in the literature since the late 1950's (Verhoogen, 1956), but no unequivocal example was found in natural oceanic rocks until recently, when we reported a prominent case of partial self-reversal in Late Cretaceous pillow lavas from Detroit Seamount in the northwestern Pacific Ocean (Doubrovine and Tarduno, 2004). Here we present new paleomagnetic, rock-magnetic and compositional data from severely oxidized mid-ocean ridge tholeiitic basalts of Late Jurassic age collected at Mesozoic magnetic anomaly M21 of the Hawaiian magnetic lineations (DSDP Site 307). Through a series of rock-magnetic, X-ray diffraction and energy-dispersive X-ray spectrometry analyses, we show that the magnetic carriers in Site 307 basalts are highly oxidized, pseudo-single domain titanomaghemites. In thermal demagnetization data for the majority of samples, we observed a small, reversed polarity magnetization component unblocking between ~200-275°C, antiparallel to the magnetizations of normal polarity isolated at higher and lower temperatures. However, alternating field demagnetization of thermally untreated sister samples showed only a single, normal polarity magnetization component, suggesting that the apparent component of reversed polarity is an artifact of the inversion of titanomaghemite, rather than the unblocking of a true self-reversed CRM. If this interpretation is correct, the compositions of titanomaghemites from Site 307 basalts constrain the self-reversal field of titanomaghemite to extremely high oxidation states, requiring almost complete maghemitization (z≥0.95) to produce a self-reversed CRM. Further rock-magnetic experiments performed to test this hypothesis will be discussed.
GP12A-02 INVITED 10:50h
Magnetic Force Microscopy and Electron Back Scatter Diffraction Study of Pseudo-Single-Domain Grains of Magnetite
Magnetic domain structures in pseudo-single-domain (PSD) grains (5-20 Îœm) of magnetite (Fe3O4) were studied using magnetic force microscopy (MFM) and correlated with grain shapes and with crystallographic orientations determined by electron back scatter diffraction (EBSD). The magnetite grains, produced by the glass-ceramic method, are randomly oriented and dispersed in a nonmagnetic silicate matrix. The studies were focused on grains with magnetization mainly parallel to the sample surface. Domain wall (DW) types and crystallographic orientation of wall planes were identified using MFM and EBSD data. For instance, for a triple wall junction where three wall types (180°, 71° and 109° DW) intersect, the wall orientations were identified as following: the 180° wall is parallel to (112) type plane, the 71° wall to (110) type plane and the 109° wall to (001). Most of the studied grains were subdivided into domains by 180° DWs, which cross the entire grain without formation of closure domain structures at grain edges. Domain structures remained qualitatively the same after repeated AF demagnetization cycles. Most of the observed 180° walls were parallel to (110) type planes, most likely because such planes contain two easy axes, which reduces wall energy density. Close to grain edges, 180° DWs often deviated from (110) orientations by twisting around a [111] axis parallel to the magnetization in adjacent domains. This allows optimization (minimization of total) of wall energy density, wall area and stray field (magnetic charge at grain edges) and indicates that stress is a subordinate factor in causing wall bending. Our wall bending analysis is consistent with published micromagnetic calculations. The combined MFM and EBSD study allows the conclusion that remanent 3-D domain structures in PSD grains result from combined magnetostatic effect of all grain surfaces (shape anisotropy of the grain) and magnetocrystalline anisotropy.
GP12A-03 11:10h
Memory of the magnetic field applied during cooling in stoichiometric magnetite: Grain-size dependence and inferences on the physical processes
A memory effect was recently discovered in stoichiometric magnetite (Fe3O4) at temperatures below its transition from a cubic to monoclinic structure (the Verwey transition, ~120 K) (e.g., Smirnov and Tarduno, EOS, 2002). The effect is expressed as an inflection point observed in magnetic hysteresis loops after cooling magnetite through the Verwey transition in the presence of a magnetic field. The location of the inflection is controlled by the strength of the applied field during cooling (for fields between 0.01 and 0.09 T). Our working hypothesis is that the reorganization of magnetic domains and the formation of monoclinic twins may interact such that they give rise to the field-memory effect. The hypothesis predicts that the effect should not exist in single-domain magnetite (no domain walls). We also expect that the effect should be less well expressed in multidomain magnetite because of the larger number of magnetic domains (i.e. only a few of which will potentially interact with monoclinic twins). We tested these predictions by studying synthetic magnetite samples, which varied from single-domain to multidomain in size. Our experimental results to date confirmed the predictions. The effect was best pronounced for grain sizes between 0.5 and 5 microns, and less well expressed in samples with larger grain sizes. No effect was observed on single-domain magnetite. We will discuss implications of our results for the physical mechanisms of the memory of the field applied during cooling. We also discuss new experimental approaches (such as three-axial low-temperature magnetometry), which may provide additional insights into these mechanisms.
GP12A-04 11:25h
The Effects of Crystal Fractionation and Magma Mixing on Remanent and Induced Magnetic Anomalies over a Layered Intrusion
The Bjerkreim-Sokndal layered intrusion lies in the Rogaland Igneous Complex (~930 Ma) within the Baltic Shield in southern Norway. This 7 km-thick intrusion is divided into six Megacyclic units topped by mangerite and quartz-mangerite units. The course of crystal fractionation punctuated by the influx and mixing of more primitive magmas produces sequences of early plagioclase norites, intermediate hemo-ilmenite norites, and late magnetite-rich norites with subordinate ilmenite. Oriented samples were collected from 46 sites through the stratigraphy of the intrusion and subjected to petrophysical, paleomagnetic and rock magnetic measurements. Magnetic properties show a large range of values, with susceptibilities ranging from 1.47x10-4 to 2.15x10-1 SI, NRM intensities ranging from 0.104 to 58.8 A/m and corresponding Q values of 0.1 to 85. When induced and remanent magnetizations are averaged for each subdivided mega-unit a pattern of remanence-dominance at the base to induced-dominance at the top of each cycle is clear. Hysteresis properties indicate PSD to MD size magnetites with a continuous trend between them, indicative of the magnetite-rich rocks. Hysteresis properties falling outside the magnetite PSD-MD ranges are interpreted as hemo-ilmenite samples, in good agreement with the observed oxide mineralogy. Distinctive differences in the magnetic mineralogy also shows up in demagnetization behavior. Thermal plots show either a loss of magnetization at 580C, or above 600C. AF demagnetization plots show two separate populations - one with high coercivity (hemo-ilmenite) and one with low coercivity (magnetite). Magnetic anomalies over the body correspond directly to the magnetic properties, with positive (induced) anomalies over the magnetite-rich layers and magnetic lows (due to reversed magnetic signal) over layers with hemo-ilmenite present.
GP12A-05 11:40h
Multiple blocking temperatures and thermoremanent magnetization in single-domain particles
The standard theory for thermoremanent magnetization (TRM) in single-domain particles, the Neel theory, assumes the particles have uniaxial anisotropy. However, some single-domain particles may have a mixture of uniaxial and cubic anisotropy. These include particles in ocean floor basalts and remagnetized carbonates. A more general theory for TRM is developed for such particles. In the Neel theory, a particle acquires TRM at a single blocking temperature. Above this temperature the magnetization vector varies randomly in response to thermal fluctuations. It spends most of its time near one of the two energy minima, but occasionally jumps from one minimum to the other. In the new theory for mixed cubic and uniaxial anisotropy there are up to eight minimum energy states, and jumps occur between neighboring states. The time dependence for the occupancy of states can be complicated, but the magnetization is only affected by two relaxation rates - one in the direction of the uniaxial axis and one perpendicular. This leads to two blocking temperatures. At the higher blocking temperature the states are divided into two groups and communication between these groups is cut off. At the lower blocking temperature all jumps cease. The probability distributions at the blocking temperatures are calculated in the small-field approximation to give the dependence of TRM on applied field. This dependence from the predictions of Neel theory, but these particles are still suitable for paleointensity measurements.
http://www4.ncsu.edu/~ajnewell/TRM_mixed_anisotropy.html