GP23B-01 INVITED 13:40h
High-field Cantilever Magnetometery as a Tool for Single Crystal Magnetocrystalline Anisotropy Determination
Torque magnetometry is utilized widely in physics and material science for the determination of magnetic properties in thin films and semiconductors. In rock magnetism, however, it has not yet been applied despite its great potential. In general, torque magnetometers are optimized for the specific problem at hand. Here, we use a cantilever magnetometer that allows measurement of samples with a volume up to 64 mm3, and that is inserted into an electromagnet with a maximum field of 2 T. The cantilever spring is suitable for torque values ranging from 7.510-7 N m to 40 10-7 Nm, and it can be replaced easily by springs covering different ranges. The torque is detected capacitively; the measured capacitance is converted into torque by using a calibrated feedback coil. The magnetometer allows in-situ rotation of the sample in both directions and is, therefore, suitable to analyze e.g. rotational hysteresis effects. The magnetocrystalline anisotropy of octahedral magnetite single crystals of different sizes is evaluated. The anisotropy constants have been determined by Fourier analysis of the torque signal on the (111) plane and the (110) plane. The absolute anisotropy constant has been computed using the extrapolation-to-infinite-field method.
GP23B-02 13:55h
Fundamental AMS for Single Crystals of Carbonate Minerals
The anisotropy of magnetic susceptibility (AMS) can serve as a good qualitative indicator of strain in deformed carbonate rocks with diamagnetic susceptibility. An open question is how well the AMS serves as a quantitative indicator of strain in rocks whose minerals undergo twinning and recrystallization with deformation. We are investigating this quantitative relationship by numerical models which requires knowledge of the AMS of carbonate minerals. There is, however, a lack of reliable values for the intrinsic AMS of carbonate crystals. Although the AMS for calcite is well defined, measurements from other carbonate minerals show a large range of anisotropies. This is due partially to the very low paramagnetic or diamagnetic susceptibility of these minerals but also partly due to the fact that the chemical composition of the crystals in these earlier studies was not well-constrained. We have investigated the room temperature AMS of important carbonate minerals; i.e., calcite, dolomite, siderite, aragonite, rhodochrosite, azurite, magnesite, cerussite. Single crystals of best quality were investigated, and the chemical composition of the crystals was analyzed by Laser Ablation Inductively Coupled Plasma - Mass Spectrometry. High-field (torsion magnetometry) and low-field (KLY-4S) anisotropies were measured on all crystal samples. Acquisition of isothermal remanent magnetization was used to detect possible ferromagnetic inclusions in the samples. The high-field torsion magnetometry allows for the ferrimagnetic and paramagnetic contributions to be separated. AMS values obtained by both low-field and high-field methods agree very well, which suggests that ferrimagnetic contamination is negligible. The high-field torsion magnetometer was more accurate in determining the AMS for small and weakly magnetic samples. The bulk susceptibility of calcite increases linearly with Fe and Mn content. Pure calcite has a perfectly oblate AMS with the minimum susceptibility subparallel to the crystallographic c-axis. With increasing content of ferrous ions the susceptibility changes to positive values with the maximum susceptibility axis subparallel to the c-axis. Pure dolomite shows a perfectly oblate AMS. For slightly impure dolomite the AMS shape changes to perfectly prolate.
GP23B-03 INVITED 14:10h
The carriers of magnetic fabric in volcanic rocks
Studies of the magnetic fabric of volcanic rocks have become an important tool to characterize flow directions in lava bodies, and to investigate dike emplacement and the plumbing of volcanic systems in general. However, depending on factors like the composition of the primary magma, the cooling history and the alteration regime, the magnetic fabric information can be carried by a large variety of ferri- and paramagnetic minerals. The composition and genesis of these minerals influence the ability of volcanic rocks to acquire information about the above mentioned dynamic processes. This presentation focuses on the relation between magnetic mineralogy, magnetic fabric and the techniques to determine the latter. Previous work on the topic will be reviewed and a case study will be discussed. We studied samples from the dike complex of the Koolau volcano on Oahu, Hawaii. These dikes are exposed as a result of the Nuuanu giant landslide. This landslide removed approximately the upper 3000 m of the volcanic edifice of the Koolau volcano. The dikes were thus emplaced deeply inside the Koolau volcano and were subjected to mild metamorphism. AMS measurements show different types of magnetic fabric for the 8 studied dikes. Rock magnetic and microscopic investigations as well as microprobe analyses indicate that the dikes can be subdivided into two groups with distinct magnetic mineralogies: 1. Samples containing almost unaltered coarse grained Ti-poor titanomagnetite and 2. highly altered samples where the primary titanomagnetite is replaced by its alteration products, maghemite and hematite. In both AF and thermal demagnetisation experiments a reversed ChRM for both groups of samples can be isolated. The strongly altered samples (second group) display an antiparallel overprint which is removed between 200 and 300$^{\circ}$C. Strong field thermomagnetic measurements show that above $400^{\circ}$C, the maghemite phase of the second group completely inverts to hematite. AMS measurements after heating indicate that the alteration processes triggered by heat treatment of the samples enhance the magnetic fabric.
GP23B-04 14:25h
Anatomy of an oceanic mantle shear zone deduced from high-field magnetic anisotropy: the Humboldt corridor, New Caledonia
The Humboldt corridor is located in the Massif du Sud of the New Caledonia ophiolite. This NW-SE trending corridor (40 x 20 km), exhibits tectonic peridotites with steep foliations and strong penetrative fabrics that grade into horizontal foliation outside the shear zone. Five N130° trending zones have been identified from SW to NE: 1) a 3-4 km-thick zone of harzburgites with pyroxenite dikes, 2) a 1-2 km-thick zone of harzburgites with gabbro dikes, 3) a 1-4 km-thick zone of harzburgites without dikes displaying a prominent centimeter-scale layering, 4) a zone similar to zone 2; 5) a zone similar to zone 1. The mineral lineation remains subhorizontal throughout the corridor. The dikes throughout the corridor are normal to the mineral lineation. Some porphyroblasts in zone 3 have an aspect ratio up to 10. The harzburgites display porphyroclastic microstructures while olivine LPOs indicate plastic flow with activation of the (010) [100] high temperature slip system. The origin of such a zonation in mantle peridotites remains uncertain. This corridor is oblique to the regionally dominant N-S mineral lineation of Massif du Sud. Although the origin of this corridor is still unclear, olivine LPO data indicates that deformation occurred at high temperature in the mantle. The large width and the orientation of the corridor with respect to the spreading ridge framework seem incompatible with a transform zone interpretation. Magnetic measurements have been performed on 40 specimens (20 mm cubes) from 10 stations. The low field magnetic susceptibilities and AMS, representing the ferromagnetic contribution of secondary magnetite, vary between 700 and 7000 $\mu$SI. The magnetite is formed during serpentinization and therefore the low field AMS does not reflect mantle deformation. The high field susceptibilities and AMS, measured using a vibrating sample magnetometer, represent the contribution of paramagnetic minerals only (olivine and orthopyroxene) and varies from 230 to 350 $\mu$SI. The magnetic saturation, measured using a vibrating sample magnetometer, was reached before 200 mT. Torsion magnetometry tests conducted at fields up to 1.8 T indicate that the saturation conditions were met and no high coercivity phases. Preliminary results show that the low field AMS and the high field AMS principal axes (K1 and K3) are statistically distinct. The degree of magnetic anisotropy correlates with the intensity of mineral fabric and suggests that mantle deformation in these peridotites is very heterogeneous at scales from a few centimeters up to 1 km.
GP23B-05 INVITED 14:40h
AMS of an Analogue Non-Scale Model Simulating Diapiric Pluton Emplacement
Development of magnetic fabric within a pluton during its diapiric ascent was investigated using an analogue non-scale model of plaster of Paris containing small amount of fine-grained (less than 0.09 mm) homogeneously mixed magnetite, with resulting bulk susceptibility being in the order of 10-3 [SI]. The apparatus for this modelling consists of a manual squeezer with calibrated spring and a perspex container. Stratified coloured to visualize internal flow geometries, weak plaster layer at the bottom of the container was forced to intrude overlying fine-grained (>0.017mm) sand through a hole in a board attached to the squeezer. A retarding compound was admixed into the plaster to postpone the solidification of plaster. After solidifying the model, small oriented cylindrical specimens (7 mm in diameter and 6 mm in height) were drilled and their anisotropy of magnetic susceptibility (AMS) was measured with the KLY-4S Kappabridge. The magnetic fabric in the margins of the vertical column of the diapir is characterized by high degree of AMS (P'=1.26-1.30), neutral to oblate AMS ellipsoid (T=0.2-0.6) and vertical magnetic lineations and foliations. In the vent area, the degree of AMS is also high, but the AMS ellipsoid being strongly prolate (T= -1 to -0.8) with vertical magnetic lineations. In the interior of the plug above the vent zone, abrupt transition into horizontal lineations and foliations take place and the low degree of AMS (P'=1.05-1.10) marks the area where strongly prolate magnetic fabric is being gradually changed into the magnetic fabric characterized by neutral to oblate AMS ellipsoid. This type of magnetic fabric extends to the apical part of the body. In the extrusive portions of the diapir, oblate magnetic fabric increases in anisotropy (T=0.8-1, P'=1.26-1.29), while the front of the radial extrusion shows horizontal lineations parallel to the margin and neutral AMS ellipsoids (T=0.2-0.6,P'=1.23-1.26). This changeover of fabric thus indicates divergent flow in the radial extrusion. The development of AMS fabric is correlated with complex flow pattern indicated by coloured and originally horizontal plaster layers. Narrow shear zones in the margins of the plug indicate non-linear behavior of plaster during experiments.
GP23B-06 14:55h
Complex Magnetic Fabrics in Igneous Rocks
Since the end of the 1980's, studies on the anisotropy of magnetic susceptibility (AMS) of dikes and lavas have largely assumed a simple relation between the principal AMS directions and magma flow. In the so-called "normal" magnetic fabric case, the magnetic foliation mimics the flow plane. The AMS minimum -- Kmin, which is normal to the foliation plane, lies perpendicular to a dike's walls or to a lava's top and bottom, and the magnetic lineation, given by the AMS maximum - Kmax, parallels the magma flow direction. However, numerous studies report anomalous magnetic fabrics for which the principal AMS directions are exchanged. Recent studies of Oligocene lava flows from the Kerguelen archipelago and the present study of 48 mid-Miocene dikes from the Roberts Mountains, located along the eastern Northern Nevada Rift (NNR), indicate that complex magnetic fabrics are much more common than is generally expected. Indeed, of the 48 NNR-dikes studied so far, only 45% possess a normal fabric (assuming a vertical flow). The relatively low number of samples possessing a normal fabric and the existence of permuted (exchanged) AMS axes occurring at the cooling unit scale (all the samples of a flow or dike yielding the same abnormal fabric) as well as at the sample scale (only one or two permuted samples observed in a cooling unit) suggest that the magnetic lineation may not always indicate the flow direction. Even when all the samples from the same cooling unit display the same magnetic fabric, an alternative flow direction indicated by Kint (AMS intermediate axis) cannot be refuted, leading to an uncertainty in the interpretation of the AMS data. Therefore, it is of great interest to study in detail the permutations of the AMS axes to 1) determine what factors control the occurrence of the various abnormal fabrics, and 2) possibly distinguish an abnormal fabric and infer from it the true flow direction. For that purpose, we performed a suite of mineralogical experiments to characterize representative samples of the different AMS fabrics, including FORC distribution analyses at different temperatures and MPMS measurements. We also studied the evolution of magnetic fabric upon AF and thermal demagnetization, and induced remanent magnetization. In addition, measurements of the anisotropy of magnetic remanence and of AMS in a high DC-field are in-progress. Surprisingly, the results obtained so far, seem to indicate that the permutations of AMS axes are not related to mixtures of different grain sizes. However, additional results are needed to check if this interpretation is statistically robust and test the other permutation mechanisms like magnetic interactions of the grains, rolling of the elongated particles in strong velocity gradients, or crystallographic particularities.
GP23B-07 15:10h
Magnetic Anisotropy as an aid to Identifying CRM and DRM in Red Sedimentary Rocks
To evaluate the usefulness of magnetic anisotropy for determining the origin of the natural remanent magnetization (NRM) in red sedimentary rocks, several new remanence anisotropy measurement techniques were investigated. The goal of the work was an accurate separation of the remanence anisotropy of magnetite and hematite in the same sedimentary rock sample. In one technique, Tertiary red and grey sedimentary rock samples from the Orera section of Spain were exposed to 13 T fields in 9 different orientations. This work was done at the High Field Magnet Laboratory of Radboud University, Nijmegen, The Netherlands. In each orientation, alternating field (af) demagnetization was used to separate the magnetite and hematite contributions to the high field isothermal remanent magnetization (IRM). Tensor subtraction was used to separate the magnetite and hematite magnetic anisotropies. Geologically interpretable fabrics did not result, probably because of the presence of goethite. In the second technique, also applied to samples from Orera, an anisotropy of anhysteretic remanence (AAR) was applied in af fields up to 240 mT to directly measure the fabric of the magnetite in the sample. IRMs applied in 2T fields followed by 240 mT af demagnetization, and thermal demagnetization at $90\deg$C to remove the goethite contribution, were used to independently measure the hematite fabric in the same samples. This approach gave magnetic fabrics with minimum principal axes perpendicular to bedding, suggesting that the hematite and magnetite grains in the Orera samples both carry a depositional remanent magnetization (DRM). In a third experiment, IRMs applied in 13 T fields were used to measure the magnetic fabric of samples from the Dome de Barrot area in France. These samples had been demonstrated to have hematite as their only magnetic mineral. The fabrics that resulted were geologically interpretable, showing a strong NW-SE horizontal lineation consistent with AMS fabrics measured in the same samples. These fabrics suggest that the rock's remanence has may have been affected by strain and could have originated as a DRM or a CRM. Our work shows that it is important to account for goethite when using high field IRMs to measure the remanence anisotropy of hematite-bearing sedimentary rocks. It also shows that very high magnetic fields ($>$10 T) may be used to measure the magnetic fabric of sedimentary rocks with highly coercive magnetic minerals without complete demagnetization between each position, provided that the field magnetically saturates the rock.
GP23B-08 15:25h
Normal and Inverse Magnetic Fabrics in Mesozoic Black Shales, Northern Siberia, Russia: Siderite Controlled?
A 28-meter-long profile situated on the coast of the Arctic Ocean, Russia (74N, 113E) was extensively sampled primarily for the purpose of magnetostratigraphic investigations across the Jurassic/Cretaceous boundary. The profile consists predominantly of marine black shales with abundant concretions and several distinct diagenetically cemented layers. Newly formed siderite can be macroscopically observed in cemented layers. Its presence was further verified using X-ray powder diffraction. Magnetic volume susceptibility of the shales is between 8 x 10-5 and 2 x 10-3 SI with concretions and cemented layers possessing susceptibilities with an order of magnitude higher compared to the neighboring rock. The intensity of NRM varies between 1 x 10-5 and 6 x 10-3 A/m. Several samples with anomalous paleomagnetic directions and extremely high values of NRM were found. Unlike susceptibility, the relatively high NRM values show no apparent correlation to the profile lithology. The anisotropy of magnetic susceptibility of the majority of samples is consistent with a sedimentary fabric (both magnetic foliation and lineation are sub-parallel to the bedding). The anisotropy degree ranges from 1.01 to 1.15, the shape of the susceptibility ellipsoid is distinctly oblate. Despite the well-preserved sedimentary fabric, in some samples the K1 and K3 principal susceptibility directions are interchanged and have distinctly prolate ellipsoid shapes. Although the majority of such inverse samples are bound to the cemented layers, some can be found within "uncemented" sections of profile. Magnetic fabric using anisotropy of anhysteretic remanent magnetization is bedding-parallel with oblate ellipsoid shapes. This suggests that the inverse magnetic fabric is due to the paramagnetic mineral siderite, which is confirmed by high-field anisotropy measurements. We conclude that the presence of normal or inverse magnetic fabric is controlled by the relative siderite content in studied rocks. The samples with high NRM values may contain oxidation products of siderite (possibly hematite) which could acquire the stable chemical remanent magnetization with anomalous directions.