Thermal Enhancement of Magnetic Fabrics in High Grade Gneisses

International audienceThe orientation-distributions of AMS and AARM ellipsoids are non-coaxial in the Kapuskasing Zone granulites. AARM is due to the preferred orientation of magnetite which mostly postdates the silicate fabric. The AMS orientation-distribution combines susceptibility anisotropies of magnetite and the silicates. Whereas heating does not increase the bulk susceptibility, it does improve the definition of the orientation-distributions of both AMS and AARM. However, the orientation-distribution of AARM is most enhanced and therefore post-heated samples emphasize the magnetite subfabric. Thus, heating does not clarify the interpretation of the silicate fabric in this study. However, it does change the samples' AMS fabric in such a way as to reveal the magnetite subfabric which can only otherwise be detected by more tedious AARM measurements

Magnetic characterization using a three-dimensional hysteresis projection, illustrated with a study of limestones

International audienceLimestones provide an important source of palaeomagnetic information despite their low content of submicroscopic remanence-bearing minerals. The chief sources of these minerals are thought to be clastic volcanic magnetite and titanomagnetite, and organic magnetite, the latter mostly from bacterial sources. Chemically remagnetized limestones carry magnetite or pyrrhotite. Three hysteresis properties prove useful in identifying and characterizing these mineralogical influences on limestones: the ratio of zero-field maximum remanence to saturation remanence (M r /M s) in an applied field, coercivity of remanence (B cr) and coercivity (B c). To a lesser extent K f /M s may be useful, where K f is the ferrimagnetic susceptibility. Traditionally, these have been plotted on a combination of 2-D graphs that of necessity only preserve two variables (Day et al. 1977; Wasilewski 1973). However, we found that magnetic discrimination and characterization of the limestones was much easier on a three-axis hysteresis projection that preserves the values of B cr , B c and M r /M s as independent variables. Using logarithmic scales, the regression surfaces through the data become almost planar and distinguish pelagic, shallow marine, shelf and remagnetized limestones on the basis of the slope and intercept of the associated regression surface. Clearly, there are sensitive sedi-mentological, geochemical or organic influences that dictate the magnetic mineralogy through sedimentary environment. Moreover, the 3-D plot of hysteresis criteria affords easy recognition of remagnetized limestones and may permit the rejection of material unsuitable for palaeomagnetic study. The 3-D hysteresis projection may be useful for the characterization of other rocks and magnetic material

Magnetic anisotropy reveals the depositional and postdepositional history of a loess-paleosol sequence at Nussloch (Germany)

International audienceAnisotropy of magnetic susceptibility (AMS) is employed as a tool to unravel the depositional history of the 17 m thick Nussloch P8 Weichselian loess sequence located 10 km south of Heidelberg, Germany. Through an AMS study, the primary aeolian depositional origin of the magnetic fabrics is evaluated, and overprinting due to postdepositional reworking and/or deformation is identified. Primary fabrics along the P8 sequence are defined by near-vertical K MIN axes and horizontal foliations. Eight intervals display secondary fabrics, characterized by either prolate orientation distributions or oblate orientation distributions with dipping foliation planes. These postdepositional fabrics are associated with laminated loess and tundra gley horizons. It is proposed that increased moisture (due to higher precipitation or enhanced snowmelt) and repeated cryogenic processes were able to reorganize and rework the accumulated loess. Primary aeolian fabrics are archived within 6.45 m of cumulated depth or 38% of the profile and dominantly within the Upper Pleniglacial units. Even though maximum susceptibility axes of primary fabrics are statistically well resolved at the specimen (ε 12 = 10.1° ± 8.6) and population (ε 12 = 6°) level, any inferred paleowind directions from the magnetic lineation remains speculative given the low concentration of ferrimagnetic minerals (<0.03 wt %). Tundra gley horizons upprofile display primary magnetic fabrics and no major changes in the degree of anisotropy or AMS orientation distributions. This suggests a weakening in gley-induced diagenesis and therefore favorable environmental conditions needed (moisture and presence of permafrost active layer) to initiate their formation in loess deposits. Finally, pedogenesis has not played an important role in modifying the magnetic fabric since paleosols display the same magnetic fabrics observed in primary loess

Neo-tectonic and rock magnetic study of the Circum Troodos Sedimentary Succession, Cyprus

The Circum Troodos Sedimentary Succession (Late Cretaceous to Recent) overlies the Troodos ophiolite of Cyprus, located in the Eastern Mediterranean. The pattern of neo-tectonic deformation was investigated through magnetic analysis of the tectonic petrofabncs. Anisotropy of magnetic susceptibility (AMS), anisotropy of anhysteretic remanence magnetization (AARM) and hysteresis loop parameters were determined in order to define the magnetic fabric and the magnetic mineralogy. The study area extends over approximately 1000 km2 mainly to the south of the exposed Troodos ophiolite. The sample suite includes 432 oriented hand samples, predominantly of the Lefkara and Pakhna Formations

Revisiting the mechanism of reversed thermoremanent magnetization based on observations from synthetic ferrian ilmenite (y = 0.7)

International audienceThis study investigates the magnetic behavior of three well-characterized synthetic single-phase ferrian ilmenite (y = 0.7) specimens over the temperature range between 10 K and 573 K. Careful experiments measuring induced and remanent magnetizations in variable temperatures, applied magnetic fields, and pretreatment conditions are conducted in order to elucidate the mechanism leading to reversed thermoremanent magnetization (RTRM). Magnetic ordering temperatures of the cation ordered domains, in all three samples, are estimated at 380 K, suggesting that their Curie temperatures (T C) are independent of the sample's thermal history. This is not the case for cation disordered boundaries resulting from quenching from high temperatures. These cation disordered domains have estimated magnetic ordering temperatures of 418 K (Q1300), 410 K (Q1050), and 425 K (Q900). The data unambiguously support a less than perfect ferrimagnetic–antiferromagnetic exchange interaction as the fundamental source of RTRM. Furthermore, the magnetic field strength of the ''effective'' exchange anisotropies in such polycrystalline samples are estimated at $2.7 mT (Q1300),$12 mT (Q1050), and 0 mT (Q900). However, from the results presented herein we conclude that favorable conditions for the acquisition of RTRM are dependent not only on the strength of the exchange anisotropy but also on the crucial role played by the size of the cation ordered domains. INDEX TERMS: 1519 Geomagnetism and Paleomagnetism: Magnetic mineralogy and petrology; 1540 Geomagnetism and Paleomagnetism: Rock and mineral magnetism; 1714 History of Geophysics: Geomagnetism and paleomagnetism; KEYWORDS: ferrian ilmenite, reversed thermoremanent magnetization, exchange anisotropy Citation: Lagroix, F., S. K. Banerjee, and B. M. Moskowitz (2004), Revisiting the mechanism of reversed thermoremanent magnetization based on observations from synthetic ferrian ilmenite (y = 0.7)

Mineral magnetic characterization of the Upper Pleniglacial Nussloch loess sequence (Germany): an insight into local environmental processes

International audience(1) First comprehensive environmental magnetism study of the Nussloch (Rhine River Valley, Germany) loess/palaeosol deposit. (2) Bulk ferrimagnetic concentration parameters are dominantly controlled by variations in coarse-grained MD ferrimagnetic particles of detrital aeolian origin. (3) The imprint of waterlogging-induced redoxomorphic processes on the magnetic record is observed by dissolution of fine-grained magnetic minerals. (4) The interpretation of magnetic susceptibility variations alone within loess and palaeosol deposits following the wind-vigour or pedogenic enhancement models will be hindered by waterlogging-induced redoxomorphic processes, if present. S U M M A R Y Presently, most loess/palaeosol magnetic susceptibility records are interpreted as following either the wind-vigour model or the pedogenic enhancement model. However redoxomorphic processes induced by waterlogging, often referred to gleying in the loess literature, are also known to alter loess deposits but their impact on loess/palaeosol magnetic susceptibility records has received little attention. The reported rock magnetic study aims to characterize the mineral magnetic response of loess to waterlogging-induced redoxomorphic processes, thus improving our understanding of mineral magnetic changes within loess deposits with respect to environmental and climate conditions. The Nussloch loess-palaeosol deposit (Rhine Valley, Germany) was targeted because it is one of the best-studied Pleniglacial deposits for Western Europe in which numerous tundra gley intervals have been identified. Moreover, a comprehensive high-resolution environmental magnetism study has never been undertaken for this site. Various rock magnetism experiments were conducted at both room and low temperatures to characterise the composition, concentration and relative magnetic grain size of the mineral magnetic assemblage. The relative changes in magnetic parameters within the investigated loess interval are primarily controlled by (1) varying concentrations of coarse-grained ferri-magnetic particles of detrital (aeolian) origin and (2) dissolution of fine-grained ferrimagnetic particles related to in situ post-depositional alteration promoted by waterlogging-induced re-doxomorphic processes. Goethite is found to be ubiquitous throughout the studied interval and is argued to have both a primary (aeolian) and secondary (in situ) origin. We conclude, that redoxomorphic processes induced by waterlogging, if present, will hinder the interpretation of magnetic susceptibility variations within loess and palaeosol deposits following the expected relationships dictated by the wind-vigour and the pedogenic enhancement magnetism models

Magnetic fabric of sheared till: A strain indicator for evaluating the bed deformation model of glacier flow

International audienceWet-based portions of ice sheets may move primarily by shearing their till beds, resulting in high sediment fluxes and the development of subglacial landforms. This model of glacier movement, which requires high bed shear strains, can be tested using till microstructural characteristics that evolve during till deformation. Here we examine the development of magnetic fabric using a ring shear device to deform two Wisconsin-age basal tills to shear strains as high as 70. Hysteresis experiments and the dependence of magnetic susceptibility of these tills on temperature demonstrate that anisotropy of magnetic susceptibility (AMS) develops during shear due to the rotation of primarily magnetite particles that are silt sized or smaller. At moderate shear strains ($6–25), principal axes of maximum magnetic susceptibility develop a strong fabric (S 1 eignevalues of 0.83–0.96), without further strengthening at higher strains. During deformation, directions of maximum susceptibility cluster strongly in the direction of shear and plunge ''up-glacier,'' consistent with the behavior of pebbles and sand particles studied in earlier experiments. In contrast, the magnitude of AMS does not vary systematically with strain and is small relative to its variability among samples; this is because most magnetite grains are contained as inclusions in larger particles and hence do not align during shear. Although processes other than pervasive bed deformation may result in strong flow parallel fabrics, AMS fabrics provide a rapid and objective means of identifying basal tills that have not been sheared sufficiently to be compatible with the bed deformation model

Persistent paléosurfaces in the basement of French Massif Central: geodynamic implications.

National audienceThe siderolithic paleoweathering surfaces of the French Central Massif have been dated to the Late Jurassic/Early Cretaceous, contrasting with previously accepted Tertiary age and implying that the Massif has never hosted a thick sedimentary cover. This contradicts with former thermochronological results. Herein, we expose the arguments for and against the proposed geodynamic evolution of the French Massif Central constrained by paleomagnetic age determinations

A case study of the internal structures of gossans and weathering processes in the Iberian Pyrite Belt using magnetic fabrics and paleomagnetic dating

International audienceIn the Rio Tinto district of the Iberian Pryrite Belt of South Spain, the weathering of massive sulfide bodies form iron caps, i.e., true gossans and their subsequent alteration and re-sedimentation has resulted in iron terraces, i.e., displaced gossans. To study the stucture and evolution of both types of gossans, magnetic investigations have been carried out with two foci: (1) the characterisation and spatial distribution of magnetic fabrics in different mineralised settings, including massive sulfides, gossans, and terraces, and (2) paleomagnetic dating. Hematite has been identified as the suceptibility carrier in all sites and magnetic fabric investigation of four gossans reveals a vertical variation from top to bottom, with: (1) a horizontal foliation refered to as "mature" fabric in the uppermost part of the primary gossans, (2) highly inclined or vertical foliation interpreted as "immature" fabric between the uppermost and lowermost parts, and (3) a vertical foliation interpreted to be inherited from Hercynian deformation in the lowermost part of the profiles. In terraces, a horizontal foliation dominates and is interpreted to be a "sedimentary" fabric. Rock magnetic studies of gossan samples have identified goethite as the magnetic remanence carrier for the low-temperature component, showing either a single direction close to the present Earth field (PEF) direction or random directions. Maghemite, hematite, and occasionally magnetite are the remanence carriers for the stable high-temperature component that is characterized by non PEF directions with both normal and reversed magnetic polarities. No reliable conclusion can be yet be drawn on the timing of terrace magnetization due to the small number of samples. In gossans, the polarity is reversed in the upper part and normal in the lower part. This vertical distribution with a negative reversal test suggests remanence formation during two distinct periods. Remanence in the upper parts of the gossans is older than in the lower parts, indicating that the alteration proceeded from top to bottom of the profiles. In the upper part, the older age and the horizontal "mature" fabric is interpreted to be a high maturation stage of massive sulfides' alteration. In the lower part, the age is younger and the inherited "imature" vertical Hercynian fabric indicates a weak maturation stage. These two distinct periods may reflect changes of paleoclimate, erosion, and/or tectonic motion

X-ray magnetic circular dichroïsm provides strong evidence for tetrahedral iron in ferrihydrite

International audience[1] Ferrihydrite is an important iron oxyhydroxide for earth and environmental sciences, biology, and technology. Nevertheless, its mineral structure remains a matter of debate. The stumbling block is whether a significant amount of tetrahedrally coordinated iron is present. Here we present the first X-ray magnetic circular dichroïsm (XMCD) measurements performed on a well characterized synthetic sample of 6-line fer-rihydrite, at both K and L 2,3 energy edges of iron. XMCD results demonstrate unambiguously the presence of tetrahedrally coordinated Fe(III) in the mineral structure, in quantities compatible with the latest extended X-ray absorption fine structure (EXAFS) analyses suggesting a concentration of 20–30%. Moreover , we find an antiferromagnetic coupling between tetrahedral and octahedral sublattices, with the octa-hedral sublattice parallel to the external magnetic field. Components: 5100 words, 3 figures