Decrypting magnetic fabrics (AMS, AARM, AIRM) through the analysis of mineral shape fabrics and distribution anisotropy

The fieldwork was supported by the DIPS project (grant no. 240467) and the MIMES project (grant no. 244155) funded by the Norwegian Research Council awarded to O.G. O.P.'s position was funded from Y-TEC.Anisotropy of magnetic susceptibility (AMS) and anisotropy of magnetic remanence (AARM and AIRM) are efficient and versatile techniques to indirectly determine rock fabrics. Yet, deciphering the source of a magnetic fabric remains a crucial and challenging step, notably in the presence of ferrimagnetic phases. Here we use X-ray micro-computed tomography to directly compare mineral shape-preferred orientation and spatial distribution fabrics to AMS, AARM and AIRM fabrics from five hypabyssal trachyandesite samples. Magnetite grains in the trachyandesite are euhedral with a mean aspect ratio of 1.44 (0.24 s.d., long/short axis), and > 50% of the magnetite grains occur in clusters, and they are therefore prone to interact magnetically. Amphibole grains are prolate with magnetite in breakdown rims. We identified three components of the petrofabric that influence the AMS of the analyzed samples: the magnetite and the amphibole shape fabrics and the magnetite spatial distribution. Depending on their relative strength, orientation and shape, these three components interfere either constructively or destructively to produce the AMS fabric. If the three components are coaxial, the result is a relatively strongly anisotropic AMS fabric (P’ = 1.079). If shape fabrics and/or magnetite distribution are non-coaxial, the resulting AMS is weakly anisotropic (P’ = 1.012). This study thus reports quantitative petrofabric data that show the effect of magnetite distribution anisotropy on magnetic fabrics in igneous rocks, which has so far only been predicted by experimental and theoretical models. Our results have first-order implications for the interpretation of petrofabrics using magnetic methods.Publisher PDFPeer reviewe

Low temperature magnetic transition of chromite in ordinary chondrites

International audienceWe present low temperature magnetic data (thermal demagnetization, hysteresis) obtained on thirty three ordinary chondrite meteorites. A magnetic transition is observed in the 40–80 K range (average 66 K), and is interpreted as a ferrimagnetic to paramagnetic phase transition. We present quantitative analyses of changes in magnetization across the magnetic phase transition and chemical elemental analyses to argue that chromite, a common accessory mineral in meteorites, is the mineral undergoing the phase transition, and not troilite or tetrataenite as proposed in previous studies. Moreover, we observe a correlation between the measured Curie temperature and chemical composition of chromites in ordinary chondrites. Low temperature magnetic measurement are thus a sensitive indicator of chromite composition in these meteorites, and as such a possible proxy to the understanding of their thermal metamorphism

Ophiolite Tectonics, Rock Magnetism and Palaeomagnetism, Cyprus

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Tracing sources of suspended sediments in fluvial systems by magnetic and geochemical particle characterization

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“Magnetic fingerprinting” of fluvial suspended particles in the context of fertile soil erosion: example of the Canche River watershed (Nord-Pas-de-Calais, France)

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Analyzing the geomagnetic axial dipole field moment over the historical period from new archeointensity results at Bukhara (Uzbekistan, Central Asia)

Since the mid-19th century, direct measurements of both intensity and direction of the Earth's magnetic field have been available, allowing an accurate determination of its spatio-temporal variations. Prior to this time, between ~1600 and 1840, only direct directional measurements are available. Therefore, the construction of global field models over this period requires either a specific treatment of the axial dipole field component or the use of archeomagnetic intensity data. In this study, we use a regional approach based on the construction of an archeointensity variation curve in Central Asia. We analyze baked clay brick fragments sampled in Bukhara (Uzbekistan), dated between the end of the 16th century and the beginning of the 19th century. This city is of particular interest for archeomagnetism due to the well-preserved old buildings accurately dated by documentary archives. A series of archeointensity results is obtained using the Triaxe experimental protocol, which shows a decreasing trend in intensity from ~1600 to ~1750, with intensities during the 18th century lower than expected from global geomagnetic field models. These new data appear consistent with other Triaxe data previously obtained in western Europe and western Russia, when transferred to Bukhara using the field geometry of the gufm1 model. Together, these data are used to recalibrate the axial dipole moment evolution provided by this model. The resulting evolution appears non-linear, with a clear relative minimum in the magnitude of the axial dipole during the late 18th century. We illustrate the fact that at present this evolution can neither be satisfactorily confirmed nor refuted by other datasets available in western Eurasia (as well as at a wider spatial scale), mainly due to the significant dispersion of the data. Our interpretation relies on the accuracy of the field geometry of the gufm1 model, which appears less reliable prior to ~1750. Nevertheless, the minimum proposed in the 18th century seems to be a true feature of axial dipole behavior

Excursions to C₄ vegetation recorded in the Upper Pleistocene loess of Surduk (Northern Serbia): an organic isotope geochemistry study

Loess sequences have been intensively studied to characterize past glacial climates of the 40–50° north and south latitude zones. Combining different approaches of sedimentology, magnetism, geochemistry, geochronology and malacology allows the general pattern of the climate and environment of the last interglacial–glacial cycle in Eurasia and America to be characterized. Previous studies performed in Europe have highlighted the predominance (if not the sole occurrence) of C₃ vegetation. The presence of C₃ plants suggests a regular distribution of precipitation along the year. Therefore, even if the mean annual precipitation remained very low during the most extensive glacial times, free water was available for more than 2 months per year. Contrarily, the δ¹³C record of Surduk (Serbia) clearly shows the occurrence and dominance of C₄ plants during at least 4 episodes of the last glacial times at 28.0–26.0 kyr cal BP, 31.4–30.0 kyr cal BP, 53.4–44.5 kyr cal BP and 86.8–66.1 kyr. The C₄ plant development is interpreted as a specific atmospheric circulation pattern that induces short and dry summer conditions. As possible explanation, we propose that during "C₄ episodes", the Mediterranean Sea would have been under the combined influence of the following: (i) a strong meridional circulation unfavorable to water evaporation that reduced the Mediterranean precipitation on the Balkans; and (ii) a high positive North Atlantic Western Russian (NA/WR)-like atmospheric pattern that favored northerlies over westerlies and reduced Atlantic precipitation over the Balkans. This configuration would imply very dry summers that did not allow C₃ plants to grow, thus supporting C₄ development. The intra-"C₄ episode" periods would have occurred under less drastic oceanic and atmospheric patterns that made the influence of westerlies on the Balkans possible

Major dust events in Europe during marine isotope stage 5 (130–74 ka): a climatic interpretation of the "markers"

At present, major dust storms are occurring at mid-latitudes in the Middle East and Asia, as well as at low latitudes in Northern Africa and in Australia. Western Europe, though, does not experience such dramatic climate events, except for some African dust reaching it from the Sahara. This modern situation is of particular interest, in the context of future climate projections, since the present interglacial is usually interpreted, in this context, as an analog of the warm Eemian interval. European terrestrial records show, however, major dust events during the penultimate interglacial and early glacial. These events are easily observed in loess records by their whitish-color deposits, which lie above and below dark chernozem paleosols in Central European records of Marine Isotope Stage (MIS) 5 age. We describe here the base of the Dolni Vestonice (DV) loess sequence, Czech Republic, as the reference of such records. The dust is deposited during intervals that are characterized by poor vegetation-manifested by high δ13C values and low magnetic susceptibility-while the fine sand and clay in the deposits shows grain sizes that are clearly different from the overlying pleniglacial loess deposits. Some of these dust events have been previously described as "Markers" or Marker Silts (MS) by one of us (G. Kukla), and are dated at about 111-109 ka and 93-92 ka, with a third and last one slightly visible at about 75-73 ka. Other events correspond to the loess material of Kukla's cycles, and are described as eolian silts (ES); they are observed in the same DV sequence and are dated at about 106-105 ka, 88-86 ka, and 78.5-77 ka. These dates are determined by considering the OSL ages with their errors measured on the studied sequence, and the comparison with Greenland ice-core and European speleothem chronologies. The fine eolian deposits mentioned above, MS as well as ES, correspond to short events that lasted about 2 ka; they are synchronous with re-advances of the polar front over the North Atlantic, as observed in marine sediment cores. These deposits also correlate with important changes observed in European vegetation. Some ES and MS events appear to be coeval with significant dust peaks recorded in the Greenland ice cores, while others are not. This decoupling between the European eolian and Greenland dust depositions is of considerable interest, as it differs from the fully glacial situation, in which the Eurasian loess sedimentation mimics the Greenland dust record. Previous field observations supported an interpretation of MS events as caused by continental dust storms. We show here, by a comparison with speleothems of the same age found in the northern Alps, that different atmospheric-circulation modes seem to be responsible for the two categories of dust events, MS vs. ES. © 2013 Author(s)