Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : International audience
The Cretaceous paleogeography and the kinematic evolution of the Iberian plate are poorly constrained. Especially problematic is to reconcile Iberian paleomagnetic data with paleomagnetic data of the neighboring plates and with Euler poles derived from seafloor magnetic anomalies. The first limitation arises from the Cretaceous Normal Polarity Superchron where paleogeographic reconstruction using marine magnetic anomalies is handicapped. The second arises from the paucity of reliable paleomagnetic poles with satisfactory statistical criteria and age. In order to address these shortcomings and provide new high quality paleomagnetic poles for Iberia, we conducted a detailed rock magnetic and paleomagnetic study of two Cretaceous magmatic sills, the Paço de Ilhas (PI) and Foz da Fonte (FF) sills, from the Lusitanian Basin, Portugal, recently dated at about 88 and 94 Ma, respectively. Our results show that the magnetic mineralogy of the sills is primary, i.e., acquired during magma cooling, and essentially represented by titanomagnetite. The corresponding paleomagnetic poles match the synthetic APWP from the African plate at 80 and 100 Ma. On the basis of a rigorous selection of Iberian Cretaceous poles, we then calculated mean paleomagnetic poles for different time intervals and found that Iberian paleomagnetic data fit well the global APWP between 70 and 120 Ma, but move far away from the APWP at pre-rift times. Our approach shows that new and better constrained paleomagnetic poles can aide in solving part of the contradiction between Iberian and African APWPs.
ISSN: 0148-0227

hal-00758103
https://hal.archives-ouvertes.fr/hal-00758103
DOI : 10.1029/2011JB009067

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
A detailed rock-magnetic and archeointensity study was carried out on materials baked by a western Mexican artisan following traditional techniques to produce faithful reproductions of archeological pieces of the Michoacan region (Western Mesoamerica). The field strength at the site (41.0 +/- 0.51 mu T) was measured with a fluxgate magnetometer and the temperature of the furnace during the baking process was monitored continually by means of a thermocouple placed in the middle of the baking cavity.;Rock-magnetic experiments performed on the raw material (clay and paste) and on in situ prepared baked ceramics and bricks included measurement of thermomagnetic curves (susceptibility and strong-field magnetization versus temperature), first-order reversal curves (FORC), anisotropy of magnetic susceptibility (AMS) and anisotropy of thermoremanent magnetization (A-TRM). Magnetite and probably hematite are present in the samples as carriers of the remanence. Hysteresis ratios suggest that the samples fall in the pseudo-single-domain grain size region, which may indicate a mixture of multidomain and a significant amount of single-domain grains.;Ceramic pieces and brick fragments were subjected to the Thellier-Coe archeointensity method and to an alternative paleointensity experiment, with a TRIAXE magnetometer, in order to check whether they are faithful recorders of the local geomagnetic field strength.;Mean raw-intensity of sample M1 (pottery) overestimates a 7% the expected site intensity, while those corresponding to the brick samples (LQ1 and LQ2) underestimate it 15%. Brick sample LNQ shows a slightly lower intensity (7%), but agrees with the expected site intensity within the experimental uncertainty. The intensity retrieved from the volcanic fragment also included closely reproduces the expected intensity. After A-TRM and cooling-rate corrections, all mean raw values move closer to the expected intensity.;Measurement of temperatures at different parts inside the kiln (bottom and upper parts of both central and peripheral parts) revealed the existence of significant thermal gradients, similar to those observed in ovens from other localities. Different cooling rates are then expected in a single oven.;The scatter in the intensity determinations observed in this study, retrieved from pieces elaborated together in the same oven, could arise from this differentiated cooling rate within the oven and thus, to an inappropriate cooling rate correction in the archeointensity protocol. As this situation was probably reproduced in the baking of ancient ceramic artifacts, a better knowledge of the temperature distribution inside these types of kiln would be desirable in order to choose the appropriate cooling rate correction.
ISSN: 0031-9201

hal-00644559
https://hal.archives-ouvertes.fr/hal-00644559
DOI : 10.1016/j.pepi.2011.06.007

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
We present new archaeointensity results from three Italian kilns situated at Ascoli Satriano, Vagnari and Fontanetto Po obtained with the Thellier modified by Coe double heating method. These data complement the directional results previously published. All sites are dated on the basis of archaeological information and/or thermoluminescence dating. The results are corrected for the anisotropy of the thermoremanent magnetization and the cooling rate effects. The new data are compared with previously published archaeointensity data from Italy and nearby countries within 900 km radius from Viterbo. An initial data set including archaeointesity data mainly coming from Italy, France, Switzerland, Czech Republic, Slovakia, Hungary, Greece and Bulgaria has been compiled. After the application of strict selection criteria, the most reliable data have been used for the calculation of a preliminary Italian intensity secular variation (SV) curve for the last 3000 yr. The new curve covers the 300 BC-400 AD and 1200-1900 AD periods. It is established by means of sliding windows of 200 yr shifted by 100 yr. The lack of reliable data for the 1000-200 BC and 400-1200 AD time intervals does not permit the calculation of a continuous curve. Clearly, more high-quality archaeointensity data from Italy and Europe are still needed to draw a robust intensity SV curve for the Italian Peninsula that could be used for archaeomagnetic dating in combination with the directional data.
ISSN: 0956-540X

hal-00828691
https://hal.archives-ouvertes.fr/hal-00828691
DOI : 10.1093/gji/ggs120

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Underground CO2 sequestration is highly recommended as an effective means of significantly decreasing CO2 concentration in the atmosphere. Mineral storage is the more secure technology, but requires the presence of high concentration of divalent cations in the pore-fluid. Results from CO2 percolation experiments through chlorite/zeolite-rich sandstone samples from the Pretty Hill Formation (Otway Basin, Australia) are presented. The dissolution of the laumontite (7 wt.%) and chamosite (7 wt.%) are the potential sources of calcium, iron and magnesium required for carbonate precipitation. The percolation experiment was setup to reproduce, at laboratory scale, the in situ temperature and pressure conditions (T = 95 degrees C and P = 10 MPa). The fluid injected at constant flow rate is a rock-equilibrated brine subsequently enriched in CO2 up to partial pressure of 6 MPa. We observe feldspars, laumontite and chamosite dissolution, kaolinite and silica precipitation and a noticeable sink of CO2 in the sample which is attributed to the precipitation of both amorphous carbon due to the reduction of CO2 and Fe-rich carbonate. Permeability decreases of about one order of magnitude due to the localization of the kaolinite precipitation in the main flow paths, while porosity increases. The high reactivity of this sandstone makes this reservoir a valuable target for CO2 mineralization, but the associated permeability decrease may limit the injection rate and the spreading of the CO2 in the reservoir.
ISSN: 0009-2541

hal-00712829
https://hal.archives-ouvertes.fr/hal-00712829
DOI : 10.1016/j.chemgeo.2011.11.018

Éditeur(s) : HAL CCSD Springer Verlag (Germany)
Résumé : International audience
The influence of magnetic interactions on the anisotropy of magnetic susceptibility (AMS) have been largely studied by several theoretical models or experiments. Numerical models have shown that when magnetostatic interactions occur, the distributions of particles over the volume rather than their individual orientations control the AMS. We have shown recently from a comprehensive rock magnetic study and from a theoretical 2-dimensional (2-D) model that single domain particles closely packed in globule aggregates could produce strong local random interaction magnetic fields which could influence the magnetic susceptibility and decrease the degree of anisotropy. In this paper, we first present in detail this 2-D theoretical model and then we extend it to the 3-D case. The possible distribution function of the magnetostatic interaction fields comprises two extreme states: it is either isotropic or ordered. The former case corresponds to the thermal-demagnetized state while the second case corresponds to the alternating field (AF) demagnetized state. We show that when easy axes of magnetization are not uniformly distributed, the degree of anisotropy decreases as the interaction field increases in both AF- and thermal-demagnetized states in 2-D and 3-D geometry. Thus we conclude that random magnetic fields generated by a random arrangement of magnetic particles over the sample volume decrease the degree of anisotropy of AMS and may alter the magnetic fabric.
ISSN: 0039-3169

hal-00758113
https://hal.archives-ouvertes.fr/hal-00758113
https://hal.archives-ouvertes.fr/hal-00758113/document
https://hal.archives-ouvertes.fr/hal-00758113/file/SGEG-2012-HAL.pdf
DOI : 10.1007/s11200-010-9097-7

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
IODP Expedition 352 cored igneous rocks from the Izu-Bonin-Mariana forearc crust. Cores from Sites U1440 and U1441 recovered Eocene basalts and related rocks and cores from Sites U1439 and U1442 recovered Eocene boninites and related rocks. We selected samples from Holes U1439C, U1440B and U1442A for palaeointensity measurements. Hysteresis measurements and high and low-temperature magnetization curves show that samples from Hole U1440B undergo magneto-chemical changes when heated and are mostly composed of single-domain (SD) or pseudo-single-domain (PSD) titanomaghemite. In contrast, the same measurements show that most selected samples from Holes U1439C and U1442A are thermally stable and are composed of either SD or PSD titanomagnetite with very little titanium content, or SD ferromagnetic grains with a large paramagnetic contribution. Thellier–Thellier palaeointensity experiments carried out on U1439C and U1442A samples give a good success rate of 25/60 and Virtual Dipole Moment (VDM) values between 1.3 and 3.5 × 1022 Am2. Multispecimen palaeointensity experiments with the domain-state corrected method carried out on 55 samples from Hole U1440B (divided into four groups) and 20 from Hole U1439C gave poor quality results, but indicated a VDM around 4–6 × 1022 Am2 in Hole U1440B forearc basalts. These results are in agreement with the few, low VDM values previously measured on Eocene rocks. However, they do not support an inverse relationship between field intensity and reversal rate for this period of time, since the Eocene reversal rate was low.
ISSN: 0956-540X

hal-01622775
https://hal.archives-ouvertes.fr/hal-01622775
https://hal.archives-ouvertes.fr/hal-01622775/document
https://hal.archives-ouvertes.fr/hal-01622775/file/carvalloGJI2017.pdf
DOI : 10.1093/gji/ggx208

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The Blake excursion was among the first recognized with directional and intensity behavior known mainly from marine sediment and Chinese loess. Age estimates for the directional shifts in sediments are poorly constrained to about 118−100 ka, i.e., at the marine isotope stage (MIS) 5e/5d boundary. Moreover, sediments at Lac du Bouchet maar, France and along the Portuguese margin reveal what may be a "post-Blake" excursion at about 105−95 ka. The excursional directions are associated with a prominent paleointensity minimum between about 125 and 95 ka in global stacked records. Lava flow recordings of the Blake excursion(s) have, however, been questionable because precise ages required for correlation with these sediment records are lacking. To establish new, independent records of the Blake excursion, and link these into a larger Quaternary GITS, we have undertaken 40Ar/39Ar incremental heating and unspiked K-Ar experiments on groundmass from the transitionally magnetized Inzolfato flow on Lipari Island. We also obtained 40Ar/39Ar incremental heating results for a lava flow on Amsterdam Island originally thought to record the Mono Lake excursion and from the transitionally magnetized El Calderon basalt flow, New Mexico that was K-Ar dated by Champion et al. (1988) at 128 ± 66 ka. Unspiked K-Ar ages of four samples from the Inzolfato flow are 102.5 ± 4.7, 101.3 ± 3.3, 97.1 ± 2.6, and 96.8 ± 3.1 ka and thus indistinguishable from one another. 40Ar/39Ar results are more complex, with three samples yielding discordant age spectra. Based on incremental heating data obtained in both the UW-Madison and Gif-sur-Yvette 40Ar/39Ar laboratories, a fourth sample yields six concordant age plateaus and a weighted mean age of 105.2 ± 1.4 ka that we take as the best estimate of time since the flow erupted. Five 40Ar/39Ar incremental heating experiments on the Amsterdam Island lava yield a plateau age of 120 ± 12 ka, whereas ages from two sites in the Calderon flow are 112 ± 23 and 101 ± 14 ka, together giving a weighted mean of 104 ± 12 ka. The age of 120 ± 12 ka from Amsterdam Island, though imprecise, correlates with the Blake excursion. In contrast, the 104-105 ka age obtained from both Lipari and New Mexico indicates that these lavas record a younger period of dynamo instability, most probably associated with the post-Blake excursion. These radioisotopic ages are consistent with the astronomical ages of two paleointensity minima in the PISO-1500 global stack. Our findings indicate that the Blake and post-Blake excursions are both global features of past geodynamo behavior and support the hypothesis that Brunhes chron excursions are temporally clustered into two groups of at least a half-dozen each spanning over 220 to 30 ka and 720 to 520 ka.
ISSN: 1871-1014

hal-01054336
https://hal.archives-ouvertes.fr/hal-01054336
DOI : 10.1016/j.quageo.2012.12.005

Éditeur(s) : HAL CCSD
Résumé : The Earth's magnetic field shows a large range of temporal variations from the year to several million years. I studied during my PhD thesis several aspects of these fluctuations, from recent secular variations to reversals. The first part of my manuscript deals with archeomagnetism, a discipline that allows to track the temporal variations of the Earth's magnetic field through millennia, mainly from archeological materials. I studied two sets of samples , one from Greece (Neolithic period 6800-3200 B.C.) and the other from Mexico (Palenque, Maya Classic period 320-840 A.D.), to acquire new archeointensity data in order to better constrain the secular variation of the geomagnetic field. By comparing my data with those available in the literature and with the various global and regional models, I showed that the secular variations during the Neolithic in Greece and during the first millennium in Central America are poorly defined. My data suggest that local components, not described by global models, may exist in these regions. They reinforce the importance of developing specific regional models. As a consequence, the acquisition of new high quality data is of main importance. The second part presents the description of a geomagnetic field reversal. This work was based on two points : first by studying transitional directions to provide new constraints on the possible preferred longitudinal paths of virtual geomagnetic poles (VGPs) and second by checking transitional paleointensities obtained on a volcanic sequence in Akaroa volcano (New Zealand). Indeed the transitional field intensity is significantly higher than the field intensity before and after the reversal. We re-sampled this sequence, and the directional results show a complex sequence of N-T-R-T-N-T-R polarity. The transitional VGPs obtained are clustered in two longitudinal bands through Australia and America. This observation seems to reinforce the assumption of a core-mantle interaction over several million years. Following a rock magnetic study, I selected samples that could provide a value for the Thellier and multispecimen paleointensity methods. The obtained paleointensity are relatively low (about 20 μT) during the polarity change and strong at the end of the sequence. Based both on the field strength values and on the radiochronological ages, showing that the sequence was erupted in a very short time, I suggest that only the C4AR.1n-C4Ar.1r reversal was recorded in this sequence. In this assumption, the reversal shows a complex path comparable to other reversals recorded in the northern hemisphere (for example the Steens Mountain ), including a rebound before stabilizing. Finally the last part is devoted to the development of a new methodology and a new apparatus to determine absolute paleointensity. Following the low success rate of paleointensity experiments from the previous study, I decided to test the multispecimen protocol, which can be applied to samples yielding a predominant multidomaine behavior. The main technical drawback of this method lies in the application of the laboratory field along the natural remanent magnetization, a difficult task to perform accurately in standard paleointensity ovens. Thus, we decided to adapt sample holders from our standard oven in order to allow the sample orientation in space and to develop an ultra-fast heating oven prototype particularly well-suited for this method, allowing to apply the laboratory field in the 3 dimensions. I checked the different multispecimen protocols on historical lavas from Reunion and Etna volcano, yielding very different magnetic mineralogies. For all flows, I obtained paleointensities very close from the expected values, regardless from the magnetic mineralogy, revealing the feasibility of our apparatus and the promising interest of the method. The application of various corrections on the statistical estimation of the intensity value or on the cooling rate had no significant impact on my results.
La première partie de ma thèse porte sur l'archéomagnétisme, discipline qui permet de retracer l'évolution temporelle du champ magnétique terrestre au travers des derniers millénaires, principalement à partir des matériaux archéologiques. J'ai étudié deux jeux d'échantillons provenant pour l'un de la Grèce (période néolithique, 6800-3200 avant J.C.) et pour l'autre du Mexique (Palenque, période Maya Classique 320-840 après J.C.), dans le but d'acquérir de nouvelles données d'archéointensité et de mieux contraindre la variation séculaire du champ magnétique terrestre. La comparaison de mes données avec celles disponibles dans la bibliographie et avec les différents modèles globaux et régionaux a mis en évidence que les variations séculaires au Néolithique en Grèce et au cours du premier millénaire en Amérique Centrale sont très mal définies. Mes données suggèrent que des composantes locales, non prisent en compte dans les modèles globaux peuvent exister au niveau de ces régions. Elles renforcent l'intérêt de développer des modèles régionaux précis. Pour ce faire, l'acquisition de nouvelles données de haute qualitéest un élément majeur. La deuxième partie traite de la description d'un renversement du champ magnétique terrestre. Cette étude a été basée sur deux points précis : étudier les directions transitionnelles afin d'apporter de nouvelles contraintes sur le possible confinement longitudinal des pôles géomagnétiques virtuels (PGV) d'une part, et d'autre part vérifier des paléointensités transitionnelles obtenues sur la séquence volcanique d'Akaroa (Nouvelle Zélande), dont l'intensité est significativement supérieure à celles des intensités avant et après le renversement. Nous avons ré-échantillonné cette séquence, et l'évolution directionnelle obtenue pour ce renversement est une succession complexe de polarités N-T-R-T-N-T-R. Les PGV obtenus semblent se regrouper sous deux bandes longitudinales sous l'Australie et l'Amérique, ce qui renforce l'hypothèse d'une interaction entre le manteau et le noyau sur plusieurs millions d'années. Suite à une étude de minéralogie magnétique, j'ai sélectionné les échantillons susceptibles de fournir une valeur de paléointensité par les méthodes de Thellier et du multispecimen. Les paléointensités obtenues sont relativement faibles (environ 20 μT) au cours du changement de polarité et forte à la fin de la séquence. Mon interprétation, basée à la fois sur les valeurs de l'intensité du champ et sur les données radiochronologiques montrant que la séquence s'est mise en place très rapidement, est de considérer que seul le renversement C4Ar.1n-C4Ar.1r a été enregistré dans cette séquence. Dans cette hypothèse, le renversement montre un cheminement complexe comparable à d'autres renversements enregistrés dans l'hémisphère nord (comme celui de la Steens Mountain par exemple), incluant un phénomène de rebond avant de se stabiliser. La troisième partie de ma thèse est consacrée au développement d'une nouvelle méthodologie et d'un nouvel appareillage pour déterminer des paléointensités. Le faible taux de réussite des expériences de paléointensité de l'étude précédente m'a poussé à m'intéresser au protocole multispecimen, qui peut s'appliquer aux échantillons possédant un comportement polydomaine. L'inconvénient technique majeur de cette méthode réside dans l'application du champ le long de l'aimantation naturelle, difficile à réaliser avec précision dans les fours standards. Pour ce faire, nous avons décidé de développer des porte-échantillons permettant d'orienter les échantillons dans l'espace pour le four standard et un prototype de four à chauffage ultra-rapide particulièrement bien adapté avec la possibilité d'orienter le champ appliqué dans les 3 dimensions. J'ai testé les différents protocoles de la méthode sur des laves historiques de la Réunion et de l'Etna, possédant des minéralogies très différentes. Pour l'ensemble des coulées étudiées, j'ai obtenu des paléointensités très proches des valeurs attendues, et ce indépendamment de la minéralogie, révélant ainsi le faisabilité de notre appareillage et le côté prometteur de la méthode. L'application de diverses corrections sur la statistique d'estimation de la valeur de l'intensité ou sur le taux de refroidissement n'ont eu aucun impact notable sur mes résultats.
https://tel.archives-ouvertes.fr/tel-00719380

tel-00719380
https://tel.archives-ouvertes.fr/tel-00719380
https://tel.archives-ouvertes.fr/tel-00719380/document
https://tel.archives-ouvertes.fr/tel-00719380/file/thesefanjat2012.pdf

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
We present new, full geomagnetic field vector results from three Neolithic ovens discovered at the archaeological site of Portonovo (Marche, Italy). The discovered structures are a rare example of very well preserved underground ovens from the Early Neolithic period. Standard thermal demagnetization procedures were used to isolate the direction of the Characteristic Remanent Magnetization acquired by the baked clay during the ovens' last firing. The corresponding archaeointensities were determined by the multi-specimen procedure (MSP-DSC) and show a clear intensity low during the Neolithic period. Both directional and intensity results are of high quality, offering the first contribution of full geomagnetic field vector data for this period in Italy. The new data are compared with other contemporaneous data from Europe and with global geomagnetic field models. Independent archaeomagnetic dating of the three ovens was also performed by means of the SCHA.DIF.14k model. The obtained results are in excellent agreement with available radiocarbon dates and confirm that all ovens belong to the Neolithic. These new data importantly enrich our knowledge of the geomagnetic field during the Neolithic period that is poorly documented by data, not only in Italy but also in the whole of Europe and show that archaeomagnetic dating can provide precise results even for prehistoric periods.
ISSN: 0012-821X

hal-01357276
https://hal.archives-ouvertes.fr/hal-01357276
DOI : 10.1016/j.epsl.2016.05.003

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The present study aims to apply the AMS method (Anisotropy of Magnetic Susceptibility) at a regional scale to track the fluid circulation direction that has produced an iron metasomatism within pre-existing dolomite host rock. The Urgonian formations hosting the Zn-Pb mineralizations in La Florida (Cantabria, northern Spain) have been taken as target for this purpose. Sampling was carried out, in addition to ferroan dolomite host rock enclosing the Zn-Pb mineralizations, in dolomite host rock and limestone to make the comparison possible between magnetic signals from mineralized rocks, where fluid circulation occurred, and their surrounding formations. AMS study was coupled with petrofabric analysis carried out by texture goniometry, Scanning Electron Microscopy (SEM) observations and also Shape Preferred Orientation (SPO) statistics. SEM observations of ferroan dolomite host rock illustrate both bright and dark grey ribbons corresponding respectively to Fe enriched and pure dolomites. SPO statistics applied on four images from ferroan dolomite host rock give a well defined orientation of ribbons related to the intermediate axis of magnetic susceptibility K2. For AMS data, two magnetic fabrics are observed. The first one is observed in ferroan dolomite host rock and characterised by a prolate ellipsoid of magnetic susceptibility with a vertical magnetic lineation. The magnetic susceptibility carrier is Fe-rich dolomite. These features are probably acquired during metasomatic fluid circulations. In Fe-rich dolomite host rock, ‹c› axes are vertical. As a rule, (0001) planes (i.e. planes perpendicular to ‹c› axes) are isotropic with respect to crystallographic properties. So, the magnetic anisotropy measured in this plane should reflect crystallographic modification due to fluid circulation. This is confirmed by the texture observed using the SEM. Consequently, AMS results show a dominant NE-SW elongation interpreted as the global circulation direction and a NW-SE secondary elongation that we have considered as sinuosities of the fluid trajectory. The second type of magnetic fabric is essentially observed in the limestone and characterised by an oblate form of the ellipsoid of magnetic susceptibility, a horizontal magnetic foliation and mixed magnetic susceptibility carriers. It is interpreted as a sedimentary fabric.
ISSN: 0012-821X

insu-00342499
https://hal-insu.archives-ouvertes.fr/insu-00342499
https://hal-insu.archives-ouvertes.fr/insu-00342499/document
https://hal-insu.archives-ouvertes.fr/insu-00342499/file/Essalhi-EPSL-2008.pdf
DOI : 10.1016/j.epsl.2008.11.011

Éditeur(s) : HAL CCSD Springer/Terra Scientific Publishing Company
Résumé : geomagnetism
International audience
Preface
ISSN: 1343-8832

hal-00413106
https://hal.archives-ouvertes.fr/hal-00413106

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Studies of paleomagnetic fields during polarity transitions recorded over the past few million years frequently suggest a configurational persistence consistent with long-lived mantle control over the pattern of flux emanating from Earth's fluid core. Fundamental to an understanding of the core-mantle dynamics involved is the question of the spatial-temporal nature of the physical conditions at the base of the mantle that may be responsible. Our analysis of the field during a mid-Cenozoic, reverse-to-normal (R-N) polarity change, recorded in a continuous sequence of lava flows in southeastern Queensland, Australia, provides strong evidence that the time span over which local mantle features retain such an influence is far longer than had thus far been observed. Specifically. the reversal is dominated by two sequential groupings of transitional magnetic vector directions, each associated with a clustering of virtual geomagnetic poles (VGPs) at locations common to several more youthful reversal datasets from about the globe. Our Ar-40/Ar-39 age determinations indicate that these Australian lavas erupted 25.56 +/- 0.48 Ma, strongly suggesting that the characteristic time for invariant control by the mantle over flux emerging from the outer core is at least on order of a few tens of Myr. The availability of transitional paleomagnetic data from still earlier times within the Cenozoic may further delineate this duration and, perhaps, provide a means to track changes in the pattern of long-standing concentrations of magnetic flux at the core-mantle boundary. (C) 2008 Elsevier B.V. All rights reserved.
ISSN: 0031-9201

hal-00411919
https://hal.archives-ouvertes.fr/hal-00411919
DOI : 10.1016/j.pepi.2008.07.012

Éditeur(s) : HAL CCSD American Geophysical Union (AGU) American Geophysical Union
Résumé : International audience
Environmental magnetism of speleothems is still in its early stage of development. Here we report on our investigation of the environmental and paleomagnetic information that has been recorded in speleothems, and what are the factors that control its preservation and reliability. To address these issues, we used a multidisciplinary approach, including rock magnetism, petrography, scanning electron microscopy, stable carbon and oxygen isotope compositions, and major and trace element concentrations. We applied this to a set of samples from different stages of speleothem evolution: present-day dripwater (glass plates), a weathered stalactite, a fresh stalagmite, cave sediments, and terra rossa soils. These samples come from the Penico and Excentricas caves, located in two distinct aquifers of the Algarve region, South Portugal. Our results show that the main magnetic carriers of the speleothems under study are primary (detrital) and consist of maghemite (and magnetite?). Similarities in coercivity and temperature dependence of the studied set of samples suggest that iron oxides are inherited from the terra rossa soils that cap the cave and were transported to the speleothems by dripwater. Hence, they represent a regional environmental signature. Interestingly, a stable and probably detrital remanent magnetization could be isolated in the fresh stalagmite, whereas the weathered stalactite yielded chaotic magnetic directions and very low remanent intensities. We propose that these low intensities can be the result from (i) different remanence acquisition mechanisms between stalagmite and stalactite and/or (ii) iron dissolution by fungal activity. We also suggest that magnetic properties and color and the content in detrital elements in the fresh speleothem inform about environmental processes acting on the interface of rock (soil)-atmosphere, while oxygen isotope composition and alkaline-earth element concentrations inform about calcite-water interaction processes. These results provide a better understanding of how environmental information is recorded in speleothems and what the factors are that control the reliability of the paleomagnetic and paleo-environmental signal.
ISSN: 0148-0227

hal-01114953
https://hal.archives-ouvertes.fr/hal-01114953
DOI : 10.1002/2014JB011381

Éditeur(s) : HAL CCSD American Association for Cancer Research
Résumé : International audience
Triple-negative breast cancers (TNBC) lacking estrogen and progesterone receptors and HER2 amplification have a relatively high risk of metastatic dissemination, but the mechanistic basis for this risk is not understood. Here we report that serum levels of CD95L are higher in TNBC patients compared to other breast cancer patients. Metalloprotease-mediated cleavage of CD95L expressed by endothelial cells surrounding tumors generates a gradient that promotes cell motility, due to formation of an unconventional CD95-containing receptosome termed the motility-inducing signaling complex. Formation of this complex was instrumental for Nox3-driven ROS generation. Mechanistic investigations revealed a Yes-Orai1-EGFR-PI3K pathway that triggered migration of TNBC cells exposed to CD95L. Our findings establish a pro-metastatic function for metalloprotease-cleaved CD95L in triple-negative breast cancers, revisiting its role in carcinogenesis.
ISSN: 0008-5472

hal-00873634
https://hal.archives-ouvertes.fr/hal-00873634
PUBMED : 24072745
DOI : 10.1158/0008-5472.CAN-13-1794

Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : International audience
We present the results of a paleomagnetic study of seven new volcanic sections (146 flows) from Kerguelen Archipelago. For two of these sections, preliminary (;40;Ar/;39;Ar) ages are reported to aid in the calibration of the paleomagnetic results. The primary contribution of this report, however, is a compilation of these new data with those already published in order to describe statistically the characteristics of the paleomagnetic field as recorded by the Kerguelen flood basalts. In total, 258 paleomagnetic directions sampled at 13 stratigraphic sections through the lava pile are available and span an approximately 5 Ma window: from 25 to 30 Ma. The composite section represents at least 11 polarity zones that are correlated to the reference geomagnetic polarity timescale. Our approach is to investigate the average normal and reversed polarity field directions over this 5 Ma window. We calculated a paleomagnetic pole found to be located at ;λ; = 85.5°N, = 189.3°E (A;95; = 2.3°, K = 16.5, N = 233). This pole is in close agreement with the coeval paleomagnetic poles obtained from different worldwide places when analyzed in the Indo-Atlantic hot spot reference frame. In the statistical analysis we tackle the specific question: Is the secular variation isotropic? This question is directly related to the occurrence of a longitudinal confinement of the virtual geomagnetic poles, which is still a matter of debate among paleomagnetists. By means of statistical tests we show that the paleomagnetic data from Kerguelen agree with an isotropic model for paleosecular variation. Finally, we present adjustments to Camps and Prévot's (1996) statistical model, developed for some Northern Hemisphere latitudes, to the Southern Hemisphere paleomagnetic data from Kerguelen.
ISSN: 0148-0227

hal-00407636
https://hal.archives-ouvertes.fr/hal-00407636
https://hal.archives-ouvertes.fr/hal-00407636/document
https://hal.archives-ouvertes.fr/hal-00407636/file/pcamps-JGR2007.pdf
DOI : 10.1029/2006JB004648

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
IODP Expedition 324 cored igneous rocks from Shatsky Rise, an oceanic plateau in the northwest Pacific Ocean that formed mainly during late Jurassic and Early Cretaceous times. We selected 60 samples from 3 different holes for Thellier-Thellier palaeointensity determinations. Induced and remanent magnetization curves measured at low- and high-temperature suggest a diverse and complex magnetic mineralogy, with large variations in Ti content and oxidation state. Hysteresis and FORC measurements show that most samples contain singledomain magnetic grains. After carrying out the palaeointensity determinations, only 9 samples satisfied all reliability criteria. These gave palaeointensity values between 16.5 and 21.5 μT, which correspond to average VDM values of (4.9 ± 0.2) × 1022 Am2 for an estimated age of 140-142 Ma. This value is lower than that for the recent field, which agrees with the hypothesis of a Mesozoic Dipole Low.
ISSN: 0956-540X

hal-00795491
https://hal.archives-ouvertes.fr/hal-00795491
DOI : 10.1093/gji/ggs100

Éditeur(s) : HAL CCSD Springer/Terra Scientific Publishing Company
Résumé : International audience
The laboratory of paleomagnetism of Montpellier (France) has developed a new one-axis vibrating thermal magnetometer dedicated to the Study of physical properties of natural rocks remanence. Among its key characteristics, this apparatus allows both measurement or the magnetization moment on the interval from room temperature to 700 degrees C with a precision of 2 x 10(-9) A m(2) and acquisition of a total or a partial thermo-remanent magnetization using a steady field from - 100 up to 100 mu T. Another point that is worth noting is that one can apply a controlled atmosphere by means of argon flux to prevent oxidation of the studied sample during heating. We report here a technical description of this new instrument and review some specific applications in absolute paleointensity surveys.
ISSN: 1343-8832

hal-00413078
https://hal.archives-ouvertes.fr/hal-00413078
https://hal.archives-ouvertes.fr/hal-00413078/document
https://hal.archives-ouvertes.fr/hal-00413078/file/Poidras-CatVTM.pdf

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
Anisotropy of low-field magnetic susceptibility (AMS) of basaltic lava flows can give some clues about post-emplacement tilting occurring in volcanic sequences. Such a study has been carried out on a sequence of 19 successive lava flows from Kerguelen Archipelago. Surprisingly, two different patterns were observed. The first one - herein called normal fabric - is consistent with the lava-flow direction inferred from geological observations, whereas the second one observed for about 70 per cent of the analysed samples - herein called abnormal fabric - appears to be unrelated to the shear history of the lava flows during their emplacements. These abnormal fabrics are not strictly inverse fabric but seem to have a more complex origin. The unusual nature of these abnormal fabrics has been confirmed by lattice preferred orientation of plagioclases obtained from high-resolution electron backscattered diffraction measurements. The aim of this study is to propose a physical interpretation of these fabrics. A first step in our understanding was to clearly identify the nature and the magnetic properties of the Fe-Ti oxides assemblage. Thus, we present a comprehensive rock magnetic analysis relying on low- and high-temperature thermomagnetic curves (K-T), crossover diagrams, first-order reversal curves, ore-microscopy, Mössbauer measurements and electron backscattered images. We found that in the present case the abnormal fabrics are probably linked to a mixture of prevailing single domain (SD) strongly interacting grains population and a subsidiary multidomain grains population. SD grains are identified as small magnetite crystallized within the interstitial glass in globular aggregates along the silicate framework. Then a question arises: to what extent such SD interacting grains in globular aggregates can affect the magnetic susceptibility and its anisotropy? To answer this question, we developed a physical model in which the total energy density of an SD grain with an uniaxial anisotropy is minimized. In this model, the distribution function of the direction of the magnetic interaction field is comprised between two boundary states. It is either isotropic, which formally corresponds to the thermal demagnetized state, or it is ordered, which formally corresponds to the Alternative Field demagnetized state. We show that in both cases the degree of anisotropy decreases as the interaction field increases. Thus, we conclude that the abnormal fabric encountered in this study can be simply explained by strong magnetostatic interactions that would explain the large scattering of AMS often observed in basaltic lava flows.
ISSN: 0956-540X

hal-00716449
https://hal.archives-ouvertes.fr/hal-00716449
https://hal.archives-ouvertes.fr/hal-00716449/document
https://hal.archives-ouvertes.fr/hal-00716449/file/gji-S-12-HAL.pdf
DOI : 10.1111/J.1365-246X.2012.05421.X

Éditeur(s) : HAL CCSD
Résumé : palaeomagnetism applied to tectonics
Geotemas

hal-00411975
https://hal.archives-ouvertes.fr/hal-00411975

Éditeur(s) : HAL CCSD G.M.P.C.a/Presses universitaires de Rennes
Résumé : Magnetic measurements (magnetic susceptibility at low and high frequency, isothermal and anhysteretic remanent magnetizations, magnetic susceptibility according to the temperature and hysteresis loop) were carried out on samples taken in the filling of Caverna delle Manie (Arma delle Manie) located in Ligurie, Italy. The filling of the site, dating from upper Pleistocene and Holocene, is characterized in its upper part by an alternance of carbonated concretions, stalagmitic floors and clays, and in its lower part by an sandy-silty-clay deposit. The results of the magnetic study reveal the abundance of magnetic grains of secondary origin (magnetite and maghemite), and more particularly in the upper part of the filling. The grains of superparamagnetic size (SP) dominate, followed by those of multi-domain (MD) and single-domain size (SD). The latter have a low coercitive force. Some hearths were located within the filling thanks to their color, with the presence of coal and the strong values of the various magnetic parameters.
Des mesures magnétiques (susceptibilité magnétique à basse et haute fréquence, aimantation rémanente isotherme et anhystérétique, susceptibilité magnétique en fonction de la température et cycle d'hystérésis) ont été réalisées sur des échantillons prélevés dans le remplissage de la Caverna delle Manie (Arma delle Manie) située en Ligurie Italienne. Le remplissage du site, d'âge Pléistocène supérieur et Holocène, se caractérise dans sa partie supérieure par l'alternance de concrétions carbonatées, de planchers stalagmitiques, et d'argiles; et dans sa partie inférieure par un dépôt sablo-limono-argileux. Les résultats de l'étude magnétique révèlent l'abondance de grains magnétiques d'origine secondaire (magnétite et maghémite). Cette abondance est plus marquée dans la partie supérieure du remplissage. Les grains de taille superparamagnétique (SP) dominent, suivi par ceux de taille poly-domaine (PD) et mono-domaine (MD). Ces derniers ont un champ coercitif faible. Quelques foyers ont été repérés au sein du remplissage grâce à leur couleur, à la présence de charbon et aux fortes valeurs des différents paramètres magnétiques.
ISSN: 1960-1360

hal-00653405
https://hal.archives-ouvertes.fr/hal-00653405