Éditeur(s) : HAL CCSD Springer/Terra Scientific Publishing Company
Résumé : International audience
We report oil the discovery of magnetite nanoparticles ranging in size from 2 to 14 full in the mineralized zones of the Pena Colorada iron-ore deposit, southern Mexico. Micrometric scale magnetite was magnetically reduced and divided into distinct size ranges: 85-56 mu m, 56-30 mu m, 30-22 mu m, 22-15 mu m, 15-10 mu m, 10-7 mu m and 7-2 mu m. Nanometric-scale magnetite in the size range 2-14 nm was identified. The magnetite was characterized by X-ray diffraction, transmitted and reflected light microscope, high-resolution transmission electron microscopy (TEM), high angle annular dark field, Mossbauer spectroscopy and its magnetic properties. Crystallographic identification of nanostructures Was performed using high-resolution TEM. Characteristic changes were observed when the particles make the size transition from micro- to nanometric sizes, as follows: (1) frequency-dependent magnetic Susceptibility percentage (chi FD%) measurements Show high Values (13%) for the 2-14 nm fractions attributed to dominant fractions of superparamagnetic particles; (2) variations of chi FD% < 4.5% in fractions of 56-0.2 mu m occur in association with the presence of microparticles formed by magnetite aggregates of nanoparticles (< 15 nm) embedded in berthierine; (3) Mossbauer spectroscopy results identified a superparamagnetic fraction; (4) nanometric and 0.2-3 mu m grain size magnetite particles require a magnetic field up to 152 mT to reach saturation during the isothermal remanent magnetization experiment; (5) coercivity and remanent magnetization of the magnetite increase when the particle size decreases, probably due to parallel Coupling effects; (6) two-magnetic Susceptibility versus temperature experiments of the same 2-14 nm sample show that the reversibility during the second heating is due to the formation of new magnetite nanoparticles and growth of those already present. during the first heating process.
ISSN: 1343-8832

hal-00413053
https://hal.archives-ouvertes.fr/hal-00413053

Éditeur(s) : HAL CCSD Springer/Terra Scientific Publishing Company
Résumé : International audience
Numerous phases of igneous activity took place in Egypt during the Mesozoic and the Cenozoic but no paleointensity results have ever been published from these rocks. Therefore a preliminary Survey was conducted in the northern part of Egypt and in Sinai to test the Suitability of these basalts for paleointensity determinations. Three Oligo-Miocene sites have been sampled north of Cairo: Abu Zaabal, Qatrani, and 6th of October City. In Sinai, we sampled Jurassic basalts in Wadi Budra and Oligo-Miocene rocks from Wadi Nukhul. The main magnetic carriers of these rocks are titanomagnetites with varying Ti content. Large secondary components are present in most middle Jurassic basalts and discrepancies remain in the directional analysis so this sill cannot be used for paleointensity experiments and the associated poles should not be considered for paleomagnetic reconstructions either. Directional analysis of the Oligo-Miocene basalts is very straightforward and updated mean VGPs have been calculated from the Cairo area (68.9 degrees N, 91.3 degrees E; Kappa = 274; A(95) = 3.7) and from Wadi Nukhul (72.7 degrees N, 13.4 degrees E; Kappa = 528; A(95) = 5.4). Pseudo-paleointensity results obtained as well from stable Egypt as from Sinai are promising, with values lower or similar (between 29 to 58 mu T) to present-day field intensity.
ISSN: 1343-8832

hal-00413086
https://hal.archives-ouvertes.fr/hal-00413086

Éditeur(s) : HAL CCSD Springer/Terra Scientific Publishing Company
Résumé : International audience
This study of the Cajati deposit provides evidence that the ore was neither purely hydrothermal, nor volcanic in origin, as previous workers have proposed. The ores were formed from magnetite-rich magmas, hydrothermally altered and intruded at an indicated crustal depth in excess of 500 m. The mineralogical and textural association between magnetite and magnesioferrite in the carbonatite, and between the titanomagnetite and magnesioferrite-Ti mineralization in the pyroxenite of hedenbergite, seems to be analog mineralizations strongly related to the ionic substitution of Fe2+ by Mg. Relatively high Q ratios (>= 5) for Jacupirangite-pyroxenite may indicate a thermo remanent magnetization (TRM) by the ore during post-metamorphic cooling, however it call also be developed from chemical remanent magnetization (CRM). Vector plots for the pyroxenite samples show reasonably linear and stable magnetic components. The intensity decay Curves show that only two components of magnetizations are likely present. Continuous susceptibility measurements with increasing temperature show that the main magnetic phase seems to be magnetite. Maghemite is probably produced during the cooling process. Susceptibility recorded from low temperature (liquid nitrogen (- 196 degrees C)) to room temperature produces typical Curves. indicating Verwey transition of magnetite. Hysteresis parameters point Out that nearly all values fill in a novel region of the Day plot, parallel to but below magnetite SD + MD mixing curves.
ISSN: 1343-8832

hal-00420922
https://hal.archives-ouvertes.fr/hal-00420922

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 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 Springer/Terra Scientific Publishing Company
Résumé : We analyse the results of Thellier palaeointensity experiments modified so that the measured magnetisation at any stage may be decomposed into components of natural remanent magnetisation (NRM) and laboratory thermal remanent magnetisation (TRM). We demonstrate that the very long high temperature tails of pTRM acquired by multidomain (MD) grains may be detected in Thellier experiments without recourse to explicit pTRM tail check measurements. This can save time in the experimental process and provides a more sensitive indication of how MD behaviour may affect that particular experiment than either pTRM tail checks or the observation of zigzagging in an IZZI experiment. We observe that the action of imparting a pTRM tail also involves some removal of NRM. However, the pTRM tails are not the analogues of classical overprints as may be intuitively expected but instead appear to favour remagnetisation over demagnetisation. This and other observed behaviour is not consistent with any phenomenological model of MD TRM but is consistent with predictions made by kinematic theory.
ISSN: 1343-8832

hal-00407946
https://hal.archives-ouvertes.fr/hal-00407946

Éditeur(s) : HAL CCSD Springer/Terra Scientific Publishing Company
Résumé : International audience
Samples were obtained from three baked contacts and one lava flow along the upper Turonian-lower Coniacian Tovuz section, two baked contacts along the upper Coniacian-lower Santonian Paravakar section in the northern part of Armenia, and three baked contacts along the Titonian-Valanginian Kafan section in southern Armenia. A total of 130 samples were studied. Updated mean paleomagnetic poles were calculated for the Upper Cretaceous Tovuz-Paravakar sections (65.6 degrees N, 162.2 degrees E, A(95) = 4.3, paleolatitude = 27.0 +/- 3.4 degrees) and the Upper Jurassic-Lower Cretaceous Kafan section (61.7 degrees N, 158.9 degrees E, A(95) = 4.8 degrees, paleolatitude = 24.7 +/- 3.8 degrees). Paleointensity determinations could be estimated from two of the upper Cretaceous and three of the Upper Jurassic-Lower Cretaceous baked contacts, corresponding to a 30% success rate. The mean virtual dipole moments obtained were low (1.7-5.5 x 10(22) A m(2)), which is in agreement with data published by Bol'shakov and Solodovnikov (1981a, 1983) for the same sections (3.0-4.4 x 10(22) A m(2)). Our results support the hypothesis of the Mesozoic Dipole Low, even though the overall data arc widely dispersed.
ISSN: 1343-8832

hal-00413047
https://hal.archives-ouvertes.fr/hal-00413047

Éditeur(s) : HAL CCSD Springer/Terra Scientific Publishing Company
Résumé : International audience
Titanian clinohumite is an accessory phase found in peridotites, and it can be a key repository of volatile and other trace elements in sub-arc mantle. To characterize spectroscopic variations due to volatile elements in the phase, we have investigated the infrared spectroscopic characteristics of a suite of naturally occurring Ti clinohumite minerals with varying F and Ti concentrations. Samples were ultramafic rocks from Cerro del Almirez, Spain and Cima di Gagnone, Switzerland. The infrared spectra corresponding to OH vibrations showed systematic variation in regard to the abundance of F in Ti clinohumite. In particular, the intensity ratios of infrared absorption spectra at 3,565 and 3,390 cm-1 correlated linearly with the abundance of F independently of the crystal orientation. Raman spectra did not corroborate the Fourier transform infrared (FTIR) results. This intensity ratio provides an alternative analytical criterion for the characterization of Ti clinohumite.
ISSN: 1343-8832

hal-01133685
https://hal.archives-ouvertes.fr/hal-01133685
DOI : 10.1186/1880-5981-66-60

Éditeur(s) : HAL CCSD Springer/Terra Scientific Publishing Company
Résumé : International audience
We carried out a detailed paleomagnetic, rock-magnetic, and Thellier paleointensity study of all similar to 3.6 My Pliocene lava flow succession in southern Georgia. An earlier study (Camps et al., 1996) revealed that several consecutive lava flows record an intermediate polarity direction at the base of the section followed by a thick reverse polarity zone. The transitional field was interpreted as an excursion within chron 2Ar or all upper Cochiti-Gilbert reversal. New paleomagnetic data reported here have been obtained from nearby lava successions. In total, about 170 standard paleomagnetic cores belonging to 22 lava flows were collected during the 2005 sample collection campaign. Rock-magnetic experiments showed that the remanence is carried by Ti-poor titanomagnetite in most of the samples. The fraction of grains with multidomain magnetic structure does not seem to be important. Characteristic remanent magnetization was successfully determined oil all samples. The direct correlation with the original (Thoki) sequence, previous preliminary measurements of natural remanent magnetization (Sologashvili, 1986), and field observations allowed us to establish a new magnetic stratigraphy. The lower part of section is characterized by intermediate magnetic polarity followed by thick reversely magnetized lavas. The upper sequence, represented by 18 consecutive flows yielded normal magnetic polarity. The mean paleointensity of the intermediate field is drastically reduced with respect to the post-transitional field strength. Based on all available radiometric ages and new paleomagnetic data, it may be speculated that Gilbert-Gauss (R-N) reversal was recorded at the upper part of sequence. Lower intermediate polarity flows possibly represent a form of precursor of this reversal that is similar to the Matuyama-Brunhes geomagnetic transition.
ISSN: 1343-8832

hal-00413099
https://hal.archives-ouvertes.fr/hal-00413099