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 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
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
Résumé : palaeomagnetism applied to tectonics
Geotemas

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

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

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
We report paleomagnetic results from four overlapping stratigraphic sections (55 flows) through the lava pile in Tjörnes peninsula (North Iceland). The initial aim of our study was to check the existence of intermediate directions previously interpreted as belonging to the Matuyama-Brunhes geomagnetic reversal (MBR), and complete this record with Thellier-Thellier paleointensity determinations and 40Ar/39Ar radioisotopic dating. The directional results corroborate the findings by Kristjánsson et al. (1988): a sequence of reversed, transitional, normal, and transitional polarity (R-T-N-T) is found in each section. The polarity change is characterized by a jump from reversed virtual geomagnetic poles (VGPs) to a quasi-cluster of transitional VGPs located over China, followed by a second jump to a cluster of normal polarity before moving again towards intermediate polarities. Reversely magnetized flows exhibit a magnetic mineralogy well suited for paleointensity determination. Of these 25 flows, 20 yielded paleointensity estimates of good quality. Unfortunately, the systematic presence of coarse multidomain (MD) titanomagnetite in the transitional and normal polarity flows hampered paleointensity estimates for these flows. The Virtual Dipole Moments (VDM) calculated for the reversed polarity flows vary from 4.9 to 7.0 × 1022 Am2 with an arithmetic mean value of 5.5 ± 0.8 × 1022 Am2. This value is identical to the mean VDM obtained for the 0.3-5 Ma time window and thus strengthens the conclusion that the recent geomagnetic field strength is anomalously high compared to that older than 0.3 Ma. 40Ar/39Ar isochron ages were obtained with the incremental heating technique on groundmass separates or phenocryst-poor whole rock samples. Six of the transitionally magnetized lavas yielded isochron ages that are indistinguishable from one another at the 95% confidence level. The weighted average age is 862 ± 51 ka. In the upper part of this volcanic sequence, the normally magnetized flows yielded a weighted average age of 787 ± 40 ka. This finding supports the conclusion that a geomagnetic excursion within the Matuyama chron is recorded in the lower part of the Tjörnes volcanic sequence, whereas only the latest portion of the MBR is recorded by flows in the upper part of the lava pile. This conclusion is supported via comparison of the VGPs obtained from the lavas with those predicted by a geomagnetic field model.
ISSN: 0012-821X

hal-00618515
https://hal.archives-ouvertes.fr/hal-00618515
https://hal.archives-ouvertes.fr/hal-00618515/document
https://hal.archives-ouvertes.fr/hal-00618515/file/Camps-epsl2011.pdf
DOI : 10.1016/j.epsl.2011.07.026