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European Geosciences Union General Assembly 2006, Vienne : Autriche
New paleo- and rockmagnetic results from La Chaine des Puys during the Laschamp event : Evidences of mineralogical disturbances
G. Plenier, J.P. Valet, G. Guérin, B. Carter-Stiglitz, D. Miallier1

Forty years ago Bonhommet and Babkine (in 1967) reported on two lava flows(Laschamp and Olby) and a scoria cone (Laschamp) from the Chaîne des Puys with almost reversed directions of magnetization which were interpreted as the record of a geomagnetic excursion named after the Laschamp village. Due to specific magnetic mineralogy, these rocks were shown to be prone to self-reversals, potentially misrecording the ambient magnetic field during their emplacement. This new discovery raised a controversy regarding the origin of the directions recorded by these lava flows. Many studies were then conducted to determine the age of the event at 40 ka, which coincides with a period of low dipole field intensity and thus a high probability to observe significant departures of the field from the axial dipolar direction. Taking advantage of new Potassium-argon age determinations, we revisited some sites and sampled several new ones in order to clarify the origin of their magnetization and hopefully improve the resolution of the Laschamp event. This study involves thermal and alternating field demagnetization of 257 samples from 20 units, including 12 new localities, and also experiments of magnetic mineralogy (susceptibility as a function of temperature, thermomagnetic curves, IRM acquisition curves and hysteresis parameters determination). The magnetic mineralogy is dominated by primary titanomagnetite with a Curie temperature of 130±60°C (0.6
1 :  LPC - Laboratoire de Physique Corpusculaire [Clermont-Ferrand]

Planète et Univers/Sciences de la Terre/Volcanologie