HR: 14:40h
AN: V13E-05    [Abstracts]
TI: A strategy for interpretation of magnetic susceptibility in lava flows: Application to the Tiretaine lava flow (Chaîne des Puys, France)
AU: * Loock, S
EM: s.loock@opgc.univ-bpclermont.fr
AF: Laboratoire Magmas et Volcans, 5 rue Kessler, Clermont Ferrand, 63038, France
AU: Diot, H
EM: herve.diot@univ-lr.fr
AF: Université de La Rochelle, Avenue M. Crépeau, La Rochelle, 17042, France
AU: Van Wyk De Vries, B
EM: B.vanwyk@opgc.univ-bpclermont.fr
AF: Laboratoire Magmas et Volcans, 5 rue Kessler, Clermont Ferrand, 63038, France
AU: Launeau, P
EM: Patrick.Launeau@univ-nantes.fr
AF: Laboratoire de Planétologie et Géodynamique, 2 rue de la Houssinière, Nantes, 44322, France
AU: Merle, O
EM: O.Merle@opgc.univ-bpclermont.fr
AF: Laboratoire Magmas et Volcans, 5 rue Kessler, Clermont Ferrand, 63038, France
AU: Vadeboin, F
EM: vadeboin@cerege.fr
AF: CEREGE, Université, d'Aix-Marseille III, Europôle Méditerranéen de l'Arbois, Aix en Provence, 13545, France
AB: Anisotropy of Magnetic Susceptibility (AMS) is a convenient method for finding strain and thus emplacement information on lava flows. However, interpretation of AMS parameters requires caution and several factors need verification to avoid misunderstandings. Verification can be divided into 2 groups: 1) what minerals create AMS? 2) how AMS behaves? 1) For the magnetic minerals, thin sections are needed for estimating concentrations, sizes, shapes and compositions. Then Curie temperature determination will indicate if magnetites are the main AMS carrier. First Order Reversal Curves (FORCs) are then performed to find the magnetic type: multi domain (MD) or single domain (SD). 2) To find AMS behaviour, a comparison between AMS ellipsoid and microlite fabric ellipsoid shows how AMS is acquired and if it is controlled by the silicate template. An example was taken on a 5m-high vertical profile from a lava flow of the Tiretaine valley. Results show that AMS is carried by interstitial titanomagnetite, with a 5 to 50 μ m size. The FORCs show that AMS is mainly due to MD magnetites. The comparison between the magnetic and microlite fabric shows that AMS fabric is controlled by the silicate template. Magnetites are therefore late formed, i.e. during the late stages of emplacement, when significant cooling had occurred. Moreover kmax is parallel to the downflow direction and thus kmax is significant in term of maximal elongation axis. The degree of anisotropy profiles can not be used as a tool for estimating lava flow deformation as titanomagnetites cover a large size and shape range, and so the magnetic fabric is assumed to be stabilized. However, degree of anisotropy is not constant and displays several breaks linked with changes in the kmax dip. This indicates several horizontal interfaces. Compartments between interfaces can display kmax dip greater than 45°, greater than the incremental elongation axis during simple shear deformation. As a consequence pure shear may be contributing. As the pure shear is assumed to be constant in the lava profile, variations of simple shear intensity will allow kmax dip greater than 45°. The lava flow section should thus be considered to be undergoing general pure shear, but compartmentalised into different simple shear bands. A way to explain such difference of the simple shear amount is the existence in the lava flow of layers with different viscosity. A more viscous lava will undergo less simple shear than a more fluid one.
DE: 1527 Paleomagnetism applied to geologic processes
DE: 1540 Rock and mineral magnetism
DE: 8425 Effusive volcanism
DE: 8429 Lava rheology and morphology
DE: 8488 Volcanic hazards and risks
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting