HR: 0800h
AN: GP21B-0157 INVITED     [Abstracts]
TI: Magnetic properties of natural and synthetic olivines: high-field measurements
AU: * Ferre, E C
EM: eferre@geo.siu.edu
AF: Southern Illinois University at Carbondale, Department of Geology - MC4324, Carbondale, IL 62901 United States
AU: Martin-Hernandez, F
EM: fatima@geo.uu.nl
AF: Utrecht University, Paleomagnetic Laboratory Fort Hoofddiijk, Utrecht, CD 3584 Netherlands
AB: Olivine [(Fe$_{x}$, Mg$_{1-x}$)$_{2}$ SiO$_{4}$] is an orthosilicate solid solution between fayalite [Fe$_{2}$ SiO$_{4}$] and forsterite [Mg$_{2}$ SiO$_{4}$]. Olivine is a major constituent of the Earth mantle that is abundant in oceanic and continental peridotites and mantle xenoliths. The magnetic properties of olivines have been previously investigated using gem quality natural crystals known as peridots (Zabargad) or using laboratory grown synthetic crystals. Magnetic investigations are generally performed using low magnetic field or neutron diffraction techniques. Optical microscopy and TEM imagery reveal that most olivine crystals host iron oxides formed by exsolution during cooling. Theoretically, the magnetic susceptibility of olivine should decrease linearly from fayalite to fayalite as a function of the Fe content. The magnetic behavior should range from antiferromagnetic at high Fe content, paramagnetic at intermediate Fe contents and diamagnetic at very low Fe contents. New magnetic measurements, performed on various high field instruments (vibrating sample magnetometer, torque magnetometer, cantilever magnetometer), both on natural and synthetic samples, display ferromagnetic behavior, interpreted as due to the systematic presence of titanomagnetite inclusions in olivine crystals. These results emphasize the need to conduct measurements in high field in order to isolate the intrinsic paramagnetic properties of olivines. These measurements demonstrate the orthorhombic nature of the intrinsic paramagnetic properties, but also yield new data concerning the relationship between crystallographic axes, magnetic anisotropy and other physical anisotropies: [100] = K$_{1}$, [010] = K$_{2}$ and [001] = K$_{3}$. Preliminary results also indicate substantial variations in degree of paramagnetic anisotropy (P) and paramagnetic shape factor (T). For Fo$_{92}$, P = 1.359 and T = -0.845. These intrinsic paramagnetic properties are used to model the magnetic behavior of olivine across a range of temperatures relevant to planetary exploration. They are also used to evaluate the possible effects of olivine deformation on its intrinsic properties. For example, deformation-induced striped iron zoning is anticipated to strongly modify crystal magnetic anisotropy.
DE: 3900 MINERAL PHYSICS
DE: 1518 Magnetic fabrics and anisotropy
DE: 1519 Magnetic mineralogy and petrology
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting