HR: 14:25h
AN: GP12A-04 INVITED     [PDF]
TI: The magnetic anisotropy of mantle peridotites
AU: * Ferre, E C
EM: eferre@geo.siu.edu
AF: Southern Illinois University at Carbondale, Department of Geology MC4324, Carbondale, IL 62901 United States
AB: Previous magnetic anisotropy studies in peridotites have shown that secondary magnetite dominates their low-field AMS. This magnetite is formed during serpentinization processes along the foliation plane and along fractures. In serpentinized peridotites from the Mid-Atlantic Ridge, the long axis of the low-field AMS ellipsoid, was reported to be parallel to the [100] olivine axis while the short axis was parallel to [001]. In extensively serpentinized peridotites of the same area, secondary pseudo-single domain (PSD) magnetite was dispersed and no relationship between olivine LPO and magnetic fabric existed. In general, the shape fabric of magnetite does not relate to either the olivine LPO or the primary silicate flow fabrics. Therefore, the low-field AMS of mantle peridotites cannot provide useful information on mantle flow fabrics. The separation of the primary silicate paramagnetic fabric component from the bulk, i.e., ferromagnetic and paramagnetic, AMS fabric is necessary for flow studies in peridotites. In a previous study of serpentinized harzburgites from an ophiolite, others proposed to isolate the ferromagnetic component from the bulk AMS by using the anisotropy of anhysteretic remanence. This approach, based on the subtraction of the AARM tensor (carried by ferromagnetic phases only) from the low-field AMS tensor, may be misleading because the AARM fabric of ferromagnetic phases does not exactly correspond to their low-field AMS fabric. This is particularly true with regards to the degree of anisotropy. The normalized AARM tensor theoretically solves this problem in the case of multi-domain magnetic fabrics. However the AARM tensor normalization cannot be easily applied to rocks with a range of grain-sizes from single domain to multi-domain. At this point the paramagnetic component of the AMS fabric can be isolated only by using torque magnetometry a technique requiring 4 to 6 hours per specimen. More recently we have developed a new magnetic fabric method, based on high-field measurements, above the saturation of ferromagnetic phases, using a vibrating sample magnetometer. This technique has been succesfully tested on the Twin Sisters dunite massif, in Washington State, which was chosen because of its petrological simplicity and moderate degree of serpentinization. The paramagnetic component of the AMS is controlled by the magnetocrystalline anisotropy of mafic silicates which, in turn, should coincide with olivine LPOs. The principal current challenge is to investigate the geometric relationship between the high-field magnetic fabrics and the olivine LPOs measured on the same specimens.
DE: 1518 Magnetic fabrics and anisotropy
DE: 1525 Paleomagnetism applied to tectonics (regional, global)
DE: 1527 Paleomagnetism applied to geologic processes
DE: 1540 Rock and mineral magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2003 Fall Meeting