HR: 1340h
AN: GP13A-0035    [Abstracts]
TI: Deciphering Magma Flow Directions From Complex Magnetic Fabrics
AU: Plenier, G
EM: plenier@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris, 4 Place Jussieu, Paris, 75252 France
AU: * Glen, J M
EM: jglen@usgs.gov
AF: US Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 United States
AB: A simple relation is generally presumed to exist between an igneous rock's anisotropy of magnetic susceptibility (AMS) fabric and magma flow. In such cases (termed " normal "), the magnetic fabric displays a magnetic foliation pole, given by the minimum AMS axis, that lies perpendicular to the flow plane (i.e., dike walls, or lava flow top or bottom) and a magnetic lineation (AMS maximum axis) aligned with the flow direction. However, abnormal fabrics differing significantly from the normal case are more common than is generally accepted, possibly biasing the interpretation of flow directions. Surprisingly, such cases often have AMS axes that are aligned with the flow coordinates but are flipped (through 90 degrees) with respect to each other. One of the most compelling causes for these " permuted " fabrics involves mixtures of uniaxial single domain (SD) and multi-domain (MD) titanomagnetites (Rochette et al., 1992; Ferre, 2002). In cases where grain size mixtures are responsible for the permuted fabrics, magnetic anisotropy techniques, that isolate the influence of different grain size fractions on magnetic fabric, might allow the true flow directions to be recovered (Plenier and Glen 2005). We will present two such methods that can be used to achieve this goal. The first method acts on the pseudo-single domain (PSD) grain size fraction. The presence of a sub-saturating field tends to saturate the PSD grains that in turn behave more like SD grains. The second method is particularly suited for samples that contain a fraction of super paramagnetic (SP) grains. It involves cooling samples below the SP-blocking temperature in order to increase their fraction of single domain grains. These treatments both result in changes of the sample's AMS fabric that might allow one to recover the true flow directions. We will discuss results from both of these methods, along with independent measurements made to assess magnetic mineralogy and grain size, to determine magma flow directions for mafic dikes and lava flows from the western US. Ferre, 2002, GRL, 29(7), 10.1029/2001GL014367 Plenier and Glen, J.G., 2005, IRM Quarterly, 14(4), 2-3 Rochette et al., 1992, Rev. Geophys., 30, 209-226
DE: 1518 Magnetic fabrics and anisotropy
DE: 1519 Magnetic mineralogy and petrology
DE: 1540 Rock and mineral magnetism
DE: 1594 Instruments and techniques
SC: Geomagnetism and Paleomagnetism [GP]
MN: Fall Meeting 2005