HR: 0800h
AN: GP31B-01    [Abstracts]
TI: Thermally Enhanced Magnetic Fabrics of Basaltic Dikes from Kapaa Quarry, Koolau Volcano, Oahu, Hawaii, USA.
AU: Lau, J
AF: SOEST-HIGP University of Hawaii at Manoa, Petrofabrics and Paleomagnetics Laboratory, Honolulu, HI 96822, United States
AU: * Herrero-Bervera, E
EM: herrero@soest.hawaii.edu
AF: SOEST-HIGP University of Hawaii at Manoa, Petrofabrics and Paleomagnetics Laboratory, Honolulu, HI 96822, United States
AU: Urrutia Fucugauchi, J
EM: juf@geofisica.unam.mx
AF: Instituto de Geofisica, UNAM, Laboratorio de Paleomagnetismo y Paleoambientes, Cd Universitaria, Mexico City, DF 04510, Mexico
AB: Progressive thermal treatment has been used to investigate the anisotropy of magnetic susceptibility (AMS) of a wide range of lithologies. Initial results on e.g., red sandstones, glacial tillites, granites and gneisses showed that laboratory stepwise heating resulted in thermal enhancement of AMS, showing the potential of thermal treatment in studying weak AMS and masked or cryptic fabrics. Studies have however shown that heating induced changes in AMS may be more complex that simple enhancement of the magnetic fabric In general, thermal induced magneto-mineralogical alterations are complex and not well understood, and further investigation of heating induced effects in mineralogy, grain size and texture systematically investigated for different lithologies is needed. For our experiment we have used a suite of samples from eight basaltic dikes from the Kappa Quarry, Koolau volcanic range in Oahu, Hawaii. The AMS fabric was determined as part of a study to investigate the influence of hydrothermal alteration by Krasa and Herrero-Bervera (2005). They found that hydrothermal alteration changes the bulk susceptibility and anisotropy degree, but AMS ellipsoid principal axes are not affected. Since hydrothermal alteration transforms the primary Ti-poor titanomagnetites into granular intergrowths of titanomagnetites, titanomaghemite and hematite, and that samples show varying degrees of alteration, the samples react differently to laboratory stepwise heating permitting study of thermal effects on the magnetic mineralogy, and AMS parameters and principal susceptibility axes. Further, thermal treatment results in fabric enhancement with reduced axial scatter associated with weak bulk susceptibilities and anisotropy degrees in the dikes. For the AMS experiment samples were heated progressively to temperatures up to 400° C or 560° C and the AMS measured after each step. AMS parameters and bulk susceptibility show changes with increasing temperature while the AMS ellipsoidal axes remain stable or with small angular changes. The dike fabrics are defined by maximum k1 axes within the dike planes and minimum k3 axes lying oblique to dike planes. Results from the thermal treatment experiment permit improved definition of directional AMS patterns, with in some cases less scatter in the principal axes.
DE: 1518 Magnetic fabrics and anisotropy
DE: 1519 Magnetic mineralogy and petrology
DE: 1533 Remagnetization
DE: 1540 Rock and mineral magnetism
DE: 1599 General or miscellaneous
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Joint Assembly