HR: 1330h
AN: T12B-0460    [PDF]
TI: Absolute Paleointensity Determinations of Oceanic Basalts Using the Thellier-Coe and Microwave Techniques
AU: Gratton, M N
EM: gratton@liverpool.ac.uk
AF: Geomagnetism Laboratory, S.A.CO.S., University of Liverpool, Liverpool, L69 3BX United Kingdom
AU: * Herrero-Bervera, E
EM: herrero@soest.hawaii.edu
AF: SOEST-HIGP, University of Hawaii, 1680 East West Road, Honolulu, HI 96822 United States
AU: Shaw, J
EM: shaw@liverpool.ac.uk
AF: Geomagnetism Laboratory, S.A.C.O.S., University of Liverpool, Liverpool, L69 3BX United Kingdom
AU: Valet, J
EM: valet@ipgp.jussieu.fr
AF: I.P.G.P., 4 Place Jussieu, Paris, 75252 France
AU: Acton, G
EM: acton@geology.ucdavis.edu
AF: Geology Department, One Shields Avenue, University of California, Davis, CA 95616 United States
AB: A selection of 43 oceanic basalts from cores recovered from ODP Leg 200 off Oahu, Hawaii (ca. 45 Ma old) have been analysed using the Thellier-Coe method to determine the absolute palaeointensity of the geomagnetic field. A very low success rate (less than 10 percent) was observed due to the onset of low-temperature alteration of the samples. Low-field susceptibility vs. temperature (k-T) analyses show this is due to the presence of a low Curie temperature magnetic mineral phase (about 150-200$^{o}$C). A higher temperature phase (about 580$^{o}$) was also observed. Magnetisation (SIRM) experiments have also been carried out to further characterize the magnetic mineralogy. The magnetic experiments indicate that the carriers of magnetisation are small grains of titanomagnetites with average volume content of 1 percent. These titanomagnetites are oxidised gradually under sub-oceanic conditions and thereby form cation deficient spinels (titanomaghaemites). This oxidation proceeds at low temperatures, most likely at bottom water temperatures. The microwave technique was then applied to six 5mm diameter samples, with five samples yielding palaeointensity results. Where comparison between Thellier-Coe and microwave results was possible, the obtained palaeointensity values were similar. A range of palaeointensity values from 23-58 micro Tesla were obtained, with a mean value of 36.8 micro Tesla, which corresponds to a mean VADM of 7.48 x 10$^{22}$ Am$^{2}$ for a latitude of 27$^{o}$ North. This study demonstrates the potential of using ODP oceanic basalts as a source of palaeointensity data for a period extending back 160 My. The reduced thermochemical alteration of the samples and use of small sample sizes makes the microwave technique a powerful tool in this area of study.
DE: 1519 Magnetic mineralogy and petrology
DE: 1521 Paleointensity
DE: 1533 Remagnetization
DE: 1540 Rock and mineral magnetism
DE: 1599 General or miscellaneous
SC: Tectonophysics [T]
MN: 2003 Fall Meeting