HR: 1340h
AN: GP23A-0028    [Abstracts]
TI: Magnetic Properties and Absolute Paleointensity of Upper Oceanic Crust Generated by Superfast Seafloor Spreading, ODP Leg 209.
AU: * Herrero-Bervera, E
EM: herrero@soest.hawaii.edu
AF: SOEST-HIGP, 1680 East West Road, Honolulu, HI 96822 United States
AU: Acton, G
EM: acton@geology.ucdavis.edu
AF: University of California, Davis, Department of Geology, One Shields Avenue, Davis, CA 95616 United States
AB: We investigate the magnetic mineralogy and absolute paleointensity of oceanic basalt samples from Hole 1256D, cored during Ocean Drilling Program (ODP) Leg 206. Hole 1256D is located on the Cocos Plate about 5 km east of the transition zone between marine magnetic anomalies 5Bn.2n and 5Br (~15 Ma). During Leg 206, the hole penetrated 502 m into basalts of the upper oceanic crust that was generated by superfast seafloor spreading (>200 mm/yr) along the East Pacific Rise. Rock magnetic investigations included continuous low field (k-T) thermomagnetic analyses, alternating field (AF) and thermal demagnetization, optical microscopy, saturation isothermal remanent magnetization (SIRM), and magnetic grain size analyses. Following the removal of a drilling overprint, AF and thermal demagnetization paths for most samples decay linearly to the origin on orthogonal vector end point diagrams, suggesting that a stable characteristic remanent magnetization component can be resolved. Optical microscopy and k-T (Curie points) identified titanomagnetites and titanomaghemites as the main magnetic carriers and grain size studies indicate that the carriers are either single domain (SD) and/or pseudosingle domain (PSD) in nature. Using the modified Thellier-Coe double heating method, we determined absolute paleointensity determinations for 51 specimens sampled from different ``stratigraphic'' levels of the core. pTRM checks were performed systematically one temperature step down the last pTRM acquisition in order to document magnetomineralogical changes during heating. The temperature was incremented by steps of 50°C between room temperature and 500°C and every 25-30°C for higher temperatures. The paleointensity determinations were obtained from the slope of the Arai diagrams. Special care was taken to interpret the Arai diagrams within the same range of temperatures lower than 300°C unless a clear and unique slope was present over a higher range of temperatures. Only about 10 percent of the samples yielded acceptable results. The paleofield estimated from these samples ranges between 28 to 16 micro Teslas (i.e., VADM of 6 to 4 x 10²² A/m²), which is concordant with the average paleofield intensity for the period between 0-160 Myr (4 ± 2 x10²² A/m²) and half of the strength of the present field (~8x10²² A/m²).
DE: 1517 Magnetic anomalies: modeling and interpretation
DE: 1519 Magnetic mineralogy and petrology
DE: 1521 Paleointensity
DE: 1540 Rock and mineral magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: Fall Meeting 2005