HR: 15:15h
AN: GP43A-06    [Abstracts]
TI: Volcanic Record of the Halawa Excursion (ca 2.514+/-0.039 Ma), Koolau Volcano, Oahu Hawaii, USA: Full Vector Analyses
AU: Lau, J
AF: SOEST-HIGP, University of Hawaii at Manoa, Paleomagnetics and Petrofabrics Laboratory, 1680 East West Rd, Honolulu, HI 96822, United States
AU: * Herrero-Bervera, E
EM: herrero@soest.hawaii.edu
AF: SOEST-HIGP, University of Hawaii at Manoa, Paleomagnetics and Petrofabrics Laboratory, 1680 East West Rd, Honolulu, HI 96822, United States
AB: New paleomagnetic measurements (directions and paleointensity determinations), coupled with precise ⁴⁰Ar/³⁹Ar radioisotopic dating, are revolutionizing our understanding of the geodynamo by providing detailed terrestrial lava records of the short-term behavior of the paleomagnetic field. As part of an investigation of the evolution of Koolau Volcano (one of the volcanoes comprising Oahu Island) and the short-term behavior of the geomagnetic field, we have sampled a long volcanic section located on the buttressed flank of the volcano within Halawa Valley. Prior paleomagnetic and K-Ar investigations of the Koolau (Volcano) Series revealed excursional directions (Site F of Doell and Dalrymple, 1973). The alkaline composition of lava flows, easy access, and close geographical proximity to K-Ar dated lava flows made this newly studied 120 m thick sequence of flows in Halawa valley an excellent candidate for detailed paleomagnetic analysis. At least eight samples collected from each of 28 successive flow-sites were stepwise demagnetized by both alternating field (5mT to 100mT) and thermal (from 28° C to 575-650°C) methods, and the mean directions obtained by principal component analysis. All samples yielded a strong and stable ChRM trending towards the origin based on no less than seven to nine steps, with thermal and AF results agreeing to a very high degree. Low field susceptibility versus temperature (k-T) analyses were conducted for individual lava flows, and the majority of them show reversible curves. Curie point determinations revealed a temperature close to or equal to 580°C, indicative of almost pure magnetite for most of the flows. Magnetic grain sizes analysis indicated SD-PSD sizes. The mean directions of magnetization of the entire section sampled indicate that about 10 m of the section are characterized by excursional directions (5 lava flows). In addition to the directional analyses we performed absolute paleointensity determinations on the 28 lavas sampled. We used the modified Thellier-Coe double heating method to determine paleointensities. 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. 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 would be present. Our paleointensity results indicate a near-zero reduced strength of the field during the excursional period ranging from 5 to 9 micro-Tesla. The corresponding VGPs are located off the southeast part of Africa, close to Madagascar. ⁴⁰Ar/³⁹Ar incremental heating experiments on groundmass from nine flow-sites located at different stratigraphic levels yielded isochron ages ranging from 2.64+/-0.25 to 2.40+/-0.46 Ma indicating that the excursion may correlate with the C2r.2r-l Cryptochron of Cande and Kent [1995]. This is potentially the first terrestrial record of the ca. 2.514 +/- 0.039 Ma Cryptochron, a finding that will place important constraints on evolution of the entire Koolau shield edifice also.
DE: 1513 Geomagnetic excursions
DE: 1520 Magnetostratigraphy
DE: 1521 Paleointensity
DE: 1522 Paleomagnetic secular variation
DE: 1535 Reversals: process, timescale, magnetostratigraphy
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Joint Assembly