NEW 40AR/39AR AGES AND FULL VECTOR RESULTS OF THE UPPER MAMMOTH TRANSITION AND NORWEGIAN-GREENLAND SEA EXCURSION FROM VOLCANIC SEQUENCES ON OAHU, HAWAII

Paleomagnetic measurements, coupled with ⁴⁰Ar/³⁹Ar radioisotopic dating of terrestrial lava sequences, are revolutionizing our understanding of the geodynamo by providing high resolution records of the paleomagnetic field. An investigation of the Wai’anae and Koolau Volcanoes, Oahu, and the short-term behavior of the geomagnetic field, we have sampled a long volcanic section located on Wai’anae 's collapsed flanks at a locality known as Pu’u Kaulakauila and we performed detailed sampling of the flows from the Kaupo vent. Prior paleomagnetic investigations of the Kamaile’unu Volcanic Series (i.e. Herrero-Bervera and Valet, 2005) as well as the Makapuu Point flows revealed transitional and excursional directions. At least 8 samples, collected from each of 45 successive flows at Pu’u Kaulakauila and 10 sites at Kaupo, were stepwise demagnetized by alternating field (5 mT to 100 mT) and thermal (from 28 °C to 575–650 °C) methods. Mean directions were obtained by principal component analysis. All samples yielded a strong and stable ChRM trending towards the origin of vector demagnetization diagrams based on seven or more demagnetization steps, with thermal and AF results differing insignificantly. k–T analysis conducted on individual lava flows indicated approximately half with reversible curves. Curie point from these analyses revealed a temperature close to or equal to 580 °C, indicative of almost pure magnetite ranging from single domain to pseudo-single domain grain sizes. The mean directions of magnetization of the section indicate a reversed polarity, with ∼10 m of the section characterized by excursional directions (5 lava flows). The corresponding VGPs of the Upper Mammoth polarity transition are located on the western part of Australia. ⁴⁰Ar/³⁹Ar incremental heating experiments on groundmass from transitional flow sites at different stratigraphic levels yields a weighted mean age of 3.201± 0.041 Ma, which, combined with the overall reversed polarity and two older polarity reversals, strongly suggests that the transitional lavas correspond to the Upper Mammoth polarity transition. The Kaupo flow VGPs are located over the eastern part of Asia (i.e. over Japan and Korea) ⁴⁰Ar/³⁹Ar ages from multiple flows give weighted mean around ~68 ka, which correlates well with the Norwegian-Greenland Sea excursion.