New 40Ar/39Ar Ages and full Vector Results of the Upper Mammoth Transition and Norwegian-Greenland Sea Excursion from Volcanic Sequences on Oahu, Hawaii

New paleomagnetic measurements, coupled with ArgonArgon (40Ar/39Ar) radioisotopic dating of terrestrial lava sequences, are revolutionizing our understanding of the geodynamo by providing high resolution records of the paleomagnetic field. As part of an investigation of the Waianae and Koolau Volcanoes, Oahu, and the short-term behavior of the geomagnetic field, we have sampled a long volcanic section located on Waianae 's collapsed flanks at a locality known as Puu Kaulakauila and we performed detailed sampling of the individual flows from the Kaupo vent. Prior paleomagnetic investigations of the Kamaileunu Volcanic Series (i.e. Herrero-Bervera and Valet, 2005) as well as the Makapuu Point flows revealed transitional and excursional directions. At least eight samples, collected from each of 45 successive flow sites at Puu Kaulakauila and 10 sites at Kaupo, were stepwise demagnetized by both alternating field (5 mT to 100 mT) and thermal (from 28 C to 575650 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. Low-field susceptibility vs. temperature (kT) analysis conducted on individual lava flows indicated approximately half with reversible curves. Curie point determinations from these analyses revealed a temperature close to or equal to 580 C, indicative of almost pure magnetite ranging from single domain (SD) to pseudo-single domain (PSD) grain sizes for most of the flows. The mean directions of magnetization of the entire section sampled indicate a reversed polarity, with 10 m of the section characterized by excursional directions (5 lava flows). The corresponding VGPs of the termination 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 northern part of the Kamchatka peninsula to the Taiwan area. 40Ar/39Ar ages from multiple flows give weighted mean around ~68 ka, which correlates well with the Norwegian-Greenland Sea excursion.