Upper Mammoth Polarity Transition Recorded in the Pu'u Kualakauila volcanic sequence, Wai'anae Volcano, Oahu, Hawaii USA: Paleomagnetic and 40Ar/39Ar Evidence

New paleomagnetic measurements, coupled with Argon-Argon (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 Wai'anae Volcano, Oahu, and the short-term behavior of the geomagnetic field, we have sampled a long volcanic section located on the volcano's collapsed flank at a locality known as Pu'u Kaulakauila. Prior paleomagnetic investigations of the Kamaile'unu Volcanic Series (i.e. Herrero-Bervera and Valet, 2005) revealed transitional directions. The silicic composition of lava flows, easy access, and close geographical proximity to K-Ar dated flows made this newly studied 214-m thick sequence of flows an excellent candidate for detailed paleomagnetic analysis. At least eight samples, collected from each of 45 successive flow sites, were stepwise demagnetized by both 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. Low-field susceptibility vs. temperature (k-T) 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 pseudosingle 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). Thellier-Coe and microwave paleointensities determinations of these flows indicate a substantial decrease of the absolute paleointensity before and during the transition and a progressive increase of it during the recovery phase of the transition. The corresponding VGPs 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 3.233×0.088 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.