On the Discovery of Cryptochron C2r.2r-l (ca. 2.42-2.44 Ma) Recorded on Koolau Volcano at Halawa: Evidence from Paleomagnetic and 40Ar/39Ar studies

New paleomagnetic measurements, 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 Koolau Volcano, Oahu, and the short-term behavior of the geomagnetic field, we have sampled a long volcanic section located on the volcanoś buttressed flank 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 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^oC to 575-650^o C) methods. Mean directions were 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 vs. temperature (k-T) analyses were conducted for individual lava flows, with approximately half showing reversible curves. Curie point determinations revealed a temperature close to or equal to 580^o C, indicative of almost pure magnetite for most of the flows. 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). The corresponding VGPs are located off the southeast part of Africa, close to Madagascar. Initial ⁴⁰Ar/³⁹Ar incremental heating experiments on groundmass from four flow-sites located at different stratigraphic levels yielded isochron ages ranging from 2.68+/-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]. Pending further ⁴⁰Ar/³⁹Ar dating underway, this is potentially the first terrestrial record of the ca. 2.4 Ma Cryptochron, a finding that will place important constraints on the evolution of the entire Koolau shield edifice.