Cryptochron C2r.2r-1 recorded 2.51 Ma in the Koolau Volcano at Halawa, Oahu, Hawaii, USA: Paleomagnetic and Ar-40/Ar-39 evidence

摘要

New paleomagnetic measurements, coupled with Argon-Argon (Ar-40/Ar-39) 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 volcano's buttressed flank within Halawa Valley. Prior paleomagnetic and Potassium-Argon (K-Ar) investigations of the Koolau Volcanic Series revealed excursional directions 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 (5 mT to 100 mT) and thermal (from 28 degrees C to 575-650 degrees 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 degrees 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 similar to 10 m of the section characterized by excursional directions (5 lava flows). The corresponding VGPs are located off the southeastern part of Africa, close to Madagascar. 40Ar/39Ar incremental heating experiments on groundmass from nine flow sites at different stratigraphic levels yields isochrons between 2.64 +/- 0.23 to 2.37 +/- 0.17 Ma with a weighted mean age of 2.514 +/- 0.039 Ma, which, combined with the overall reversed polarity and the absence of polarity reversals, strongly suggests that the excursion corresponds to Cryptochron C2r.2r-1 [S. Candle and D.V. Kent, A New Geomagnetic Polarity Time Scale for the Late Cretaceous and Cenozoic, Jour. Geophys Res., 100(1995), 6093-6095.]. Current dating is insufficiently precise to unambiguously define this excursion as a Microchron (< 10 kyr). This is potentially the first terrestrial record of Cryptochron C2r.2r-1 and the age is 2-3% older than in the reported timescale [S. Candle and D.V. Kent, A New Geomagnetic Polarity Time Scale for the Late Cretaceous and Cenozoic, Jour. Geophys Res., 100(1995), 6093-6095.]. Our finding places important constraints on the evolution of the entire Koolau shield edifice, since most of the lavas recorded a reversed polarity. (c) 2006 Elsevier B.V. All rights reserved.