First terrestrial geomagnetic record of the Norwegian-Greenland Sea excursion in the Kaupo flow, Koolau volcano, Oahu, Hawaii: Insights from 40Ar/39Ar, NRM and absolute paleointensity determinations

Abstract

New paleomagnetic measurements, coupled with ⁴⁰Ar/³⁹Ar 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 short-term behavior of the geomagnetic field, we performed detailed sampling of ten cooling units of the Kaupo vent belonging to the Honolulu Volcanic series of the Koolau Volcano, Oahu Hawaii. At least eight samples, collected from each of 10 successive cooling units at Kaupo, 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 pseudo-single domain (PSD) grain sizes for most of the flows. The mean directions of magnetization of the entire section sampled indicate a normal polarity, with ∼10 m of the section characterized by excursional directions (∼6 cooling units). Paleomagnetic investigations revealed a series of excursional directions. Absolute paleointensity determinations were performed by means of the modified Thellier-Coe protocol, the most salient results indicate absolute P.I. as low as ∼26.9 μ-Teslas (VADM 5.887 × 10²² Am²) (i.e. 22° Lat. North) and as high as ∼87.2 μ-Teslas (VADM 19.082 × 10²² Am²) at high latitudes (i.e. 87° Lat. North). The results of both the directional results (i.e. Declination, Inclination and VGPs) of the 10 cooling units in question in general terms correlate well to the GAD (i.e. +38°). 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 a weighted mean of 64.2±2.7 ka, which correlates well with the Norwegian-Greenland Sea excursion recorded in sediments.