FULL VECTOR AND 40AR/39AR STUDY OF AN EXCURSION RECORDED BY THE KAUPO FLOW, HONOLULU VOLCANIC SERIES, OAHU, HAWAII, USA: A VOLCANIC RECORD OF THE MONO LAKE EXCURSION

As part of an investigation of the Koolau volcano, Oahu, and the short-term behavior of the geomagnetic field at Makapuu Point, we have sampled a sequence of thin lava flows known collectively as the Kaupo flow. These lavas are part of the Honolulu volcanic series and are associated with rejuvenated-stage volcanism on Oahu. Full vector and ⁴⁰Ar/³⁹Ar investigations of the Kaupo lavas reveal transitional/excursional directions. Thus far, we have studied in detail the first ~30 meters of the sequence. The NRM study of about 10 flows has been determined and at least 10 samples, collected from each of the successive sites, were stepwise demagnetized by both a.f. (5 to 100 mT) and thermal (28 to 700 °C) methods. Mean directions were obtained by p.c. analysis. All samples yielded a strong and stable ChRM vector diagrams based on 7 or more demagnetization steps, with thermal and a.f. results differing insignificantly i.e. high magnetic stability. k-T analysis conducted on individual lava flows indicated 50% with reversible curves. Curie points from these analyses revealed a temperature close to or equal to 150-250°C, 575°C and 620°C, indicative of Ti-poor and Ti-rich magnetite as well as titanomaghemite. The directions as well as the VGPs of the base of the section sampled up to ∼14m of section are excursional (~10 flows). We have conducted absolute paleointensity (PI) determinations of the excursional flows using the Thellier-Coe protocol yielding PI values as low as 32 mT and up to 40 mT within the sequence. ⁴⁰Ar/³⁹Ar incremental heating experiments on the groundmass from a flow near sea level and one near the vent yield plateau ages of 33.9 + 4.0 ka indicating that is without any doubt the Mono Lake excursion recorded by lavas on the Koolau Volcano.