A Full Vector Study of a Terrestrial Geomagnetic Record of the Porcupine Excursion (ca. 2737 ka), Gauss-Matuyama (ca. 2.61 Ma) Polarity Transition and C2r.2r.l Cryptochron (ca. 2.461 Ma) Recovered From a Long Volcanic Sequence at Makapuu Point, Koolau Volcano, Hawaii, USA.
Abstract
As part of an investigation of the Koolau volcano, Oahu, and the short-term behavior of the geomagnetic field, we have sampled the exposed flows of a long volcanic section ( 191m) located on the volcano's southwest collapsed flank at a locality known as Makapuu Point. Paleomagnetic and 40Ar/39Ar investigations of the Koolau Volcanic Series have revealed transitional/excursional directions for lavas ranging from 2-3 Ma. The easy access and close geographical proximity to the Ar/Ar dated flows made this newly studied 191-m thick sequence of flows an excellent candidate for detailed paleomagnetic analysis. Thus far, we have studied the first 85 meters of the sequence. The NRM study of about 30 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 150-250oC, 575°C and 620oC, indicative of Ti-poor and Ti-rich magnetite as well as titanomaghemite. The directions of the base of the section sampled up to ∼14m of section are excursional ( 10 flows) and the rest of the flows up to 85 m recorded the Gauss-Matuyama polarity transition and the C2r.2r-1, Cryptochron. We have conducted absolute paleointensity (PI) determinations of the excursional flows using the Thellier-Coe protocol yielding PI values as low as 19 mT and up to 88 mT within the sequence. 40Ar/39Ar incremental heating experiments on the groundmass from four flow sites spanning 9 m to 183 m above sea level yield plateau ages of 2.60±0.13 Ma, 2.49+/-0.07 Ma, 2.47+/-0.08 Ma, 2.19_+/-0.12 Ma suggesting that this sequence records the Porcupine excursion (ca. 2737 ka), the G/M polarity transition and the C2r.2r-1 l Cryptochron reported by Channell et al., (2016). These are the first back-to-back terrestrial records of the Porcupine excursion, the G/M reversal boundary and a Cryptochron, the ages are 2-3% younger than previously suggested.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMGP43A0756S
- Keywords:
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- 1503 Archeomagnetism;
- GEOMAGNETISM AND PALEOMAGNETISMDE: 1522 Paleomagnetic secular variation;
- GEOMAGNETISM AND PALEOMAGNETISMDE: 1535 Reversals: process;
- timescale;
- magnetostratigraphy;
- GEOMAGNETISM AND PALEOMAGNETISMDE: 1560 Time variations: secular and longer;
- GEOMAGNETISM AND PALEOMAGNETISM