Further K-Ar dating and paleomagnetic study of the Auckland geomagnetic excursions

Abstract

Three different excursional paleomagnetic directions were reported from eight volcanoes of the Auckland volcanic field in New Zealand: north-down (ND) directions obtained from five volcanoes, west-up (WU) from two volcanoes, and south-up (SU) from one volcano. K-Ar ages have been reported for two of these volcanoes: 27±5 (1σ ) ka for the Wiri volcano of the ND group and 55±5 ka for the Hampton Park volcano of the WU group. In the present study, we have carried out further K-Ar age determinations on three other volcanoes and obtained reliable ages for two of them: 30±5 ka for the Puketutu volcano of the ND group and 50±6 ka for the McLennan Hills volcano of the SU group. The age of Puketutu agrees well with that of Wiri, and these two ages give a weighted mean age of 29±3 (1σ ) ka for the ND group. The age of the ND group is distinguishable from those of the SU and WU groups at the 2σ level, confirming that excursions occurred at two different times separated by a few tens of thousands of years. The age of the SU group is indistinguishable from that of the WU group, and a weighted mean age of 53±4 ka can be calculated for this combined group (SU-WU group). The age of the ND group and that of the SU-WU group are distinguishable from the latest age estimate of the Laschamp excursion. Overall, these age data from volcanic rocks show that at least three excursions occurred between approximately 30 and 60 ka. These three excursions are likely to be confined in the weak dipole interval of 20-70 ka, and all of these excursions yield particularly low virtual dipole moments (VDMs) of 2×10 22 A m 2 or less. Since it is suggested that the larger virtual geomagnetic pole (VGP) deviations from the geographic pole are related to the lower VDMs, the excursional fields possibly have resulted from a significantly reduced dipole field and comparable non-dipole components.

Global and Regional Assessments of Paleosecular Variation Activity Over the Past 100 ka

“…The number of records that exhibit the Mono Lake excursion is smaller than for the Laschamp excursion records: when combined with the wide range of ages for the Mono Lake excursion, this contributes to a smooth signal over the time period. However, we do note that the Mono Lake is recorded in lavas from New Zealand (Cassata et al, 2008;Cassidy & Hill, 2009;Mochizuki et al, 2004Mochizuki et al, , 2007 with ages in the range from 30 to 35 ka. Other excursions reported over the 0-100 ka period are Hilina Pali (17 ka) (Coe et al, 1978;Singer et al, 2014), Norwegian-Greenland Sea excursion (60 ka) (Laj & Channell, 2015;Nowaczyk et al, 1994), and Sk alamaelifell or post-Blake excursion (100 ka) (Jicha et al, 2011;Singer, 2014).…”

Age of the Auckland Volcanic Field: a review of existing data

“…Given the difficulty with excess Ar highlighted in the McDougall et al (1969) study that produced these results, we do not think this represents a reliable age for this centre. Mochizuki et al (2007). We consider these most recent analyses the most reliable for this centre, and the ArÁAr age is more precise ( (Table 4).…”

“…Cassata et al (2008) carried out ArÁAr experiments on this centre; their weighted mean isochron age of multiple analyses (26.698 ka) provided an age younger than the KÁAr age (5595 ka) of Mochizuki et al (2007); however, the analyses were problematic and did not yield a reliable weighted mean plateau age. Based on discussions by Cassata et al (2008), neither the ArÁAr nor KÁAr ages determined so far should be relied upon.…”

Amplitude and timing of the Laschamp geomagnetic dipole low from the global atmospheric 10Be overproduction: Contribution of authigenic 10Be/9Be ratios in west equatorial Pacific sediments

“…The Mono Lake RPI low is recorded at ∼34.7 ka according to the correlation of the sedimentary paleoclimatic proxies with oxygen isotopes records from Greenland (GICC05 age model) [ NGRIP Dating Group 2006, and references therein]. Volcanic records of the Mono Lake excursion were also retrieved from New Zealand [ Shibuya et al , 1992; Mochizuki et al , 2004, 2006, 2007; Cassata et al , 2008; Cassidy and Hill , 2009], Hawaii [ Laj et al , 2002; Teanby et al , 2002] and the Canary Islands [ Kissel et al , 2011]. The case of the Mono Lake excursion, therefore, remains controversial.…”