McGee L. 2020. U-Th-Ra analyses of samples from the 2016 drill core of the Rangitoto lava field: Devora report. Lower Hutt (NZ): GNS Science. 9 p. (GNS Science report; 2020/03). doi:10.21420/QBHS-EW37.
A recent drill core into the western flank of Rangitoto volcano in the Auckland Volcanic Field (AVF) found possible evidence that its eruptive history may have begun as long ago as 6000 years BP (Linnell et al. 2016). This evidence was in the form of a layer of fossiliferous sedimentary material that returned radiocarbon ages of 650–7400 years between basaltic lavas. The potential longevity of Rangitoto volcano was addressed with the analysis of U-Th-Ra isotopes in two lava samples from either side of the sedimentary material. 230Th decays to 226Ra, which has a half-life of 1600 years, and once a system is perturbed disequilibria is present for approximately five half-lives (c. 8000 years). Excesses of 226Ra over 230Th in the two samples are similar to previously analysed 500–550 years BP samples from Rangitoto and, importantly, the two samples are indistinguishable from each other in their U-Th-Ra isotopic composition. An age model suggests that the new samples are either the same age or up to 500 years older than the previously analysed samples. This negates the claim that the lowermost lava flow may be 6000 years old. (auth)