40Ar/39 ages and palaeomagnetism of transitionally magnetized volcanic rocks in the Society Islands, French Polynesia: Raiatea excursion in the upper-Gauss Chron

Abstract

Volcanic rock samples from several sites in the Society Islands, French Polynesia were found to be transitionally magnetized. To better characterize these geomagnetic events, we performed laser-heating 40Ar/39Ar dating and additional palaeomagnetic measurements on these samples. The measurements give virtual geomagnetic pole (VGP) latitudes of 0.7-52.4°N and virtual dipole moment (VDM) values of 0.433-0.682 × 1022A m2 which are typical for the transitional geomagnetic field. These transitional events occurred at 2.77 ± 0.02 Ma (Raiatea), 3.20 ± 0.03 Ma (Tahaa) and 4.40 ± 0.01 Ma (Maupiti). Considering the reported radioisotopic ages for known geomagnetic events, it is suggested the first event is a previously unknown geomagnetic excursion (Raiatea excursion) while the latter two probably correspond to known polarity transitions (upper boundary ages of the Mammoth reversed subchron and the Nunivak normal subchron). The existence of the proposed Raiatea excursion is supported by the contemporaneous palaeointensity low recognized in the relative palaeointensity stack from the equatorial Pacific. © 2007 The Authors Journal compilation © 2007 RAS.