Mono Lake or Laschamp geomagnetic event recorded from lava flows in Amsterdam Island (southeastern Indian Ocean)

• Published in: Geophysical Journal International
• Main Authors: Carvallo, Claire, Camps, Pierre, Ruffet, Gilles, Henry, Bernard, Poidras, T.
• Format: Text
• Language: English
• Published: Oxford University Press 2003
• Subjects: Articles; Indian; Shcherbakova; Watkins; Amsterdam Island
• Online Access: http://gji.oxfordjournals.org/cgi/content/short/154/3/767; https://doi.org/10.1046/j.1365-246X.2003.01993.x

We report a survey carried out on basalt flows from Amsterdam Island (Southeastern Indian Ocean) in order to check the presence of intermediate directions interpreted as belonging to a geomagnetic field excursion within the Brunhes epoch, completing this palaeomagnetic record with palaeointensity determinations and radiometric dating. Because the palaeomagnetic sampling was done over a few hours during the resupply of the French scientific base Martin du Viviers by the Marion Dufresne vessel, we could collect only 29 samples from four lava flows. The directional results corroborate the findings by Watkins & Nougier: normal polarity is found for two units and an intermediate direction, with associated virtual geomagnetic poles (VGPs) close to the equator, for the other two units. A notable result is that these volcanic rocks are well suited for absolute palaeointensity determinations. 50 per cent of the samples yields reliable intensity values with high-quality factors. An original element of this study is that we made use of the thermomagnetic criterion PTRM-tail test of Shcherbakova et al. to help in the interpretation of the palaeointensity measurements. Doing this, only the high-temperature intervals, beyond 400 °C, were retained to obtain the most reliable estimate of the strength of the ancient magnetic field. However, not applying the PTRM-tail test does not change the flow-mean values significantly because the samples we selected by conventional criteria for estimating the palaeointensity carry only a small proportion of their remanence below 400 °C. The normal units yield virtual dipole moments (VDM) of 6.2 and 7.7 (1022A m2) and the excursional units yield values of 3.7 and 3.4 (1022A m2). These results are quite consistent with the other Thellier determinations from Brunhes excursion records, all characterized by a decrease of the VDM as the VGP latitude decreases. 40Ar/39Ar isotopic age determinations provide an estimate of 26 ± 15 and 18 ± 9 kyr for the transitional lava flows, which could ...