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

Abstract

We report a survey carried out on basalt flows from Amsterdam Island in order to check the presence of intermediate directions interpreted to belong to a geomagnetic field excursion within the Brunhes epoch, completing this paleomagnetic record with paleointensity determinations and radiometric dating. The directional results corroborate the findings by Watkins and Nougier (1973) : normal polarity is found for two units and an intermediate direction, with associated VGPs close to the equator, for the other two units. A notable result is that these volcanic rocks are well suited for absolute paleointensity determinations. Fifty percent of the samples yields reliable intensity values with high quality factors. An original element of this study is that we made use of the PTRM-tail test of Shcherbakova et al. (2000) to help in the interpretation of the paleointensity measurements. Doing thus, only the high temperature intervals, beyond 400 degres C, were retained to obtain the most reliable estimate of the strength of the ancient magnetic field. The normal units yield Virtual Dipole Moments (VDM) of 6.2 and 7.7 10e22 Am2 and the excursional units yield values of 3.7 and 3.4 10e22 Am2. These results are quite consistent with the other Thellier determinations from Brunhes excursion records, all characterized by a decrease of the VDM as VGP latitude decreases. 40Ar/39Ar isotopic age determinations provide an estimate of 26+-15 Kyr and 18+-9 Kyr for the transitional lava flows, which could correspond to the Mono Lake excursion. However, the large error bars associated with these ages do not exclude the hypothesis that this event is the Laschamp

Controlled atmosphere vibrating thermo-magnetometer (CatVTM): a new device to optimize the absolute paleointensity determinations

“…Hence, the number of determinations of absolute paleointensity available is extremely low: To date, approximately 3000 data, representative of instantaneous recordings of the paleomagnetic field, have been published for the whole of . Continuous thermal demagnetization curves of NRM and artificial total TRM acquired in a 50-μT field for a lava sample from Amsterdam Island (Carvallo et al, 2003). In the present case, the similarity between the two curves suggests that the NRM is a TRM.…”

“…A similar shape of these two continuous demagnetization curves is qualitative evidence that the NRM is actually a TRM (Fig. 5) (Camps et al, 1996;Carvallo et al, 2003). This involves the corollary that a difference between the two curves suggests that the NRM is not a pure TRM, which is a sufficient condition to turn down samples from the selection.…”

“…1) The selection of well-suited samples for paleointensity determinations (Camps et al, 1996;Carvallo et al, 2003).…”

Hot spot activity and tectonic settings near Amsterdam–St. Paul plateau (Indian Ocean)

“…(a) Distance from the current position of ASP plume (in km) versus K‐Ar ages (in Ma) of samples related to ASP plume activity. The previous studies samples are Amsterdam and St. Paul islands and Boomerang seamount [ Watkins et al , 1975; Carvallo et al , 2003; Johnson et al , 2000]. Dotted black line corresponds to the correlation equation y = 0.0107 x for a correlation factor R 2 = 0.963.…”

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

“…It has been suggested that at least three excursions oc- Table 2), the Laschamp excursion (triangles: Bonhomet and Zähringer, 1969;Roperch et al, 1988;Chauvin et al, 1989;Guillou et al, 2004), the Skalamaelifell excursion (squares: Marshall et al, 1988;Levi et al, 1990), the Amsterdam excursion (inverse triangles: Watkins and Nougier, 1973;Carvallo et al, 2003), and the Ontake excursion (diamonds: Tanaka and Kobayashi, 2003). ND, SU, and WU denote north-down, south-up and west-up, respectively.…”