Paleointensity of the Earth's magnetic field recorded by two Late Quaternary volcanic sequences at the Island of La Réunion (Indian Ocean)

Abstract

A paleomagnetic study has been undertaken on two late Quaternary volcanic sequences from the Piton de la Fournaise volcano (island of La Réunion) in order to provide absolute paleointensities of the Earth's magnetic field from a site in the southern hemisphere. The pattern of secular variation recorded by both sequences clearly indicates that the extrusion rate of the lava flows was not constant with time. A detailed investigation of the magnetic properties of the samples was carried out in parallel with paleointensity experiments which were performed with the Thellier method. Samples with minimum overprint, composed by single‐domain or pseudo‐single‐domain magnetite showing good thermal stability and a narrow blocking temperature spectrum were the most successful during Thellier's experiments. Values of the paleofield strength from 19 to 55 mT have been obtained on 23 flows among the 30 which were sampled. Few variations (7.5–9.9×10 22 A m 2 ) are observed in the virtual dipole moments (VDMs) determined on the youngest sequence (5–12 ka). Those values are within the range of fluctuations of the worldwide variations of VDMs for this period. On the other hand, data from the oldest sequence (82–98 ka) are the first paleointensity values obtained in that time period, and the observed VDMs extend from 4.1 to 8.8×10 22 A m 2 . These results suggest that for this time interval, the field did not depart from its average behavior.