Palaeointensity results from Ethiopian basalts: implications for the Oligocene geomagnetic field strength

From a large collection of Ethiopian flood basalts (~30 Myr old) sampled for magnetostratigraphy, 40Ar/39Ar geochronology and geochemical investigations, 47 samples were selected in order to test their suitability for Thellier palaeointensity experiments. Only 17 samples from eight individual flows yielded reliable palaeointensity estimates, with flow-mean virtual dipole moments ranging from 3.0 to 10.5×1022 Am2. A critical review of the Oligocene palaeointensity data set, including these new Ethiopian data, indicates an Oligocene mean virtual dipole moment of 5.1±2.5×1022 Am2 for the complete data set. After applying mild selection criteria, the reduced data set yields a mean value of only 4.6±1.9×1022 Am2. This value is significantly lower than the present-day field strength but is higher than the Mesozoic dipole low mean field. This low Oligocene field might be in agreement with the high palaeosecular variation and rather high non-dipole field invoked around 30 Ma. However, the Oligocene data set is largely dependent on the palaeointensity determinations from Armenia, obtained mainly from baked contacts, which show amazingly low dispersion at both flow and between-flow levels. More data are needed to reduce the weight of these determinations on the mean value and avoid a possible bias.