Absolute paleointensities and paleodirections of miocene and pliocene lavas from Fernando de Noronha, Brazil

Forty-four volcanic units were sampled on the Equatorial Atlantic Island of Fernando de Noronha, Brazil. Twenty-seven sites are from the 1.8–3.3 Myr old Quixaba formation, and the others are from the approximately 8–12 Myr old Remedios formation. Analyses of paleodirections yielded normal and reverse polarities in both formations with virtual geomagnetic poles (VGPs) and VGP dispersion (Sf) of VGPlat: 86.9°, VGPlong: 136.3°, Sf: 11.2° for the Quixaba formation and VGPlat: 86.8°, VGPlong: 196.7°, Sf: 15.5° for the Remedios formation. Successful paleointensity determinations were obtained for 9 of the 27 sampled units in the Quixaba formation. A modified Thellier method was used that includes checks to monitor alterations and repetition of demagnetization steps to detect multidomain tails of partial thermoremanent magnetizations (pTRM) and magnetization stabilization processes. Furthermore, the additivity of pTRMs was monitored during this modified technique of the Thellier experiment. These additivity checks provide a quality test for correcting the paleointensity determination for magnetomineralogical changes which occur during laboratory treatment. Such correction methods were used for analyzing approximately 50% of the determinations and lead to a reduction of the within-site scatter. The mean paleointensity result of 29.3 μT, corresponding to a virtual axial dipole moment (VADM) of 7.5×1022 A m2, yields approximately the present day field strength. Mean values of the paleodirections for volcanic units of reversed and normal polarity are closer to a dipolar field configuration than previously observed. Today’s geomagnetic field in the Equatorial Atlantic shows a significant dipole anomaly. Our results indicate that this anomaly was far less pronounced in the geological past.