In Search of the Full-Vector Paleomagnetic Dipole: Multidomain-Corrected Paleointensities of Kenya Lavas from the Equator

Abstract:

Present-day and especially time-averaged geomagnetic field directions are highly consistent with the geocentric axial dipole (GAD) model, which also predicts an overall factor-of-two increase in equatorial-to-polar paleointensities (presently ~30 mT to ~60 µT). However, this latitudinal relationship has not been observed in compilations from paleointensity databases, even in the most studied interval of the past 5 Myr. We believe this failure is mostly due to inadequacies in dealing with non-single-domain behaviors that are prevalent in most paleomagnetic recorders. Notable exceptions are recent paleointensity results that focused on near-single-domain samples from rapidly-cooled lavas from Antarctica (78ºS) [Lawrence et al., 2009] and Iceland (~64ºN) [Cromwell et al., 2015]. These high latitude studies show mean polar paleointensities of only ~35 µT, about 50% of the present-day value. In support of these low values as representative of the time-averaged GAD field, a study of Pleistocene lavas from the equatorial Galapagos Islands (~1ºS) that used a correction for multidomain effects [Wang and Kent, 2013] obtained a mean paleointensity of ~20 µT [Wang et al., PNAS in revision]. In a pilot study to check this result, we utilized previously thermally demagnetized Plio-Pleistocene lavas from Mount Kenya (~0º) [Opdyke et al., 2010]. We chose 3 specimens from each of 30 lava sites with coherent directions and not exhibiting any indications of having been struck by lightning. We gave the specimens a total TRM at 580ºC in 15 µT lab-applied field, then thermally demagnetized them using the same steps as for NRM, which allowed us to apply the multidomain-correction. Because of the lack of in-experiment thermal alteration checks, such as partial TRM or total TRM checks, we applied a second total TRM at 580ºC, and then total TRMs at 600ºC, 625ºC and 650ºC and used the differences between the 625ºC TRM and the first 580ºC TRM as our alteration indicator. In total, 17 specimens from 12 sites passed our data qualification criteria, yielding a mean paleointensity of ~15 µT. Along with paleointensities from Antarctica, Iceland and Galapagos, our preliminary Kenya results support a GAD model of the time-averaged field in both direction and paleointensity but which is only ~50% of the present-day field strength.