A Neogene geomagnetic polarity transition record from lavas of the Canary Islands, Spain: episodic volcanism and/or metastable transitional fields?

Abstract

SUMMARY We present results from a study of a partial magnetic polarity transition record from a 9.7 Ma 40Ar/39Ar dated volcanic section on the island of Gomera (Canary Islands, Spain). The record provides results that bear on the debate over whether long-term persistent features of field behaviour exist during transitions. The sampled section, containing more than 20 flows, begins after the onset of the transition. The nine lowermost flows in the section, which are all transitional, yield virtually identical directions and similar low paleointensities. Following the recovery of virtual geomagnetic pole positions (VGPs) to near the spin axis, paleointensities progress to very high values. Transitional directions lay due west, reflecting the presence of a non-zonal intermediate field. VGPs for these flows cluster in the west Atlantic, which according to Hoffman would correspond to a zone of fastest P-wave propagation in the lower mantle and to a radial flux centre of the present-day non-dipole field. Hoffman had noted a similar occurrence at two other locations. He found that in a select set of volcanic records VGPs tended to group over two areas (Australia and South America), which also corresponded to fast P-wave and non-dipole field anomalies. Given the discontinuous and irregular character typical of volcanic sequences, it is natural to question whether these clusters are merely artefacts of episodic volcanism or actually reflect standstills of the reversing field. We address this question for our record by comparing chemical and magnetomineralogic characteristics of the flows. We find that all of the transitional flows have very similar chemical and rock magnetic properties. Furthermore, these flows differ significantly from the overlying reverse flows, which give distinctly different values and much greater flow-to-flow variation. From this we conclude that the packet of transitional flows was derived from the same source and had erupted over a relatively short period of time, as reflected by a magma source that presumably did not have time to evolve significantly. While the VGP cluster in the Gomera record is consistent with long-term mantle control on transitional fields, we cannot conclude that the cluster represents a standstill of the field. These results highlight the difficulties of interpreting temporal changes in field behaviour from volcanic sequences. In addition, they provide a clear incentive for investigating physical characteristics of lavas to assess relative eruptive rates.