Combined paleointensity and 40Ar/39Ar age spectrum data from volcanic rocks of the West Eifel field (Germany): Evidence for an early Brunhes geomagnetic excursion

Abstract

Results of virtual geomagnetic pole position, paleointensity and isotopic age determinations have been derived from lavas of the Quaternary West Eifel volcanic field (Germany). The characteristic remanent magnetization (CARM) directions and corresponding positions of virtual geomagnetic poles (VGP) have mainly been determined by Böhnel et al. They are exclusively of normal polarity. The VGPs form a strongly non‐Fisherian distribution characterized by about 30% pole positions with latitudes below 60°N, which are mainly confined to the longitude sector from 20°W to 120°E. Eight independent poles are clustered at 40°N and 45°E. Geomagnetic paleointensity values from 37 volcanic units of the West Eifel have been determined. Paleointensities range from 7 to 62 μT with an unexpectedly large number of paleointensity values below 30 μT. About half of the low paleointensity values are associated with intermediate VGP latitudes. Isotopic age determinations from West Eifel volcanic rocks have been published (Fuhrmann and Lippolt). Additional isotopic ages are presented here. The West Eifel volcanic field is usually devided into an older part in the NW and a younger one in the SE, based on petrological models and geological evidence. All available isotopic ages of the West Eifel volcanic rocks which are exclusively from the northwestern part of the field lie in the interval between 460 and 660 kyr. For seven of the clustered low‐latitude VGPs it was possible to determine paleointensities and 40Ar/39Ar ages. Within their error limits six poles have identical virtual dipole moments (VDM) at 3.5×1022A m2. Furthermore, the individual isotopic age intervals of the 40Ar/39Ar data overlap. The assigned age of 510±30 kyr is considered to be the age of a geomagnetic excursion. For higher‐latitude VGP positions (>45°N), VDM values range from 2 to 10×1022A m2. The mean VDM value (about 7.5×1022A m2) of the younger volcanoes (ages estimated to be below 100 kyr) is not remarkably different from the present dipole moment of the Earth. In contrast, the older volcanoes with higher‐latitude VGP positions yielded a mean value of only 5.3×1022A m2 and a bimodal frequency distribution of the VDM values. Isotopic ages determined for two of these volcanic units are close to the inferred excursion age suggesting a decrease of the geomagnetic dipole moment during a time interval which surpassed the duration of the excursion.