COMBINED PALEOINTENSITY AND AR-40 AR-39 AGE SPECTRUM DATA FROM VOLCANIC-ROCKS OF THE WEST EIFEL FIELD (GERMANY) - EVIDENCE FOR AN EARLY BRUNHES GEOMAGNETIC EXCURSION/

Results of virtual geomagnetic pole position, paleointensity and isoto pic age determinations have been derived from lavas of the Quaternary West Eifel volcanic field (Germany). The characteristic remanent magne tization (CARM) directions and corresponding positions of virtual geom agnetic poles (VGP) have mainly been determined by Bohnel et al. They are exclusively of normal polarity. The VGPs form a strongly non-Fishe rian distribution characterized by about 30 % pole positions with lati tudes below 60-degrees-N, which are mainly confined to the longitude s ector from 20-degrees-W to 120-degrees-E. Eight independent poles are clustered at 40-degrees-N and 45-degrees-E. Geomagnetic paleointensity values from 37 volcanic units of the West Eifel have been determined. Paleointensities range from 7 to 62 muT with an unexpectedly large nu mber of paleointensity values below 30 muT. About half of the low pale ointensity values are associated with intermediate VGP latitudes. Isot opic age determinations from West Eifel volcanic rocks have been publi shed (Fuhrmann and Lippolt). Additional isotopic ages are presented he re. The West Eifel volcanic field is usually devided into an older par t 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 th e field lie in the interval between 460 and 660 kyr. For seven of the clustered low-latitude VGPs it was possible to determine paleointensit ies and Ar-40/Ar-39 ages. Within their error limits six poles have ide ntical virtual dipole moments (VDM) at 3.5 x 10(22) A m2. Furthermore, the individual isotopic age intervals of the Ar-40/Ar-39 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-degrees -N), VDM values range from 2 to 10 X 10(22) A m2. The mean VDM value ( about 7.5 x 10(22) A m2) of the younger volcanoes (ages estimated to b e below 100 kyr) is not remarkably different from the present dipole m oment of the Earth. In contrast, the older volcanoes with higher-latit ude VGP positions yielded a mean value of only 5.3 X 10(22) A m2 and a bimodal frequency distribution of the VDM values. Isotopic ages deter mined for two of these volcanic units are close to the inferred excurs ion age suggesting a decrease of the geomagnetic dipole moment during a time interval which surpassed the duration of the excursion.