Geomagnetic paleointensities derived from volcanic rocks of the Quaternary East Eifel volcanic field, Germany

Abstract

Geomagnetic paleointensities have been determined for 18 rock units of basanitic, thephritic or nephelinitic composition from the Quaternary East Eifel volcanic field (Germany). In general the rocks are well suited for paleointensity determination and gave consistent results of paleointensity. For each rock unit at least five individual values have been measured with the Thellier method. Compared with the present day field of 48 μT the unweighted mean paleointensity values range from 29 to 62 μT with relative standard errors mostly below 10%. Paleointensity values are concentrated at 30 and 50 μT with a gap at 40 μT. The distribution is bimodal rather than Gaussian, although all paleopoles lie close to the geographic pole. This cannot be explained by the very discontinuous distribution of volcanic activity in time which took place mainly around 200 and 400 ka. In each age group both low and high paleointensity values are found. However, a further temporal separation within each age group cannot be excluded. Furthermore, the paleointensity values of the West Eifel volcanic field show a similar bimodal distribution which indicates that this case seems to be connected with a geomagnetic excursion recorded in those rocks. Therefore it might be considered that the East Eifel rocks recorded a characteristic feature of the earth's magnetic field namely a quasi-cyclic variation of paleointensity. On the other hand, the now available global data set of 128 virtual dipole moment values from the Brunhes chron and older than 30 ka shows an almost Gaussian distribution with a mean value of 7.4 ± 3.4 · 10 22 Am 2, if 14 values with transitional pole positions are excluded. A quasi-cyclic variation of the dipole moment therefore does not seem to be a persistent global characteristic of the earth's magnetic field.