Determination of geomagnetic palaeointensities from the Quaternary West Eifel volcanic field, Germany

SUMMARY Determination of geomagnetic palaeointensities has been attempted for 418 specimens from 51 volcanic units of the Quaternary West Eifel volcanic field. This paper describes the measuring procedure using a modification of the Thellier method, the analysis of the derived data points, the determination of linear segments and the quality of the individual palaeointensity values calculated from the corresponding slopes. According to a series of acceptance and rejection criteria, finally 312 reliable individual palaeointensity results have been obtained. About 73 per cent of the accepted regression lines have correlation coefficients better than 0.990, and less than 10 per cent of the accepted Thellier experiments have quality factors less than 1.0. The carriers of magnetization of the West Eifel volcanic rocks are titanomagnetites with a broad range of titanium contents and oxidation states in a more or less unweathered, mostly foiditic rock matrix. Accompanying rock magnetic experiments allowed a rough classification of the specimens corresponding to the oxidation state of the titanomagnetites. Specimens with highly oxidized titanomagnetites have been the most successful candidates for the Thellier experiments. However, a prediction of the suitability of the specimens for the Thellier experiments by sample selection according to this classification was not always appropriate. Instead, for most volcanic units pilot specimens were first measured to determine the suitability of a specific sample type, the temperature interval from which the evaluation of palaeointensity was possible, and the best laboratory field intensity to keep the slope errors of the Thellier experiments to a minimum. Palaeointensity determination was not possible for 14 volcanic units because only unsuitable specimens were available. For each of the 37 remaining volcanic units at least five successful Thellier experiments have been carried out, leading to a final mean palaeointensity result. These 37 values have relative standard errors of between 4 and 23 per cent, mostly around 10 per cent. Specimens with differing rock magnetic properties from the same volcanic unit yielded consistent palaeointensity values. Therefore, all West Eifel palaeointensity results have to be regarded as reliable estimates of the Earth’s magnetic palaeofield strength. Mean palaeointensities range from 7 to 62 pT with an unexpectedly large number of palaeointensity values less than 30 pT. About half of the low palaeointensity values are associated with intermediate virtual geomagnetic pole latitudes, which may be due to a geomagnetic excursion.