Paleointensity experiments using alternating field demagnetization

Abstract

Experiments for absolute paleointensity involve a large number of heatings and thus increase the probability of mineralogical changes which considerably reduce the success rate. Magnetic granulometry also plays an important role. In the presence of multidomain grains thermal demagnetization of natural remanent magnetization (NRM) is not linearly related to acquisition of thermal remanence (TRM) in the laboratory, so that the NRM–TRM diagrams are characterized by two slopes, neither of them providing a correct field determination. After briefly reviewing most current techniques for paleointensity and various attempts to improve the results, the present paper investigates approaches relying on alternating field (a.f.) demagnetization. Measurements have been performed on at least eight samples from seven contemporary flows from Hawaii. Classical thermal experiments were also conducted with poor success. In contrast, 30% of the NRM–TRM curves obtained after a.f. demagnetization were perfectly linear over the entire spectrum of coercivities and thus appropriate to determine a paleointensity. This difference was caused by the possibility of measuring a much larger number of samples. The mean field is less than 10% higher than the present field and in better agreement for a subset of samples with NRM and TRM heated at 300°C prior to a.f. demagnetization. These values are acceptable within the uncertainties inherent in any study of absolute paleointensity. Thus a.f. techniques can be used with confidence provided that the whole spectrum of coercivities is involved in the field determination and exempt from any correction. This condition is necessary and sufficient to demonstrate the absence of alteration after heating. Because of the rapidity of such experiments, an approach combining a.f. and thermal experiments is certainly the best way to increase considerably the number of field determinations per lava flow and hence the overall quality of paleointensity studies.