FURTHER EVIDENCE FOR LOW INTENSITY OF THE GEOMAGNETIC FIELD DURING THE EARLY CRETACEOUS TIME: USING THE MODIFIED SHAW METHOD AND MICROWAVE TECHNIQUE

Abstract

We report new absolute palaeointensity estimates using basalts from northeastern China (K/Ar age, 125-120 Ma) using the modified Shaw method in conjunction with the microwave technique. Samples for the palaeointensity experiments were selected mainly based on their good reversibility of thermomagnetic curves and single primary magnetization characteristics. Using the modified Shaw method, 28 out of 45 measured samples from 10 cooling units give a virtual dipole moment of (3.1 +/- 1.0) × 1022 Am2, and the microwave technique using 14 acceptable determinations (out of 20 measured) give an average value of (2.9 +/- 0.9) × 1022 Am2. Results using both the modified Shaw method and the microwave technique demonstrate that the geomagnetic field strength recorded by these lavas was low. This is in agreement with previous results of the same time interval obtained by the Thellier method with partial thermal remanence (p-TRM) checks. The fact that different techniques give qualitatively compatible low palaeointensity results provides greater confidence that the weak field features seen just prior to the Cretaceous normal superchron (CNS) are the result of the actual field recorded by the basalts as opposed to artefacts of the method/analysis. This study also demonstrates that the microwave technique can be used for very old basalts.