Earth Planets Space, Vol. 59 (No. 7), pp. 727-746, 2007
Y. Yamamoto1, H. Tsunakawa2, J. Shaw1, and M. Kono2
1Geomagnetism Laboratory, Department of Earth and Ocean Sciences,
University of Liverpool, L69 7ZE, United Kingdom
2Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo 152-8551, Japan
(Received October 7, 2006; Revised December 16, 2006; Accepted January 6, 2007; Online published July 20, 2007)
Paleomagnetic measurements were conducted on Datong volcanic rocks from China, which are thought to have formed in the mid to early Brunhes Chron. Meaningful site-mean paleodirections were obtained from 21 sites which are considered to represent 17 independent cooling units. They give a mean VGP (virtual geomagnetic pole) position of (76.5°N, 7.9°E) with A95=7.7° (N=17), which is statistically distinct from geographic north. This dataset also yields an ASD (angular standard deviation) of 17.2° around the mean VGP position. Because the paleodirections form two clusters, the samples may record the paleomagnetic field during two different short periods and therefore may not average out paleosecular variation. Paleointensity measurements were conducted using three different methods. The DHT and LTD-DHT Shaw methods, the Thellier method, and the microwave Thellier method were applied to 119, 29 and 73 specimens respectively, and they give 66, 16 and 12 successful results (success rates are 55, 55 and 16%). From the LTD-DHT Shaw dataset, eight acceptable site-mean paleointensities are obtained. They give an average VDM (virtual dipole moment) of 3.79±1.94×1022A m2. This is 56% lower than the average VDM of 5.91±1.74×1022 A m2 (N=14) calculated from the selected Thellier data from the latest paleointensity database using the same criteria. One possible reason for this difference might be systematic overestimations of paleointensities by the Thellier method on volcanic rocks.
Key words: Paleosecular variation, paleointensity, LTD-DHT Shaw method, Datong Volcano, Brunhes Chron.