Abstract
Abstract A paleointensity study using the Thellier’s method was made on Tertiary basaltic rocks in Inner Mongolia and Hebei Province, northeastern China. K-Ar ages were previously reported for all the rocks, which range around 6–8 Ma and 28–32 Ma. Sample selection was based on total quality of remanence behavior and rock magnetism. High stability to AF and thermal demagnetization, small difference between the heating and cooling curves of magnetic susceptibility vs. temperature measurements, and PSD to SD characteristics in the Day plot of the hysteresis parameters were required. Experiments in vacuum using Coe’s procedure were applied to 54 specimens from nine flows. Experiments were successful for 34 specimens, giving seven flow mean paleointensities. Excluding two flow means (in one flow, only two specimens from the same sample survived and in the other, the within-site error amounts to 42%) the final success rate was 29 out of 54 (54%), which is not low. The obtained results range from 54 to 65 μT, except for one lava which gave 23 μT. These results indicate that although the paleointensity in the Tertiary was generally smaller than the present-day value, there were large fluctuations in the dipole moment, and paleointensities of the present-day level were often attained.
Highlights
Recent comprehensive focus by the research community to paleointensity studies on volcanic rocks has resulted in a remarkable increase in the database, which has 3128 data from 215 references in the most updated version of Pint03 by Perrin and Schnepp (2004) compared to 1123 data from 83 references in the first version by Tanaka et al (1995)
Sample directions were combined for CJ01 and CJ02, which are only 50 m apart and considered to be from the same flow
An anomalous paleodirection of I=−44.2◦, D=95.3◦, α95=11.6◦ is obtained from CD01, which was not reported in Zheng et al (1991)
Summary
Recent comprehensive focus by the research community to paleointensity studies on volcanic rocks has resulted in a remarkable increase in the database, which has 3128 data from 215 references in the most updated version of Pint by Perrin and Schnepp (2004) compared to 1123 data from 83 references in the first version by Tanaka et al (1995). Paleointensity data filtered with recent stringent criteria indicate that the paleointensity was much weaker than the present-day value for most of geological time (Selkin and Tauxe, 2000). This is still a matter of debate, and the research community is far from attaining a consensus (e.g., Biggin and Thomas, 2003; Heller et al, 2003). Samples were taken from the collection of Zheng et al (1991). K-Ar ages of the samples were reported by Zheng et al (2002) and range around 6–8 Ma and 28–32 Ma
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