Paleointensity study of the historical andesitic lava flows: LTD-DHT Shaw and Thellier paleointensities from the Sakurajima 1914 and 1946 lavas in Japan

Correct determination of absolute paleointensities is essential to investigate past geomagnetic field. There are two types of methods to obtain the paleointensities: the Thellier-type and Shaw-type methods. Many paleomagnetists have so far regarded the former method as reliable. However, there are increasing evidences that it is sometimes not robust for basaltic lavas resulting in systematic high paleointensities (e.g. Calvo et al., 2002; Yamamoto et al., 2003). Alternatively, the double heating technique of the Shaw method combined with low temperature demagnetization (LTD-DHT Shaw method; Tsunakawa et al., 1997; Yamamoto et al., 2003), a lately developed paleointensity technique in Japan, can yield reliable answers even from such basaltic samples (e.g. Yamamoto et al., 2003; Mochizuki et al., 2004; Oishi et al., 2005). In the Japanese archipelago, there are not only basaltic lavas but also andesitic lavas. They are important candidates of the absolute paleointensity determination in Japan. For a case study, we sampled oriented paleomagnetic cores from three sites of the Sakurajima 1914 (TS01 and TS02) and 1946 (SW01) lavas in Japan. Several rock magnetic experiments revealed that main magnetic carriers of the present samples are titanomagnetites with Curie temperatures of about 300-550 C, and that high temperature oxidation progresses in the order of SW01, TS01 and TS02. The LTD-DHT Shaw and Coe-Thellier experiments were conducted on 72 and 63 specimens, respectively. They gave 64 and 60 successful determinations. If the results are normalized by expected field intensities calculated from IGRF-9 (Macmillan et al., 2003) and grouped into LTD-DHT Shaw and Thellier datasets, their averages and standard deviations (1 sigma) resulted in 0.98+/-0.11 (LTD-DHT Shaw) and 1.13+/-0.13 (Thellier). Considering the standard deviations, we can say that both paleointensity methods recovered correct geomagnetic field. However, it is apparent that the LTD-DHT Shaw method has higher reliability than the Thellier method.