Paleomagnetic intensity of Aso pyroclastic flows: Additional results with LTD-DHT Shaw method, Thellier method with pTRM-tail check

For the sake to calibrate the absolute value of the ’relative paleointensity variation curve’ drawn from sediment cores, Takai et al. (2002) proposed to use pyroclastic flows co-bearing with wide spread tephras. The pyroclastic flows prepare volcanic rocks with TRM, which let us determine absolute paleointensity, and the tephras prepare the correlation with sediment stratigraphy. While 4 out of 6 pyroclastic flows are consistent with Sint-800 paleointensity variation curve, two flows, Aso-2 and Aso-4, show weaker and stronger than Sint-800 beyond the error, respectively. We revisited the paleointensity study of Aso pyroclastic flows, adding LTD- DHT Shaw method, the pTRM-tail check in Thellier experiment, and LTD-DHT Shaw method by using volcanic glasses. We prepared 11 specimens from 3 sites of Aso-1 welded tuff for LTD-DHT Shaw method experiments, and obtained 6 paleointensities satisfied a set of strict criteria. They yield an average paleointensity of 21.3±5.8uT, which is smaller than 31.0±3.4uT provided by Takai et al. (2002). For Aso-2 welded tuff, 11 samples from 3 sites were submitted to Thellier experiments, and 6 passed a set of pretty stringent criteria including pTRM-tail check, which is not performed by Takai et al. (2002). They give an average paleointensity of 20.2±1.5uT, which is virtually identical to 20.2±1.0uT (27 samples) given by Takai et al. (2002). Although the success rate was not good in LTD-DHT Shaw method, 2 out of 12 specimens passed the criteria, and gave 25.8±3.4uT, which is consistent with Takai et al. (2002). In addition, we obtained a reliable paleointensity from a volcanic glass in LTD-DHT Shaw method, it gives a paleointensity of 23.6 uT. It is also consitent with Takai et al. (2002). For Aso-3 welded tuff, we performed only LTD-DHT Shaw method for one specimen from one site yet. It gives a paleointensity of 43.0uT, which is higher than 31.8±3.6uT given by Takai et al. (2002). Eight sites were set for Aso-4 welded tuff, and 42 specimens were submitted to Thellier experiments. Twelve specimens from 4 sites passed the same criteria as Aso-2, and yield a mean paleointensity of 43.1±1.4uT. It again agrees with the value (45.6±1.7uT) of Takai et al. (2002). LTD-DHT Shaw method experiment is also applied for 12 specimens from 3 sites, and 4 passed the criteria giving 38.2±1.7. Although it is a little smaller than Thellier results, it is way larger than the Sint-800 at the time of Aso-4. Aso-1 result in this study is more consistent with the Sint-800 at that time than Takai et al. (2002). But for Aso-2 and Aso-4, their new reliable paleointensity results suggest that the discrepancy from the Sint-800 is not attributed to the experimental problems.