Multi-level consistency tests in paleointensity determinations from the welded tuffs of the Aso pyroclastic-flow deposits

摘要

We have performed multi-level consistency tests in an LTD-DHT Shaw paleointensity study on the welded tuffs of the Aso pyroclastic-flow deposits (Aso-1, Aso-2, Aso-3, and Aso-4) in order to obtain reliable mean paleointensities from each unit. For a unit-level consistency test, we collected samples at three or more sites in each unit. To enhance a site-level consistency test, we conducted paleointensity experiments on both whole-rock and glass samples from the same sites. Thermal demagnetization curves of the samples indicate that the main magnetic carrier is titanium-poor titanomagnetite to pure magnetite. Hysteresis parameters are different among the sites of the same units. Also, differences in unblocking spectra and hysteresis parameters are recognized between the whole-rock and glass samples from the same sites. Therefore, these samples of various properties are expected to work effectively for multi-level consistency test. Twenty-nine out of 65 whole-rock samples and 10 out of 31 glass samples gave accepted paleointensity results. At the site-level, consistency of paleointensity estimates is observed with the exception of one site. Two paleointensity estimates from glass samples of a site of Aso-4 are inconsistent and are excluded from further analysis. At the unit-level, consistency or inconsistency among the sites is observed for individual units. For Aso-1, inconsistency in paleointensity estimates among the sites is observed and we can recognize more reliable results from two sites. For Aso-2, consistency in paleointensity estimates from glass and whole-rock samples supports the acceptance of a site. For Aso-3, all paleointensity estimates from two sites are thought to be reliable because consistency between the site-means of the two sites as well as site-level consistency between the glass and whole-rock samples for each site is recognized. For Aso-4, some difference among the sites is observed, but there is no obvious reason to exclude the results of particular sites. Overall, both the site-level and unit-level consistency checks work effectively in these samples which confirm the reliability of the paleointensity estimates or highlights the erroneous ones. On the basis of the reliable paleointensity estimates, unit-mean values are obtained as 24.9. ±. 2.2 (1. σ). μT (N=9) for Aso-1, 22.9. ±. 1.6. μT (N=5) for Aso-2, 28.3. ±. 3.4. μT (N=10) for Aso-3, and 40.1. ±. 6.2. μT (N=7) for Aso-4.