14:45 〜 15:00
[SCG57-11] 高温マグマプロセス解明に向けたU-Th-Pb年代測定法の再編
キーワード：高温マグマプロセス、U-Th-Pb系列年代測定、若い鉱物の年代測定、LA-ICP-MS/MS
U-Th-Pb dating of accessory minerals for the Quaternary period is an important method for understanding the high-temperature cooling history of magmas and constraining the ages of tephras as important chronostratigraphic marker horizons (Schmitt, 2011). Independent of the analytical difficulty associated with measuring small amount of radiogenic nuclides (e.g., 230Th, 206Pb, 207Pb, 208Pb), to obtain accurate crystallization ages from young minerals, it is necessary to correct for the effect of initial disequilibria caused by intermediate nuclides in the 238U and 235U decay series (i.e., 230Th and 231Pa) (Wendt and Carl, 1985) and common Pb. To correct for these effects, modified 207Pb method is proposed (Sakata et al., 2017; Sakata, 2018). In this correction method, estimating the magnitude of disequilibria from Th/U and Pa/U partitioning in mineral-melt system is required. However, it is widely recognized that Th and U could have been heterogeneously distributed in the magma and, moreover, that (Th/U)melt could change with time due to the crystallization of U-Th-bearing minerals within the melts (Amelin and Zaitsev, 2002). These factors make it difficult to estimate the (Th/U)melt and (Pa/U)melt at the time and site of mineral crystallization. As another approach, 238U-230Th dating method is widely used for young mineral dating as well. In this method, however, it is also difficult to discern the potential multi-stage crystallization history of a single crystal if, for example, Th/U in the melt is variable (Boehnke et al., 2016). Therefore, more rigorous dating method is highly desired for revealing the history of magmas before eruption. In this study, we would like to return to the principle of U- and Th-decay series, and reconstruct a better dating method which overcomes the aforesaid problems.
After deforming the formula in Wendt and Carl (1985) which represents relationship among the numbers of atoms in 238U- decay series, we introduced a relationship associated with mineral's crystallization age, which including information about intermediate nuclides in U- decay series and 206Pb. From this relationship, crystallization age can be obtained by measuring 238U, 230Th, and 206Pb. This indicates that accurate crystallization age can be calculated without depending on uncertain estimation of Th/U partitioning in mineral-melt system.
We also provide analytical technique for this method using laser ablation ICP tandem quadrupole mass spectrometry (LA-ICP-MS/MS). To measure 230Th/238U in minerals accurately by LA-ICP-MS, great care must be taken for abundance sensitivity and polyatomic interferences due to the existence of tailing from 232Th mass peak. We achieved high abundance sensitivity (<0.3 ppb) and successfully removed interferences of polyatomic ions using tandem quadrupole and collision/reaction cell devices. Hence, in this presentation, we would like to demonstrate first application of new dating method to Quaternary mineral samples, and discuss its effectiveness for magma process study.
After deforming the formula in Wendt and Carl (1985) which represents relationship among the numbers of atoms in 238U- decay series, we introduced a relationship associated with mineral's crystallization age, which including information about intermediate nuclides in U- decay series and 206Pb. From this relationship, crystallization age can be obtained by measuring 238U, 230Th, and 206Pb. This indicates that accurate crystallization age can be calculated without depending on uncertain estimation of Th/U partitioning in mineral-melt system.
We also provide analytical technique for this method using laser ablation ICP tandem quadrupole mass spectrometry (LA-ICP-MS/MS). To measure 230Th/238U in minerals accurately by LA-ICP-MS, great care must be taken for abundance sensitivity and polyatomic interferences due to the existence of tailing from 232Th mass peak. We achieved high abundance sensitivity (<0.3 ppb) and successfully removed interferences of polyatomic ions using tandem quadrupole and collision/reaction cell devices. Hence, in this presentation, we would like to demonstrate first application of new dating method to Quaternary mineral samples, and discuss its effectiveness for magma process study.