12:00 〜 12:15
[SCG64-12] 高温マグマプロセス解明に向けたジルコンウラン系列年代測定法の再編
キーワード：ジルコン、ウラン系列年代測定、マグマプロセス
Zircon U–Pb age data for the samples of the Quaternary period can provide key information to unveil the high-temperature cooling history of magmas and also to define chronological constrains for the tephras as an important chronostratigraphic marker horizons (Schmitt, 2011). Independent of the analytical difficulty associated with measuring small amount of radiogenic Pb, to obtain accurate crystallization ages from young zircon, 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). Common Pb contribution is also non-negligible in the case of Quaternary zircon dating. To correct these effects, the modified correction method using the 207Pb has been proposed (Sakata et al., 2017; Sakata, 2018). In this approach, estimating the magnitude of disequilibria from Th/U and Pa/U partitioning in zircon–melt system is highly required. However, it is widely recognized that Th and U could have been heterogeneously distributed in the magma and, more importantly, 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 zircon crystallization. Faced with this, another approache can be applied. As a second approach, 238U–230Th dating method is widely used for young zircon 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 have introduced a new equation associated with zircon crystallization age, which including information about 232Th- decay series and common Pb. In equation 1, the 't' denotes zircon crystallization age, and 'c' and 'm' represent common Pb and measured isotopic ratios, respectively. This indicates that accurate crystallization age can be derived by measuring isotopic ratios in equation 1 without depending on uncertain estimation of Th/U and Pa/U partitioning in zircon–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 zircon 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 and Zr2O3+ (Guillong et al., 2015). With the present LA-ICP-MS/MS technique, high abundance sensitivity of better than <0.3 ppb can be achieved, and thus the possible interferences from polyatomic ions can be successfully removed. Several reference zircon samples were used to evaluate the reliability of the measurement of 230Th/238U and other isotopic ratios in equation 1. The resulting values showed good agreement with the reference values. In this presentation, we would like to demonstrate first application of new dating method to Quaternary zircon samples, and discuss its effectiveness for magma process study.
References
Amelin, Y., Zaitsev, A.N. (2002) Geochim. Cosmochim. Acta 66, 2399–2419.
Boehnke et al. (2016) Quat. Geochronol., doi: 10.1016/j.quageo.2016.03.005.
Guillong et al. (2015) J. Volcanol. Geotherm. Res. 296, 101–103.
Sakata, S. (2018) Geochem. J. 52, doi:10.2343/geochemj.2.0508, in press.
Sakata et al. (2017) Quat. Geochronol. 38, 1–12.
Schmitt, A.K. (2011) Ann. Rev. Earth Pl. Sci. 39, 321–349.
Wendt, I., Carl, C. (1985) Earth Planet. Sci. Lett. 73, 278–284.
After deforming the formula in Wendt and Carl (1985), which represents relationship among the numbers of atoms in 238U- decay series, we have introduced a new equation associated with zircon crystallization age, which including information about 232Th- decay series and common Pb. In equation 1, the 't' denotes zircon crystallization age, and 'c' and 'm' represent common Pb and measured isotopic ratios, respectively. This indicates that accurate crystallization age can be derived by measuring isotopic ratios in equation 1 without depending on uncertain estimation of Th/U and Pa/U partitioning in zircon–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 zircon 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 and Zr2O3+ (Guillong et al., 2015). With the present LA-ICP-MS/MS technique, high abundance sensitivity of better than <0.3 ppb can be achieved, and thus the possible interferences from polyatomic ions can be successfully removed. Several reference zircon samples were used to evaluate the reliability of the measurement of 230Th/238U and other isotopic ratios in equation 1. The resulting values showed good agreement with the reference values. In this presentation, we would like to demonstrate first application of new dating method to Quaternary zircon samples, and discuss its effectiveness for magma process study.
References
Amelin, Y., Zaitsev, A.N. (2002) Geochim. Cosmochim. Acta 66, 2399–2419.
Boehnke et al. (2016) Quat. Geochronol., doi: 10.1016/j.quageo.2016.03.005.
Guillong et al. (2015) J. Volcanol. Geotherm. Res. 296, 101–103.
Sakata, S. (2018) Geochem. J. 52, doi:10.2343/geochemj.2.0508, in press.
Sakata et al. (2017) Quat. Geochronol. 38, 1–12.
Schmitt, A.K. (2011) Ann. Rev. Earth Pl. Sci. 39, 321–349.
Wendt, I., Carl, C. (1985) Earth Planet. Sci. Lett. 73, 278–284.