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[SCG51-P03] 238U–230Th Disequilibrium Dating Method for Quaternary Zircons using Laser Ablation-Multiple Collector-ICP-Mass Spectrometry
Keywords:zircon, LA-ICP-MS, radioactive disequilibrium, Quaternary
Geochronological data derived from Quaternary zircons can provide piercing information to understand volcanic processes including growth of magma chamber, eruption, and cooling. Typical time intervals of the volcanic activity are ranging from 10 to 105 years,[1] and the pre-eruptive processes proceed at the corresponding time scales. Hence, crystallization ages of minerals are especially important to derive time constraints on these processes.
In this context, various methods have been developed to obtain formation and eruption ages of Quaternary volcanic minerals. Utilizing chronological information obtained from various isotopic geochronometers with different closure temperatures enables the acquisition of multiple age data from a single sample. Specifically, in the case of zircon, U-Th-Pb dating (closure temperature of more than 900 ℃) and fission-track dating (closure temperature of 200 ℃) on a single grain provides chronological information concerning the multiple evolutional stages with different temperatures.[2] Notably, the U-Th-Pb isotopic system for zircon can reveal the crystallization process in a magma chamber.
To unveil hidden geological events beneath volcanoes, rapid and in situ analysis of individual zircon grains is necessary to derive further detailed information concerning the thermal history of volcanic activities. Hence, this study focuses on the U-Th-Pb age analysis of Quaternary zircon using LA-ICP-MS.
Previous studies using LA-ICP-MS have attempted to obtain 232Th–208Pb ages[3] and 238U–230Th ages[4,5] for Quaternary zircons. These measurements, however, use single-collector ICP-MS, which requires mass scanning of a single detector to measure multiple isotopes, causing a low duty cycle of analytes and poor tracking performance to transient signals. This can deteriorate the accuracy and precision of isotope measurements, especially those obtained from samples with high internal heterogeneity. Furthermore, in 232Th–208Pb ages, contribution of initial lead (i.e., common Pb) can cause systematic errors. In this study, simultaneous detection of isotopes utilizing multiple-collector ICP-MS (MC-ICP-MS) is conducted to obtain 238U–230Th ages for Quaternary zircons.
The 238U–230Th age determinations require wide dynamic range covering more than 7 orders of magnitude, and thus, the combination of two different ion detectors is necessary: a secondary electron multiplier (SEM) and a Faraday detector. For the major isotopes (i.e., 232Th and 238U), Faraday detectors were used. For the minor isotopes (i.e., 230Th), SEM was employed.
To derive reliable 238U–230Th ages, the precise measurement of trace amounts of 230Th is crucial. Factors that may contribute to systematic errors include the tailing of the 232Th signal to m/z 230 and the presence of interfering ions (i.e., Zr2O3+) at m/z 230. In this study, the contribution of tailing to m/z 230 was verified using NIST SRM 610. Zr2O3+ is separated from 230Th at a mass resolution of about 1200, resulting in more accurate measurements compared to previous studies using sector field based ICP-MS.[6] To evaluate the data quality, the 230Th/238U values for reference zircons in radioactive equilibrium were measured, and also, the 238U–230Th age determinations were conducted on Quaternary zircons.
[1] White S. M. et al., 2006, Geophys. [2] Iwano H. et al., 2013, Island Arc. [3] Sakata S. et al., 2017, Quat. Geochronol. [4] Guillong M. et al., 2016, GGR. [5] Niki S. et al., 2022, GGR. [6] Guillong et al., 2015, J. Volcanol. Geotherm. Res.
In this context, various methods have been developed to obtain formation and eruption ages of Quaternary volcanic minerals. Utilizing chronological information obtained from various isotopic geochronometers with different closure temperatures enables the acquisition of multiple age data from a single sample. Specifically, in the case of zircon, U-Th-Pb dating (closure temperature of more than 900 ℃) and fission-track dating (closure temperature of 200 ℃) on a single grain provides chronological information concerning the multiple evolutional stages with different temperatures.[2] Notably, the U-Th-Pb isotopic system for zircon can reveal the crystallization process in a magma chamber.
To unveil hidden geological events beneath volcanoes, rapid and in situ analysis of individual zircon grains is necessary to derive further detailed information concerning the thermal history of volcanic activities. Hence, this study focuses on the U-Th-Pb age analysis of Quaternary zircon using LA-ICP-MS.
Previous studies using LA-ICP-MS have attempted to obtain 232Th–208Pb ages[3] and 238U–230Th ages[4,5] for Quaternary zircons. These measurements, however, use single-collector ICP-MS, which requires mass scanning of a single detector to measure multiple isotopes, causing a low duty cycle of analytes and poor tracking performance to transient signals. This can deteriorate the accuracy and precision of isotope measurements, especially those obtained from samples with high internal heterogeneity. Furthermore, in 232Th–208Pb ages, contribution of initial lead (i.e., common Pb) can cause systematic errors. In this study, simultaneous detection of isotopes utilizing multiple-collector ICP-MS (MC-ICP-MS) is conducted to obtain 238U–230Th ages for Quaternary zircons.
The 238U–230Th age determinations require wide dynamic range covering more than 7 orders of magnitude, and thus, the combination of two different ion detectors is necessary: a secondary electron multiplier (SEM) and a Faraday detector. For the major isotopes (i.e., 232Th and 238U), Faraday detectors were used. For the minor isotopes (i.e., 230Th), SEM was employed.
To derive reliable 238U–230Th ages, the precise measurement of trace amounts of 230Th is crucial. Factors that may contribute to systematic errors include the tailing of the 232Th signal to m/z 230 and the presence of interfering ions (i.e., Zr2O3+) at m/z 230. In this study, the contribution of tailing to m/z 230 was verified using NIST SRM 610. Zr2O3+ is separated from 230Th at a mass resolution of about 1200, resulting in more accurate measurements compared to previous studies using sector field based ICP-MS.[6] To evaluate the data quality, the 230Th/238U values for reference zircons in radioactive equilibrium were measured, and also, the 238U–230Th age determinations were conducted on Quaternary zircons.
[1] White S. M. et al., 2006, Geophys. [2] Iwano H. et al., 2013, Island Arc. [3] Sakata S. et al., 2017, Quat. Geochronol. [4] Guillong M. et al., 2016, GGR. [5] Niki S. et al., 2022, GGR. [6] Guillong et al., 2015, J. Volcanol. Geotherm. Res.