09:15 〜 09:30
[SCG51-02] Uranium - Lead Isotopic Geochronology on Quaternary Zircons using Multiple Collector-ICP-MS equipped with Faraday and Daly Ion Detectors
キーワード：第四紀ジルコン、多重検出方式のICP質量分析法
Precise and accurate geochronological data derived from Quaternary zircons can provide piercing information to understand the process from generation of magmas to eruption. Time interval of volcanic eruptions is ranging from 10 to 105 years [1], and therefore pre-eruptive processes, such as magma storage and cooling in magma chambers, proceed at the corresponding time scales. Hence, determination of the crystallization ages of Quaternary volcanic minerals is especially important because this imposes time constraints on the process through which magma stored in a magma chamber is erupted. In this context, various methods have been developed to obtain formation and eruption ages of Quaternary volcanic minerals.
Based on chronological information obtained from several isotopic geochronometers having different closure temperatures, multiple age data can be obtained from a single sample. For zircon, in particular, multiple age information with widely different closure temperature can be acquired from a single grain by applying U-Th-Pb dating (closure temperature 900°C) and Fission-Track (FT) dating (closure temperature 200–300°C) methods. Coupling of these chronometers allows us to gain insight into the thermal evolution of each zircon grain [2]. Among them, U-Th-Pb isotopic system for zircon with a high closure temperature can reflect the crystallization process in a magma chamber. In previous studies, some zircon crystallization ages demonstrate tens to hundreds of thousands of years younger than the eruption age [3], and the time difference is regarded as a magma residence time.
To obtain such knowledge from in situ analysis of individual zircon grains and within a zircon grain is necessary, and secondary ion mass spectrometry (SIMS) and laser ablation ICP mass spectrometry (LA-ICP-MS), which can analyze samples with high spatial resolution, have been used. In particular, LA-ICP-MS can extract information from tens to hundreds of spots in a short time. This rapidity is important for obtaining precise data from a large number of zircon grains and treating them statistically to derive more objective thermal history information. Therefore, 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 [4] and 238U–230Th ages [5,6] for Quaternary zircons. However, these measurements use single-collector ICP-MS, which requires mass scanning of a single detector to measure multiple isotopes, resulting in 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.
To do this, in this study, simultaneous detection of isotopes utilizing multiple-collector ICP-MS (MC-ICP-MS) is conducted to obtain 232Th–208Pb ages and 238U–230Th ages for Quaternary zircons. The dating methods for Quaternary samples require a wide dynamic range covering 6–7 orders of magnitude, and the combination of two different types of detectors is necessary: a Daly detector [7] and a Faraday detector. However, the Faraday detector has an internal capacitor, which requires time duration for signal stabilization when the signal intensity changes. Therefore, the Faraday detector takes about 0.1 second longer for the stabilization of isotopic signals than the Daley detector. For accurate isotope ratio measurements, the deviation derived from the time difference should be corrected. In this presentation, we will examine the difference in signal response of the two detectors to create a correction method and evaluate the accuracy of the resulting age data through the analysis of reference materials.
[1] White S. M. et al., 2006, Geophys. [2] Iwano H. et al., 2013, Island Arc. [3] Costa F., 2021, Annu. Rev. Earth Planet. Sci. [4] Sakata S. et al., 2017, Quaternary Geochronology. [5] Guillong M. et al., 2016, GGR. [6] Niki S. et al., 2022, Geostandards and Geoanalytical Research. [7] Obayashi H. et al., 2017, J. Anal. Atom. Spectrom.
Based on chronological information obtained from several isotopic geochronometers having different closure temperatures, multiple age data can be obtained from a single sample. For zircon, in particular, multiple age information with widely different closure temperature can be acquired from a single grain by applying U-Th-Pb dating (closure temperature 900°C) and Fission-Track (FT) dating (closure temperature 200–300°C) methods. Coupling of these chronometers allows us to gain insight into the thermal evolution of each zircon grain [2]. Among them, U-Th-Pb isotopic system for zircon with a high closure temperature can reflect the crystallization process in a magma chamber. In previous studies, some zircon crystallization ages demonstrate tens to hundreds of thousands of years younger than the eruption age [3], and the time difference is regarded as a magma residence time.
To obtain such knowledge from in situ analysis of individual zircon grains and within a zircon grain is necessary, and secondary ion mass spectrometry (SIMS) and laser ablation ICP mass spectrometry (LA-ICP-MS), which can analyze samples with high spatial resolution, have been used. In particular, LA-ICP-MS can extract information from tens to hundreds of spots in a short time. This rapidity is important for obtaining precise data from a large number of zircon grains and treating them statistically to derive more objective thermal history information. Therefore, 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 [4] and 238U–230Th ages [5,6] for Quaternary zircons. However, these measurements use single-collector ICP-MS, which requires mass scanning of a single detector to measure multiple isotopes, resulting in 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.
To do this, in this study, simultaneous detection of isotopes utilizing multiple-collector ICP-MS (MC-ICP-MS) is conducted to obtain 232Th–208Pb ages and 238U–230Th ages for Quaternary zircons. The dating methods for Quaternary samples require a wide dynamic range covering 6–7 orders of magnitude, and the combination of two different types of detectors is necessary: a Daly detector [7] and a Faraday detector. However, the Faraday detector has an internal capacitor, which requires time duration for signal stabilization when the signal intensity changes. Therefore, the Faraday detector takes about 0.1 second longer for the stabilization of isotopic signals than the Daley detector. For accurate isotope ratio measurements, the deviation derived from the time difference should be corrected. In this presentation, we will examine the difference in signal response of the two detectors to create a correction method and evaluate the accuracy of the resulting age data through the analysis of reference materials.
[1] White S. M. et al., 2006, Geophys. [2] Iwano H. et al., 2013, Island Arc. [3] Costa F., 2021, Annu. Rev. Earth Planet. Sci. [4] Sakata S. et al., 2017, Quaternary Geochronology. [5] Guillong M. et al., 2016, GGR. [6] Niki S. et al., 2022, Geostandards and Geoanalytical Research. [7] Obayashi H. et al., 2017, J. Anal. Atom. Spectrom.