1:45 PM - 2:00 PM
[SCG57-07] Test for high-precision double dating of 238U–206Pb and 238U–230Th zircon ages with LA-ICPMS: a case study from Ohda pyroclastic flow in the Sanbe volcanic system.
Keywords:LA-ICPMS, Sanbe volcanic system, Crystal residence time, 230Th–238U dating, 238U–206Pb dating
Recent advances in a mass spectrometry allow the application of a zircon geochronology to late Quaternary samples. The dating of a young zircon conventionally follows an U–Th–Pb decay system. Among the radionuclides in this system, the contrasting decay schemes on long-lived 238U (half-life, T1/2, = 4.47 Gy) and intermediate 230Th nuclides (T1/2 = 75.4 ky) are key to understanding the crystal growth rate and residence time in a silicic magma. To date, only a limited number of zircon data, which include both nuclides, have been reported mainly due to the low abundances of 206Pb and 230Th products in the Quaternary zircons. In a response to these challenges, a progress on the laser ablation ICP-MS (LA-ICPMS) technique has resulted in about a tenfold improvement in the sensitivity and the signal-to-noise ratio compared to previous works. At present, the double dating of 238U–206Pb and 238U–230Th ages can be performed at craters smaller than 35 × 35 μm2 using LA-ICPMS. The application of the spot double dating has the potential to shed light on the new magma geochemistry. As a case study, we applied the new dating technique to an Ohda pyroclastic flow in the Sanbe volcanic system.
The Sanbe volcanic system has remained active since the first eruption at 105 ka. Its prolonged volcanic activity, lasting up to about 105 years, culminated in a period of the caldera formation. Based on a fission track dating on zircons, an eruption of the Ohda pyroclastic flow was contemporaneous with the caldera formation at 70 ka, and postdates the oldest identified pumice of 105 ka in the Kusuki area. During the interval of 70–105 ka, no distinct magmatic activity has been found. In this study, the Ohda pyroclastic flow was crushed into zircon separates. 238U–206Pb and 238U–230Th measurements with trace element analysis were carried out on the mounted zircons using LA-ICPMS at the University of Tokyo and Kyoto University. The 238U–206Pb data are corrected for 230Th disequilibrium, and they range at 0.05–0.27 Ma with large uncertainties. The 238U–206Pb age distribution shows the presence of distinct xenocrystic grains that predate the Kusuki pumice. The majority of the age population is interpreted to result from the superposition of 2 or more age peaks. Precise estimate of a crystal residence time is based on the 230Th–238U data. The activity ratios of (238U/232Th) vary in the range of 1.1–15.4. This variation is closely correlated with the Ti abundance in zircon, which has the potential to constrain the temperature of the crystallisation. Ti-in-zircon temperatures obtained from the same pits as the 230Th–238U analysis range at 620–850 ℃. Their temperature decrements coincide with the increase in (238U/232Th) of zircons. Following the clustering analysis, the 230Th–238U dataset is preliminary categorised into two clusters, yielding isochron ages of 68.5 ± 4.0 ka and 98.9 ± 7.0 ka. The younger of these ages overlaps with the age of the caldera formation
The analysed zircons show a negative correlation between the (238U/232Th) and Ti-in-zircon temperature. This observation suggests that a decrease in magma temperature controlled a crystal fractionation in the Sanbe volcanic system. Combined with the evidence that 230Th–238U data document two concordant arrays on an isochron diagram, zircon crystallisation should be completed on an even shorter timescale than 230Th decay in a single event of the Sanbe volcanic system. This timescale is inferred to be < 104 years. In addition, a crystal residence time in the Sanbe volcanic system is constrained to ca 30 ky. Despite of the limited sample analysed, our high-precision dating has allowed us to distinguish autocrysts of 68.5 ka from antecrysts of 98.9 ka, and also from xenocryst of >105 ka for the first time. In this talk, we will present the above outline of high-precision double dating of 238U–206Pb and 238U–230Th zircon ages.
The Sanbe volcanic system has remained active since the first eruption at 105 ka. Its prolonged volcanic activity, lasting up to about 105 years, culminated in a period of the caldera formation. Based on a fission track dating on zircons, an eruption of the Ohda pyroclastic flow was contemporaneous with the caldera formation at 70 ka, and postdates the oldest identified pumice of 105 ka in the Kusuki area. During the interval of 70–105 ka, no distinct magmatic activity has been found. In this study, the Ohda pyroclastic flow was crushed into zircon separates. 238U–206Pb and 238U–230Th measurements with trace element analysis were carried out on the mounted zircons using LA-ICPMS at the University of Tokyo and Kyoto University. The 238U–206Pb data are corrected for 230Th disequilibrium, and they range at 0.05–0.27 Ma with large uncertainties. The 238U–206Pb age distribution shows the presence of distinct xenocrystic grains that predate the Kusuki pumice. The majority of the age population is interpreted to result from the superposition of 2 or more age peaks. Precise estimate of a crystal residence time is based on the 230Th–238U data. The activity ratios of (238U/232Th) vary in the range of 1.1–15.4. This variation is closely correlated with the Ti abundance in zircon, which has the potential to constrain the temperature of the crystallisation. Ti-in-zircon temperatures obtained from the same pits as the 230Th–238U analysis range at 620–850 ℃. Their temperature decrements coincide with the increase in (238U/232Th) of zircons. Following the clustering analysis, the 230Th–238U dataset is preliminary categorised into two clusters, yielding isochron ages of 68.5 ± 4.0 ka and 98.9 ± 7.0 ka. The younger of these ages overlaps with the age of the caldera formation
The analysed zircons show a negative correlation between the (238U/232Th) and Ti-in-zircon temperature. This observation suggests that a decrease in magma temperature controlled a crystal fractionation in the Sanbe volcanic system. Combined with the evidence that 230Th–238U data document two concordant arrays on an isochron diagram, zircon crystallisation should be completed on an even shorter timescale than 230Th decay in a single event of the Sanbe volcanic system. This timescale is inferred to be < 104 years. In addition, a crystal residence time in the Sanbe volcanic system is constrained to ca 30 ky. Despite of the limited sample analysed, our high-precision dating has allowed us to distinguish autocrysts of 68.5 ka from antecrysts of 98.9 ka, and also from xenocryst of >105 ka for the first time. In this talk, we will present the above outline of high-precision double dating of 238U–206Pb and 238U–230Th zircon ages.