5:15 PM - 6:45 PM
[SCG51-P02] Hybrid geochronology using U-Pb, U−Th disequilibrium and fission-track for Late Pleistocene zircon
Keywords:U-Pb, U-Th disequilibrium, fission track, zircon, geochronology, hybrid
Approximately 10% of the world's active volcanoes are located in the Japanese islands, making it one of the world's most volcanically active countries, although Japan covers only 0.25% of the world's land area. The eruption histories of Japan's major Quaternary volcanoes have been clarified and catalogued by dating. For example, according to dating studies of Japanese widespread tephras over the past 100,000 years, 14C ages play the most important role up to 50,000 years ago, while K-Ar, FT and luminescence ages, delta18O stratigraphy, and paleomagnetic stratigraphy have been mainly applied for those older than 50,000 years [1]. As this review shows, that only the 14C method can cover an age range younger than 50,000 years, and that this method can only be applied to charcoal materials. In fact, dating of small volcanoes other than major widespread tephras has rarely been reported except by the 14C method, which is an obstacle to Late Pleistocene volcanic studies and volcanic eruption countermeasures.
Because zircons contain relatively high concentrations of uranium, three types of age information can be obtained from a single grain: U-Pb, U-Th disequilibrium, and fission track (F T). The first two indicate crystallization ages, while the latter refers to cooling ages. Double dating of zircon by LA-ICP-MS using the U-Pb and FT methods has become popular in the last decade, but the recently developed LA-ICP-MS U-Th method can cover the age range of the 14C method and is expected to be a breakthrough in Late Pleistocene geochronology.
Although the U-Pb method has the advantage of providing highly accurate grain-by-grain ages, the lower limit of its applicable age range is limited to about 100,000 years ago. With the U-Th method, the applicable age range (from 350,000 to 10,000 years ago) can cover the 14C age range. However, since a single age (isochron age) is determined from a homogeneous group of zircon crystals, it is not easy to identify detrital crystals one grain at a time; the FT method can, in principle, also date to 10,000 years ago, and detrital grains older than a few Ma can be easily identified and excluded from the high FT density. However, it requires hundreds to thousands of grains to reduce the age error. At this time, the measurement foundation is in place for crystallization and cooling age determination up to 10,000 years for homogeneous zircon grain groups [2][3].
By taking advantage of and combining the above features of the three methods, this study aims to further improve the method for accurate dating of heterogeneous volcanic rock samples containing detrital zircon grains. In this presentation, we report zircon ages for the Sanbe Kisuki volcanic ash (105 ka), the Sanbe Oda pyroclastic flow (50 ka), and the Sanbe Ikeda pyroclastic flow (46.3 ka), all of which are ejecta from Sanbe-san volcano, Shimane Prefecture, and discuss differences with the eruption age (estimated with 14C dating). Furthermore, we will clarify the mixing of disequilibrium zircons and explore the possibility of dating magmatic processes that are not captured by the 14C method.
[1] Okuno M. (2019) The journal of the Geological Society of Japan, 125, 41-53. [2] Niki S., Kosugi S., Iwano H., Danhara T. and Hirata T. (2022) Geostandards and Geoanalytical Research. doi: 10.1111/ggr.12458. [3] Iwano H., Danhara T., Danhara Y., Hirabayashi S., Nakajima T., Sakai H. and Hirata T. (2020) Island Arc, 29, doi.org/ 10.1111/iar.12348
Because zircons contain relatively high concentrations of uranium, three types of age information can be obtained from a single grain: U-Pb, U-Th disequilibrium, and fission track (F T). The first two indicate crystallization ages, while the latter refers to cooling ages. Double dating of zircon by LA-ICP-MS using the U-Pb and FT methods has become popular in the last decade, but the recently developed LA-ICP-MS U-Th method can cover the age range of the 14C method and is expected to be a breakthrough in Late Pleistocene geochronology.
Although the U-Pb method has the advantage of providing highly accurate grain-by-grain ages, the lower limit of its applicable age range is limited to about 100,000 years ago. With the U-Th method, the applicable age range (from 350,000 to 10,000 years ago) can cover the 14C age range. However, since a single age (isochron age) is determined from a homogeneous group of zircon crystals, it is not easy to identify detrital crystals one grain at a time; the FT method can, in principle, also date to 10,000 years ago, and detrital grains older than a few Ma can be easily identified and excluded from the high FT density. However, it requires hundreds to thousands of grains to reduce the age error. At this time, the measurement foundation is in place for crystallization and cooling age determination up to 10,000 years for homogeneous zircon grain groups [2][3].
By taking advantage of and combining the above features of the three methods, this study aims to further improve the method for accurate dating of heterogeneous volcanic rock samples containing detrital zircon grains. In this presentation, we report zircon ages for the Sanbe Kisuki volcanic ash (105 ka), the Sanbe Oda pyroclastic flow (50 ka), and the Sanbe Ikeda pyroclastic flow (46.3 ka), all of which are ejecta from Sanbe-san volcano, Shimane Prefecture, and discuss differences with the eruption age (estimated with 14C dating). Furthermore, we will clarify the mixing of disequilibrium zircons and explore the possibility of dating magmatic processes that are not captured by the 14C method.
[1] Okuno M. (2019) The journal of the Geological Society of Japan, 125, 41-53. [2] Niki S., Kosugi S., Iwano H., Danhara T. and Hirata T. (2022) Geostandards and Geoanalytical Research. doi: 10.1111/ggr.12458. [3] Iwano H., Danhara T., Danhara Y., Hirabayashi S., Nakajima T., Sakai H. and Hirata T. (2020) Island Arc, 29, doi.org/ 10.1111/iar.12348