10:00 AM - 10:15 AM
[SCG51-11] Development of apatite U-Th disequilibrium system for geochronology of Late Pleistocene volcanics
Keywords:apatite, U-Th disequilibrium, Late Pleistocene, geochronology
In geochronology and volcanology, there is a universal and urgent need to develop a method that can be applied to dating volcanic product younger than 100,000 years, especially andesitic to basaltic rocks that do not contain zircons or sanidines. In this study, we have developed a new method, 238U-230Th disequilibrium dating[1,2] using apatite. The advantages of this method are that (1) apatite is widely present in basaltic, andesitic, and rhyolitic magmas, (2) apatite crystallizes earlier in magmas, and (3) the crystallization period is assumed to be relatively short, which are superior to those of zircon in estimating the timing of early crystal differentiation in magma chamber. We aim to apply this method to Japanese volcanoes and construct it as a dating method for volcanic eruptions younger than 100,000 years ago and/or for magmatic processes prior to eruptions.
Niki et al. (2022)[3] focused on the 238U-230Th disequilibrium dating method (hereafter referred to as the U-Th method) using thorium-230 (half-life 75,000 years), an intermediate isotope of the uranium series, as a radiometric dating method applicable to geological samples from the Late Pleistocene. The authors have developed a new technique to remove mass spectrometric interferences for the mass number 230 using a quadrupole ICP mass spectrometer equipped with a collision cell. As an application of this technique, they reported zircon ages from 100,000 to 20,000 years ago, including the Toya pyroclastic flow and the Sanbe-Kisuki pumice. In this study, we applied the LA-ICP-MS U-Th dating basis for zircon established by Niki et al. to apatite. The following three improvements were made: (1) A fast multi-spot laser ablation sampling condition was determined to obtain about 10 times higher sampling volume than that of zircon, due to the lower uranium concentration in apatite (assumed to be about 1/10 of that in zircon). (2) Durango apatite megacryst (Th/U ratio ~20) was used as a reference material with a known Th/U ratio. (3) Th- and U-free synthesized apatite was used for ablation blank correction to maintain the extremely low-level background noise for 230Th. In this presentation, we report the results of U-Th disequilibrium dating using the Sanbe-Ikeda pyroclastic flow, a tephra from Sanbe-san, Shimane Prefecture, Japan. The eruption age of this tephra (46.3 ka) is constrained by a 14C dating study in Lake Suigetsu sediments, Fukui Prefecture [4]. This tephra contains both zircon and apatite. Our preliminary analysis confirmed that the sample contains a mixture of detrital old zircon grains. Therefore, 3-1 cm diameter pumices (30 g dry weight) were collected, and separated zircon and apatite grains are considered to be essential ones. In this presentation, we will compare the zircon and apatite U-Th ages obtained from the pumice with those from other methods (14C, zircon FT), discuss the ages and their accuracy, and offer the role of the apatite U-Th disequilibrium system for the Late Pleistocene volcanochronology.
[1] Kigoshi K. (1967) Science 156, 932-934. [2] Guillong M., Sliwinski J.T., Schmitt A., Forni F. and Bachmann O. (2016) Geostandards and Geoanalytical Research, 40, 377– 387. [3] Niki S., Kosugi S., Iwano H., Danhara T. and Hirata T. (2022) Geostandards and Geoanalytical Research. doi: 10.1111/ggr.12458. [4] Maruyama S.,Takemura K., Hirata T., Yamashita T. and Danhara T. (2019) Journal of Geography (Chigaku Zasshi), 128, 879-903.
Niki et al. (2022)[3] focused on the 238U-230Th disequilibrium dating method (hereafter referred to as the U-Th method) using thorium-230 (half-life 75,000 years), an intermediate isotope of the uranium series, as a radiometric dating method applicable to geological samples from the Late Pleistocene. The authors have developed a new technique to remove mass spectrometric interferences for the mass number 230 using a quadrupole ICP mass spectrometer equipped with a collision cell. As an application of this technique, they reported zircon ages from 100,000 to 20,000 years ago, including the Toya pyroclastic flow and the Sanbe-Kisuki pumice. In this study, we applied the LA-ICP-MS U-Th dating basis for zircon established by Niki et al. to apatite. The following three improvements were made: (1) A fast multi-spot laser ablation sampling condition was determined to obtain about 10 times higher sampling volume than that of zircon, due to the lower uranium concentration in apatite (assumed to be about 1/10 of that in zircon). (2) Durango apatite megacryst (Th/U ratio ~20) was used as a reference material with a known Th/U ratio. (3) Th- and U-free synthesized apatite was used for ablation blank correction to maintain the extremely low-level background noise for 230Th. In this presentation, we report the results of U-Th disequilibrium dating using the Sanbe-Ikeda pyroclastic flow, a tephra from Sanbe-san, Shimane Prefecture, Japan. The eruption age of this tephra (46.3 ka) is constrained by a 14C dating study in Lake Suigetsu sediments, Fukui Prefecture [4]. This tephra contains both zircon and apatite. Our preliminary analysis confirmed that the sample contains a mixture of detrital old zircon grains. Therefore, 3-1 cm diameter pumices (30 g dry weight) were collected, and separated zircon and apatite grains are considered to be essential ones. In this presentation, we will compare the zircon and apatite U-Th ages obtained from the pumice with those from other methods (14C, zircon FT), discuss the ages and their accuracy, and offer the role of the apatite U-Th disequilibrium system for the Late Pleistocene volcanochronology.
[1] Kigoshi K. (1967) Science 156, 932-934. [2] Guillong M., Sliwinski J.T., Schmitt A., Forni F. and Bachmann O. (2016) Geostandards and Geoanalytical Research, 40, 377– 387. [3] Niki S., Kosugi S., Iwano H., Danhara T. and Hirata T. (2022) Geostandards and Geoanalytical Research. doi: 10.1111/ggr.12458. [4] Maruyama S.,Takemura K., Hirata T., Yamashita T. and Danhara T. (2019) Journal of Geography (Chigaku Zasshi), 128, 879-903.