Dating late Quaternary events by the combined U‐Pb LA‐ICP‐MS and (U‐Th)/He dating of zircon: A case study on Omachi Tephra suite (central Japan)

Abstract

Dating of young (<1 Ma) geological events has long been a challenge for geochronologists. Combining (U‐Th)/He with U‐Pb or U‐Th‐disequilibrium dating methods offers a unique dating tool that can address this important period. We present a new methodology that combines U‐Pb LA‐ICP‐MS and (U‐Th)/He dating of zircon and use it to date two Pleistocene marker tephras (A1Pm and DPm) from the Omachi Tephra suite (Japan). A1Pm and DPm yield U‐Pb ages in the range of 350–850 and ~140–630 ka, respectively, documenting protracted periods of zircon crystallisation (100's of k.y.) prior to eruption. (U‐Th)/He ages constrain the eruption ages of the A1Pm and DPm tephras to 375 ± 13 and 97.1 ± 7.3 ka, respectively, and are in agreement with published estimates. This study demonstrates the potential of combined zircon U‐Pb LA‐ICP‐MS and (U‐Th)/He dating to constrain magmatic and eruption events in the critical ~100 ka–1 Ma interval.

4D volcanic geology of Hachijo-jima islet, Izu-Bonin arc

“…Note that U-Pb dating combined with (U-Th)/He dating suggested protracted periods of zircon crystallization (100's of k.y.) prior to eruption for the Quaternary Omachi tephra, central Japan (Ito and Danišík 2020), and a weighted mean U-Pb age of 0.17 ± 0.05 Ma obtained for the Kikai-Tozurahara tephra in Yaku-shima island (Ito et al 2017b) was ~70 kyr older than its estimated eruption age of 0.095 Ma (Table 1; Danhara et al 2010).…”

“…U-Pb ages were calculated following methodology of Sakata (2018). See details of the method in Ito (2014) and Ito and Danišík (2020).…”

“…Danhara (1995) noted that for the Kikai-Tozurahara tephra, the total numbers of zircons with ssion tracks were only 14 of 144 zircons, with 130 zircons lacking ssion tracks. Ito (2014), Ito et al (2017a, b), and Ito and Danišík (2020) attempted to estimate more reliable U-Pb ages of the Quaternary (but younger than 1 Ma) regional tephras in Japan than the previously-applied ssion track dating. Whereas Sugihara (1998) misinterpreted the glassy tuffs as the Aira-Tn tephra (0.0275 Ma), he presented many radiocarbon ages for lignite fragments contained in the Mihara-yama strata (a dated lignite sample may be from that near HAC-UP in Fig.…”

“…Our zircon analyses obtained a weighted mean U-Pb age of the lower glassy tuff (HAC-Ata-Th) at 0.46 ± 0.12 Ma, which is older than the widely-accepted Ata-Torihama age of 0.24 Ma. Although it is di cult to obtain the eruption date precisely by the zircon U-Pb method (Ito and Danišík 2020), the obtained U-Pb age can constrain that the tephra is younger than ~0.5 Ma. Accordingly, the obtained age is consistent with the lower glassy tuff being the Ata-Torihama.…”

Magmatic history of the Oldest Toba Tuff inferred from zircon U–Pb geochronology

“…Zircon U–Pb dating was performed at the Central Research Institute of Electric Power Industry, using LA-ICP-MS (a Thermo Fisher Scientific ELEMENT XR magnetic sector-field ICP-MS coupled to a New Wave Research UP-213 Nd-YAG laser) with experimental conditions following Ito et al 35 and Ito and Danišík 36 (Supplementary Table S1 ). Zircon interior ages were obtained as follows: A 30 μm laser beam with ~ 7 J/cm 2 energy density and 10 Hz repetition rate was used to ablate the sample for 30 s following a 30 s background measurement.…”