Considerations for double dating zircon in secular disequilibrium with protracted crystallisation histories

Ruby C. Marsden, Martin Danišík, Hisatoshi Ito, Christopher L. Kirkland, Noreen J. Evans, Daisuke Miura, Bjarne Friedrichs, Axel K. Schmitt, Shimpei Uesawa, Matthew L. Daggitt

Research output: Contribution to journalArticlepeer-review

Abstract

Zircon double-dating utilises (U–Th)/He dating coupled with U–Th disequilibrium or U–Pb dating to determine eruption ages for volcanic rocks between ca. 2 ka to 1 Ma. This approach depends on understanding the crystallisation history of each zircon crystal analysed. For lack of better constraints, zircon crystallisation is generally assumed to be represented by a single crystallisation age, which is routinely determined on the rim of the grain by spot analyses. While zircon crystallisation is often protracted, interrogating the crystallisation history of a zircon crystal usually requires grinding the grain, which can introduce uncertainty to the alpha ejection (Ft) correction, critical for accurate (U–Th)/He ages. Grinding a zircon crystal to exactly 50% of its original width, to a plane of symmetry, leaves the Ft correction factor unchanged relative to that of the whole crystal. This is verified by a new computer program – GriFt, which also allows the calculation of accurate Ft correction factors for a range of different grinding depths, opening the opportunity to measure both the core and rim crystallisation ages and integrate these into a more robust disequilibrium correction of (U–Th)/He data. The feasibility of this approach is tested here in a case study of zircon crystals with protracted crystallisation histories from the Shikotsu-Toya volcanic field in Hokkaido, Japan. A maximum of 15% difference in overall eruption age is calculated between rim- and core-corrected (U–Th)/He ages. Eruption ages were determined for two tephras – Kimobetsu 1 (59–79 ka) and Kimobetsu 2 (96 ± 5 ka, 2σ). The geological implication from these dates is that a regionally important tephra, Toya, may be younger (<96 ± 5 ka) than previously reported (109 ± 3 ka). In addition, the maximum eruption ages determined from crystallisation age distributions calculated for samples from eruptions at Shikotsu and Kuttara (48 ± 17 and 49 ± 21 ka, respectively) are within uncertainty of previous measurements (44–41 ka and >43 ka, respectively).

Original languageEnglish
Article number120408
JournalChemical Geology
Volume581
Early online date29 Jun 2021
DOIs
Publication statusE-pub ahead of print - 29 Jun 2021

Keywords

  • (U-Th)/He dating
  • Japan
  • Quaternary
  • Tephrochronology
  • Zircon double-dating

ASJC Scopus subject areas

  • Geology
  • Geochemistry and Petrology

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