Considerations for double-dating zircon in secular disequilibrium with protracted crystallisation histories
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, z...
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| Main Authors: | , , , , , , , , , |
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| Format: | Article (Journal) |
| Language: | English |
| Published: |
29 June 2021
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| In: |
Chemical geology
Year: 2021, Volume: 581, Pages: 1-13 |
| ISSN: | 1872-6836 |
| DOI: | 10.1016/j.chemgeo.2021.120408 |
| Online Access: | Verlag, lizenzpflichtig, Volltext: https://doi.org/10.1016/j.chemgeo.2021.120408 Verlag, lizenzpflichtig, Volltext: https://www.sciencedirect.com/science/article/pii/S000925412100351X 
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| Author Notes: | 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 |
| Summary: | 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). |
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| Item Description: | Gesehen am 01.12.2021 |
| Physical Description: | Online Resource |
| ISSN: | 1872-6836 |
| DOI: | 10.1016/j.chemgeo.2021.120408 |