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Paper Number: 156
Ito, H. 1
1
Central Research Institute of Electric Power Industry, Abiko, Japan.
E-mail: ito_hisa@criepi.denken.or.jp
___________________________________________________________________________
Zircon U-Pb dating is now widely used to unravel Earth’s history. It
can accommodate a timeline from Hadean to Quaternary. Dating Quaternary
tephras is invaluable because it includes dating on the historic
timescale. Here LA-ICP-MS U-Pb dating was performed on Quaternary
zircons from tephras produced during caldera-forming gigantic eruptions
in Japan. First, the Toya Tephra, which created a ~13-km-wide caldera at
~0.1 Ma in northern Japan, was dated [1]. The obtained U-Pb ages were
tightly clustered at ~0.1 Ma, suggesting that the caldera-forming
eruption occurred only once at ~0.1 Ma. On the contrary, dating the
Kikai-Tozurahara Tephra (K-Tz), which was sampled at Yakushima Island,
southern Japan, tells a different story. The K-Tz, which is assumed to
have erupted at ~0.1 Ma from the Kikai Caldera, contained many older
(mostly ~1.0 Ma and 0.6 Ma) zircons in addition to ~0.1 Ma zircons
(Figure 1). The ~1.0 Ma and 0.6 Ma zircons were assumed to be identical
to two other tephras (Anbo and Ksd) distributed on Yakushima Island, ~25
km south of the Kikai Caldera. If this is the case, it is plausible that
the K-Tz and the other two tephras were all derived from the Kikai
Caldera and the caldera should have experienced at least three (~1.0 Ma,
0.6 Ma, and 0.1 Ma) gigantic eruptions. This implication is important in
that tephras of unknown source can be identified by the zircon U-Pb
method. In conclusion, zircon U-Pb dating on Quaternary tephras will
contribute to the understanding of gigantic volcanic eruptions reaching
into the historic time scale. Thus, this research is directly relevant
to human society.
Figure 1: Zircon Th/U vs. U-Pb age diagram for the three tephras
in Yakushima Island
Reference:
[1] Ito H (2014) J Volcanol Geotherm Res 289: 210-223
