Ion microprobe U–Pb dating of Quaternary zircon: implication for magma cooling and residence time

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An ion microprobe U–Pb dating method of Quaternary zircon is developed using the sensitive high resolution ion microprobe (SHRIMP) and applied to igneous rocks from Japan. Total Pb/U isochron age of 1.36±0.23 Ma (95% confidence limit (CL); MSWD=1.03) was obtained for zircons extracted from Takidani granodiorite in central Japan. This may provide a constraint on magma cooling when coupled with fission-track ages of the zircon as well as Rb–Sr and K–Ar ages on co-existing minerals. The closure temperature determined by each method has lead to exponentially declining cooling history. The history is consistent with a simple conductive cooling model of a small magma body (10 2–10 3 km 3). A Pb/U isochron age of 271±58 ka (95% CL; MSWD=0.85) has been determined for volcanic zircons from Torihama dacite, SW Japan. The age is significantly older than the 238U– 230Th (Ionium) age of 145±33 ka and fission-track age of 163±8 ka found in the literature, suggesting either the magma residence time of ∼100 kyr or a rapid cooling rate of about 6°C/kyr. In contrast a tephrochronological estimate of 240±10 ka agrees well with the observed Pb/U age, implying that the Ionium and fission-track ages are incorrect. The Pb/U isochron age of 90±29 ka (95% CL; MSWD=1.04) is obtained for zircons in pyroclastic rocks from the Unzen eruption in 1991, SW Japan. The age is again older than the eruptive age, which suggests a magma residence time of ∼90 kyr in a shallow reservoir.