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

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 (102–103 km3). 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.