Geological partial annealing zone of zircon fission-track system: additional constrains from the deep drilling MITI-Nishikubiki and MITI-Mishima

Fission track (FT) analysis of zircon from rhyolite and sandstone samples of Ministry of International Trade and Industry (MITI)-Nishikubiki and MITI-Mishima boreholes has given additional constraints on the temperature range of the zircon partial annealing zone (ZPAZ) over a geological time scale. The advantages of these samples are the following: (1) they have been exposed to a stable temperature for ∼1 million years, and (2) it is possible to obtain data at successive temperatures. Ages and confined track lengths were measured in samples from depths of ∼3.5–5.6 km with a current environmental temperature of ∼223 °C at the deepest. Track lengths in samples below 200 °C are still long and FT ages are compatible with their sedimentary ages, showing no evidence of annealing for a heating duration of 1 million years. Samples at 205 °C seem to yield shortened tracks as a result of annealing. Because of a low number of zircon crystals, only four track lengths were measured for the deepest sample whose mean length was shortest among analysed samples. These data are consistent with previous results from ultra-deep boreholes.