ESR Dating of Quartz Veins : Cooling History of a Hydrothermal System

Abstract

The only method to date directly quartz crystals and veins is counting unpaired electrons in samples created by natural radiation and accumulated at some traps in proportion to their age, such as electron spin resonance (ESR) and thermo luminescence (TL) methods. In this study, the ESR dating method was employed on quartz veins which are encoded at the floor of Minase River, Akita Prefecture, and Northeast Japan. 40 specimens of quartz veins were tested. ESR signals from Al impurity center were detected in all specimens, and no other signal was observed. 5 specimens showed too much paramagnetic impurity signals to calculate Al center ages, and 35 ESR ages were yielded; 6.4-24 ka for 34 specimens, and 0.47 ka for one. Mo systematic change in the ESR ages due to the sampling localities was observes. The ages 7.9 ka and 18 ka were calculated for 2 samples from the same vein. Slices from one vein of 20 cm thick yielded ages of 14 ka, 13 ka, 24 ka, 9.1 ka, 12 ka, and 8.5 ka, in which no systematic change was observed. Therefore, the difference between these ages has no effect, and thus the ages 6.4-24 ka for 34 specimens are roughly correlative. The field evidences suggest that the .47 ka quartz has been annealed by the younger hydrothermal activity. In conclusion, two phases of hydrothermal activity are found in this field; the main phase occurred in 12+ ka, and minor activity occurred after it.