Sample assessment and interpretation of zircon ages measured by fission track method

Abstract

How to deal with and evaluate fission track data of zircons from the point of view of users is presented. Analytical diagrams, including a grain-age histogram, a correlation between spontaneous and induced track counts, a correlation between spontaneous and induced track densities, an age spectra, a radial plot, and a uranium-concentration histogram, are explained to read grain data. Grain-age distributions were simulated for a sample with an identical age but various conditions of track counts per grain. Data from ca 2,500 samples previously determined were presented and categorized for assessment of unknown samples. These are important as a guide to evaluate fission track data with relatively large uncertainties. Fission track dating of zircon is performed as follows: (1) assessment of a sample; (2) dating and analysis of grain data; and (3) interpretation of ages. For sample assessment, it is important that grouping of zircons in the sample is based on their appearances, such as color, morphological features, and fission track densities on external surfaces. For interpretation of fission track ages, thermal history analysis with track length distributions is useful. Crosscheck of fission track ages between external and internal surfaces of zircon is also important. For examples, three samples from deep boreholes of the Niigata sedimentary basin are investigated, and the meaning of zircon ages is discussed. In Japanese