STATISTICAL-MODELS FOR MIXED FISSION-TRACK AGES

In fission track analysis it is common to find that the true ages of d ifferent crystal grains vary within a sample, and this may be importan t for geological interpretation. There are at least two well-recognize d geological processes that lead to mixed ages: grains from multiple s ources, and differential annealing between grains of differing composi tion. Data from multiple sources may be represented statistically by a finite mixture model, usually with two or three components, but data arising from the multicompositional annealing process may be better mo delled as an infinite mixture. We discuss finite mixtures and two new infinite mixture models: a random effects model whose parameters descr ibe the location and spread of the population grain ages, and a more g eneral model encompassing both two-component mixtures and random effec ts. We illustrate with case studies how to use these models to estimat e various features of interest such as the minimum age, the other comp onent ages and the age dispersion.