SELF-INDUCED FRACTURE GENERATION IN ZIRCON

The mineral zircon has particular geological significance because of i ts use in the age determination of rocks and as a structural analogue phase for radioactive waste forms. Despite its widespread utility, how ever, radiation damage of the crystal structure (metamictization) caus es a volume expansion of the crystal lattice and the generation of int ernal stresses which can induce fractures in the crystal. A model has been developed which describes the state of stress in n concentric sph erical shells, each of which may have different material properties; t he case of a three-shelled composite sphere has been examined explicit ly. In combination with fracture mechanics theory, the model predicts that spatially distinct radial and/or concentric fracture sets can be produced, and such fracture patterns accurately describe the distribut ion and types of self-induced fractures observed in natural zircons. I n general, the formation of such microfractures in zircon is a functio n of the degree of metamictization, shell thickness, and the confining pressure. The model predicts that the maximum depths in the Earth's c rust at which radial versus concentric fractures can form in a crystal are different, suggesting that the types of fractures found in a suit e of zircons from a particular rock might be potential indicators of t he depth of burial. Because metamictization-induced fractures may serv e as potential pathways for the rapid leaching of various elements fro m the zircon crystal, this may also have important implications in int erpreting the U/Pb ages of fractured zircons or in evaluating the suit ability of related crystalline phases as hosts for nuclear waste.