Fission Tracks Simulated by Swift Heavy Ions at Crustal Temperatures and Pressures

The fission-track dating technique is an important tool to constrain the history of samples from small archaeological specimens to large geological formations. The method is based on analyzing particle tracks produced by spontaneous fission of 238 U nuclei and accumulated over time. In laboratory experiments, fission tracks can be simulated by performing irradiations with energetic heavy ions provided at accelerator facilities [1]. However, a complete realistic simulation of this natural process has never been achieved, because fission tracks also form below the Earth’s surface, involving elevated pressure and temperature conditions. Until now, ion-track studies investigated the effects of pressure [2] or temperature [3] separately, but did not address the direct influence of both parameters simultaneously. In this study, fission-track formation has been simulated, for the first time, under crustal conditions by exposing natural zircon, at a pressure of 7.5 kbar and a temperature of 250 °C to a relativistic heavy ion beam. The irradiation experiment was performed at the ion synchrotron (SIS) using 35-GeV Pb ions of fluence 1×10 11 ions/cm 2 . The experimental approach was similar to the one used in previous high-pressure irradiations [2], where a microscopic sample is pressurized by squeezing two opposing diamond anvils against the sample chamber drilled in a steel gasket and filled with distilled water as pressuretransmitting medium. Sample heating was provided by molybdenum wires wrapped around each diamond.