Thermal annealing of implanted ²⁵²Cf fission tracks in monazite

Abstract

A series of isochronal heating experiments were performed to constrain monazite fission track thermal annealing properties. The 252Cf fission tracks were implanted into monazite crystals from the Devonian Harcourt granodiorite (Victoria, Australia) on polished surfaces oriented parallel to (100) pinacoidal faces and perpendicular to the crystallographic c axis. Tracks were annealed over 1, 10, 100 and 1000 h schedules at temperatures between 30 and 400 ∘C. Track lengths were measured on captured digital image stacks and then converted to calculated mean lengths of equivalent confined fission tracks that progressively decreased with increasing temperature and time. Annealing is anisotropic, with tracks on surfaces perpendicular to the crystallographic c axis consistently annealing faster than those parallel to the (100) face. To investigate how the mean track lengths decreased as a function of annealing time and temperature, one parallel and two fanning models were fitted to the empirical dataset. The temperature limits of the monazite partial annealing zone (MPAZ) were defined as length reductions to 0.95 (lowest) and 0.5 (highest) for this study. Extrapolation of the laboratory experiments to geological timescales indicates that for a heating duration of 107 years, estimated temperature ranges of the MPAZ are −44 to 101 ∘C for the parallel model and −71 to 143 ∘C (both ±6–21 ∘C, 2 standard errors) for the best-fitting linear fanning model (T0=∞). If a monazite fission track closure temperature is approximated as the midpoint of the MPAZ, these results, for tracks with similar mass and energy distributions to those involved in spontaneous fission of 238U, are consistent with previously estimated closure temperatures (calculated from substantially higher energy particles) of < 50 ∘C and perhaps not much higher than ambient surface temperatures. Based on our findings we estimate that this closure temperature (Tc) for fission tracks in monazite ranges between ∼ 45 and 25 ∘C over geological timescales of 106–107 years, making this system potentially useful as an ultra-low-temperature thermochronometer.