A JEOL JEM-2000FX analytical transmission electron microscope, equipped with a cold stage and anticontamination device, has been used to study the physical characteristics and annealing behavior of artificially induced fission tracks in fluorapatite.;Near the atomic level, unetched fission tracks are not continuous, but are comprised of segments of extended damage that are separated by gaps of undamaged microstructure. From dark-field transmission electron microscopy (TEM) images, it appears that the crystalline damage around tracks, although intensive, is not extensive. As such, the defect density may be represented by a Gaussian-type distribution function. The disordered nature of the track core and defect distribution geometry supports the Ion-Explosion Theory that has been proposed for track formation. TEM analysis reveals that track width is crystallographically controlled. Parallel to the c-axis, tracks display widths of 5 to 13 nm and hexagonal faceting on the (0001) plane. Tracks perpendicular to the c-axis display widths of 3 to 9 nm and prismatic faceting on the (1000) plane. The track cross-section facets mimic etch-pit morphologies and provide a relative measure of the crystal's surface free energy. A consequence of differential bond strengths and elastic properties in the fluorapatite structure, track-width anisotropy resolves etching- and annealing-rate anisotropy that has been reported for fission tracks in fluorapatite.;TEM observation of the behavior of fission tracks in response to electron beam exposure (i.e., radiolytic annealing), and temperature increase (i.e., thermal annealing), yields a physical and a kinetic description of the annealing process. Annealing commences with bulging at the track's tapered ends, followed by detachment of a single sphere. This process is replicated until a critical track radius is encountered at which the track geometry approaches an ideal right cylinder. A sinusoidal boundary develops at the track-matrix interface and increases in amplitude until the track spontaneously collapses into a row of spheres and small rods. The rods continue to evolve into spheres until the track remnant is comprised solely of a row of spheres. Although the spheres possess a stable surface energy geometry, ultimately they are restored to the original microstructure and the track disappears. Documentation of annealing suggests that the process is analogous to that of drop detachment, ovulation, and spheroidization. From these better known processes, it is possible to formulate a kinetic equation that describes fission-track annealing. Unlike the empirically-derived or physically-based kinetic equations that are presently employed in the reconstruction of thermo-tectonic histories from apatite fission-track data, the equation proposed in this study accurately predicts fission-track behavior over all of the scales of interest (i.e., microscopic to macroscopic dimensions, high to low temperatures, laboratory to geologic timescales). Furthermore, the equation reveals that surface interface diffusion is the primary mass transport mechanism that controls fission-track annealing.
展开▼
