ASSESSMENT OF OVERETCHING FACTOR FOR CONFINED FISSION-TRACK LENGTH MEASUREMENT IN ZIRCON

The effect of prolonged etching provides a fundamental bias in observed confined track lengths in minerals, particularly those having highly anisotropic etching characteristics, such as zircon. In this paper, the anisotropic prolonged-etching factor around the ends of horizontal confined tracks (HCT's), which are practically used for track length analysis, was assessed quantitatively by step-etch of spontaneous tracks in zircon. The observed etching characteristics are two-fold: (1) the track etch rate for the direction perpendicular to the crystallographic c-axis is significantly smaller than that parallel to it; and (2) for a particular crystallographic direction, the growth rate of track length is consistently greater than the bulk etch rate, measured as the growth rate of track width for the direction perpendicular to that of interest. To further constrain the etching characteristics of HCT's, the shape of their ends were compared and contrasted with that of track etch pits on a prismatic surface. In contrast to the diamond shape of surface tracks, the ends of HCT's are generally convex outward with the core of etched track longest, regardless of track orientation. This suggests that at the ends of HCT's, the core of track has a growth rate greater than the surrounding bulk region, probably with decreasing rate outward. Hence the traditional model of track etching, dividing a latent track into the ''etchable'' and ''unetchable'' ranges, should be refined: etch rate of a latent track decreases rather continuously loward both ends. This model is concordant with the ionization pattern in a mineral detector produced by nuclear fission, and explains reasonably the experimental results. Because the correction of such a complicated overetching factor for an individual sample is extremely laborious for routine analysis, it is recommended that HCT's nearly perpendicular to the crystallographic c-axis (e.g., greater-than-or-equal-to 70-degrees) should be used for track length measurement. Nevertheless, the reduction in number of tracks to be measured may be rather small since HCT's with smaller angles to the c-axis are less detectable due to the etching anisotropy.