FISSION-TRACK DATING USING DIGITAL IMAGE ANALYSIS AND OPTICAL REFLECTED LIGHT

Fission-track dating is a valuable stratigraphic tool, particularly in deformed subduction complexes where age-definitive fossils are often difficult to find. Fission-track dating usually involves manual counting of three-dimensional images of spontaneous tracks in zircon or apatite crystals and induced tracks in mica monitors, viewed in transmitted light at high magnification. However, we have successfully imaged fission-tracks in zircon crystals and mica monitors in reflected light, and measured track densities using digital photography and computer-assisted image analysis. In addition, fission-tracks imaged in reflected light also are exceptionally easy to count when displayed on a large screen television monitor.

Images of fission tracks in reflected light are greatly enhanced by coating samples with thin films of carbon or gold, thus highlighting individual tracks as black diamonds on a white background. Track images are enhanced by using objectives with built-in iris condensers that increase both contrast and depth of field. Image size and counter area are easily constrained by inserting a field stop in the optical train behind the objective. Track size is critical, but can be controlled by varying etch times. Images can be easily manipulated using programs such as NIH Image/J, available as freeware at http://rsbweb.nih.gov/ij/ Programs for calculating fission-track grain age parameters including BINOMFIT, ZFACTOR, ZETAMEAN and CHI2COMP are available at http:/geology.yale.edu/~brandon

Reflectance experiments on zircons and mica monitors from the Fish Canyon Tuff at 1600x are quite comparable to those obtained in transmitted light. Zeta values for four experiments in reflected light range from 390.59 ±30.2 (± 2 SE, 2 Standard Error) to 306.91 ±12.6. Average zeta in reflected light is 362.41 ± 14.8. All samples pass the Chi^2 test. Zeta values obtained by manually counting tracks in transmitted light at the same magnification on the same surfaces range from 434.39 ± 29.0 to 367.00 ± 26.6 and average 401.94 ±15.6. All samples again pass the Chi^2 test. Lower magnification lenses permit counting an entire grain surface, but increases the standard error. For example, at 640x, zeta values for three experiments in reflected light range from 481.51 ±49.8 to 357.67 ±44.3 and average 414.10 ±27.4.