Bal et al. (1983)
reported a reappraisal of the thermal stability of fission tracks in zircon and sphene, which they believe indicates a higher fission-track closure temperature for zircon than for sphene. We feel compelled to comment on this work because it is replete with errors in both experimental design and procedure. We feel that these errors have led Bal et al. to calculate closure temperature estimates which are geologically meaningless.
Bal et al. reference only three works in a review of published dating studies in which fission track ages of coexisting sphene and zircon are compared. They state that this review shows that sometimes sphene and sometimes zircon retains fission tracks at a higher temperature (their Table 3). Drawing support from a review of published laboratory annealing studies, they attribute this to "differences in the etching conditions used to etch tracks for age determination and/or annealing conditions".
Three comments are appropriate here. First, Bal et al. give no consideration to the role of chemical composition on annealing properties of either of the minerals in question. This is obviously a possible variable between the different studies referenced and has previously been shown to be a significant source of variation in the annealing properties of sphene (Gleadow 1978). Thus it cannot be considered proven that any differences in apparent thermal stability are introduced solely by the different etchants used. The most direct procedure to prove the authors' suggestion would have been to perform an annealing experiment in which the same sphene or zircon sample, having been annealed under various temperature-time conditions, is etched in the different etchants in question. This would yield direct results, positive or negative. However, in our view Bal et al. merely complicated their study by introducing two completely new etchants, neither of which has been used before for age determinations. In doing so, they rendered meaningless any comparison of their results with those of previous annealing studies of sphene and zircon.
Second, the list of references given by Bal et al. comparing fission-track ages of coexisting zircon and sphene pairs is neither comprehensive nor representative. Our own experience, and that of other workers, is that sphene ages are without fail equal to or older than the ages of coexisting zircon (e.g.