The PIs have developed and tested a method whereby the mean etchable fission track (FT) lengths parallel and perpendicular to the crystallographic c-axis in apatite may be precisely determined. The lengths so determined differ for a single population of FTs (parallel tracks > perpendicular) but, because of their equivalent thermal histories, they are defined as "equivalent lengths". The PIs plane to use this new method to determine whether the physical process of FT annealing in apatite varies with crystallographic orientation or whether we are observing experimental artifacts due to anisotropic etching. Similarly, they will determine whether the FT annealing process varies with crystal chemistry (e.g. Cl-rich track lengths> F- rich) or whether this is also an etching phenomenon. The method proposed, in order to address these questions and to better calibrate this important thermochronometer, is a series of isochronal laboratory annealing experiments (ranging from O.3- 1OOOO hours and 23-400oC). Specifically, they will investigate: 1) the correlation of observed FT length anisotropy with etching anisotropy in apatite, 2) the variation of mean etchable FT length with time, temperature and crystallographic orientation for a range of apatite compositions ;(F-, Cl-, OH-, and Mn-rich) at low temperature (23oC) and elevated temperatures, 3) how activation energy for FT annealing varies a) with mean FT length at fixed orientation, b) between equivalent lengths, and c) with host crystal chemistry, 4) the relationship of the above results with currently published results (Galbraith and Laslett, 1988) relating the dependence of FT density (tracks/unit area) to the mean etchable FT length. These new data will then be used to derive a mathematical expression relating the "equivalent length" to the time- temperature (t-T) history of the apatite. This new expression should result in a significant improvement in estimates of t-T paths experienced by geologic samples.
