Determination of Paleoheat Flux from Fission Scar Tracks in Apatite

Ian Lerche, Zhiyong He

Three methods exist for determining paleoheat flux from fission scar tracks in detrital apatite collected from wells. The annealing of the fission scar tracks is modeled using time-temperature integrals and assuming first order kinetics in agreement with the laboratory experiments over the last fifteen years.

The three methods invert the present fission-track well data along the burial paths of the sedimentary units.

The first two methods involve inverting the depth dependent: (a) areal track number density per gram of uranium and (b) mean track length to obtain an estimate of the temporal dependence of the paleoheat flux. Since these methods tend to overlook short tracks
[method (a)], or to be biased to longer tracks [method (b)], they tend to provide paleoheat flux variations that are too high [method (a)] and too low [method (b)], respectively. The uncertainty in determining paleoheat flux variation is also quite high, as shown by using data from wells Kambara 1 and Curringa 1.

The third method inverts all of the individual distributions of track lengths at each sampled depth in a well. The sensitivity of this method is much higher than methods (a) or (b). In fact, we are able to determine not only the rate of change of heat flux with time but, for the first time, variations in the direction of change of heat flux, allowing us to quantify heating and cooling events as opposed to just heating or cooling events. In addition, we are able to determine the rate constant for track annealing and, in line with qualitative arguments advanced by Gleadow, Duddy, and Lovering in 1983, we find that the rate constant appropriate to the multi-million year time scale of subsurface apatite track annealing is lower by about a factor three than the value advocated from short-l ved (on a geologic time scale) laboratory experiments. Detailed track data from Kambara 1 and Curringa 1 wells yield essentially identical values for the rate constant but different thermal history behaviors.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.