Compositional Influences on Fission Track Annealing in Apatite and Improvements in Routine Application of AFTA®

Paul F. Green, Kerry A. Hegarty, and Ian R. Duddy

Use of Apatite Fission Track Analysis (AFTA®) to reconstruct thermal histories in sedimentary basins relies critically on detailed knowledge of fission track annealing kinetics. The influence of apatite Cl content on annealing rates has long been recognised. Initial quantitative information on annealing rates came from a monocompositional apatite with approximately ~0.45 wt% Cl. Subsequent work showed that annealing rates decrease with Cl content, so that pure-fluorapatites are totally annealed at temperatures 10 to 20°C less than those containing 0.5 wt% Cl.

We have recently completed a major investigation of compositional influences on fission track annealing, using lab oratory experiments and data from geological conditions using apatites from sedimentary sequences currently at their maximum burial depth in which AFTA parameters are dominated by the present-day temperatures. Incorporation of VR data from the same sequences has assisted in confirming the simple nature of the thermal history in each well. This study has included full investigation of compositional influences and has resulted in kinetic models for annealing which allow quantitatively for the influence of Cl content.

This work has emphasised the importance of quantitatively allowing for compositional influences on fission track annealing in apatite. In routine application of AFTA, Cl contents are now determined in every grain analysed, with data grouped into 0.1 wt% Cl divisions and interpreted using the kinetic models described above. Paleotemperatures and timing determined using this approach have improved precision, and provide a well-calibrated scale with which other paleotemperature indicators can be compared.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California