Abstract:

Apatite Fission-Track Analysis(AFTA)is emerging as an important new tool for ther-mal history analysis in sedimentary basins.At temperatures between approximately 20℃ and 150℃ over times of the order of 1 to 100 million years,fission tracks in apatite are an-nealed.This is due to a rearrangement of the damage present in unetched tracks,with theresult that less of a track is etchable than in fresh,newly created tracks.Because of this,thelength of an etched fission track reduces with increasing annealing,and in turn,the track den-sity(and hence the fission-track age) is also decreased.In selected boreholes in the Otway Basin,southeastern Australia,apatites from the Otway Group show reduction in confinedfission-track length and apparent fission-track age,in a fashion characteristic of a simple ther-mal history in which samples are at or near their maximum temperatures at the present day.Track lengths show a steady decrease from lengths of approximately 15μm in outcrop ornear surface samples,to zero at about 125℃.Fission-track ages however show little or nodecrease in age until tempera tures exceed about 70℃.Above this temperature,ages rapidlyreduce to zero at about 125℃.Fission-track data from the Otway Basin contain more information than the simplede crease of age and length.The distributions of single grain ages show characteristic pat-terns,particularly above 90℃.The distribution of track lengths is also diagnostic of tempera-ture.In particular,in samples at present temperatures between 102℃ and 110℃,the distribu-tion of lengths is almost flat,with tracks of all lengths from approximately 1μm to 16μm.The temperature-sensitive,fission-track parameters observed in the Otway Basin may beapplied in other basins to elucidate paleo temperature details.In cases of mixed provenance,individual grain ages may be identified using the external detector method.Fission-tracklengths in apatites containing a significant track record at the time of deposition are generally characterised by one of two types of distributions,greatly simplifying inter pretation of distri-butions of track lengths in samples showing significant down-hole annealing.Presence of an inherited track component,or conversely of a total loss of tracks at some time since deposition,can be identified by a comparison of the stratigraphic age with thelength-corrected fission-track age.Investigation of five fission-track parameters then allowssemi-quantitative constraints to be placed on thermal history.Experiments are in progress toplace this procedure on a more rigorous,quantitative basis.AFTA offers numerous advantages over the other thermal history analysis techniques including the ability to provide a chronology of events.The method is now established in hydrocarbon exploration as a quantitative maturation indicator,and should find common application.

Key words: Apatite fission track, Paleotemperture indicator, Hydrocarbon exploration