L'apatite de Durango (Mexique) : analyse d'un minéral standard pour la datation par traces de fission

Durango apatite is analyzed by four analysts using three, different dating techniques. Both the annealing technique and the external detector technique are applied to basal and prismatic sections cut from single crystals. The population technique is applied to mounts of randomly oriented grains. Provided an individual determination of the geometry factor is performed, all ages are indistinguishable from the reference age of 31.4+/-0.5 Ma. There is no noticeable influence of either the dating technique or the crystallographic orientation of the dated section on the FT age. Based on the projected lengths of surface tracks, spontaneous track length reduction varies from approximately 4% to approximately 10% (mean approximately 6%), depending on the analyst. Two simple corrections are applied to eliminate the net effect of the missing short tracks on the mean projected length. After correction the amount of shortening of the spontaneous tracks is found to be slightly larger (mean approximately 8%) but is still dependent on the analyst (approximately 4% to approximately 12%). The mean is in fairly good agreement with the value obtained from confined track length measurements (approximately 9%). Finally, an isochronal (1 h) plateau annealing procedure was applied to a separate set of basal and prismatic sections cut from the same single crystals. Annealing temperatures up to 300-degrees-C do not affect the FT age. Erratic ages at 350-degrees-C are ascribed to the occurrence of unetchable gaps. It is argued that the absence of a distinct plateau up to temperatures where it could be expected from theoretical considerations indicates that the FT age of Durango apatite needs no correction.

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