Standardless fission-track dating of the Durango apatite age standard

Abstract

Five dating strategies were used for determining the standardless fission-track age of the Durango apatite. These use the same fossil-track densities but differ in the manner in which the induced-track densities are determined. A conventional age calculation, without correction for experimental factors, gives inconsistent ages with method-related differences >15%. Correcting for these factors brings the ages in line with each other and with the reference age but leaves no room for a partial-annealing correction based on the confined-track lengths. Three further reasons suggest that a length correction is not appropriate. (1) The evidence for length-based corrections is inconclusive. (2) The plateau age of the Durango apatite is consistent with its apparent fission-track age to within 1%. (3) The calculated effective etchable length of the fossil tracks agrees within error with that of the induced tracks; both are further consistent with the measured mean length of confined induced tracks. The circumstance that the (U,Th)/He ages of the accepted and proposed apatite age standards are consistent with their reference ages leaves no margin for a lowered fission-track age resulting from partial annealing, although the case of the Durango apatite itself is inconclusive because of its exceptional crystal size. It is conjectured that the shortening of the fossil tacks in the Durango apatite is due to a lowering of the track etch rate over time. In this case, annealing equations fitted to induced-track data underestimate the extent of confined-track-length reduction in geological samples.