The relationship between track shortening and fission track age reduction in apatite: combined influences of inherent instability, annealing anisotropy, length bias and system calibration

Abstract

The relationship between reduced track density, ϱ/ϱ 0 , and reduced mean confined track length, L/L 0 , during annealing has been studied for both spontaneous and induced tracks in four apatite samples with different chemical composition, compositional range, and origin, and with spontaneous tracks giving mean confined track lengths varying from ∼ 14.5 μm down to 12.3 μm. In all four apatites, induced tracks show a relationship between L/L 0 and ϱ/ϱ 0 composed of two regions; at higher values the two appear to be equal, while at lower values L/L 0 is greater than ϱ/ϱ 0 . This can be understood qualitatively in terms of the anisotropy of annealing and the bias against revelation of short tracks. Data from three apatites of uniform composition show near-identical relationships. In a sample showing a range of apatite compositions, the point of divergence from the 1:1 relationship is somewhat higher than in the uniform composition apatites, because individual grains anneal at different rates, and the presence of highly annealed grains affects the track density more strongly than the mean track length. Spontaneous tracks show similar relationships to those observed for induced tracks in each sample, when the data are renormalised to allow for the natural annealing present in the spontaneous tracks. Previous studies which led to the idea of length reduction without density reduction for spontaneous tracks did not renormalise density data as well as length data, thereby distorting the apparent relationship. Data for the multi-compositional apatite sample where annealing has taken place in geological conditions also agrees with the laboratory annealing data for both spontaneous and induced tracks. This consistency suggests that a single process is responsible in all three cases, implying that a laboratory-based description of annealing can be applied to geological conditions with confidence. Recognition that spontaneous tracks in apatite are commonly annealed suggests that, in general, the fission track age should be interpreted as a reflection of both the time over which tracks have been retained and the distribution of etchable track lengths. Since the latter is a manifestation of the thermal history, the fission track age should be interpreted in similar terms.