Influence of etching conditions on apatite fission-track etch pit diameter

Abstract

The temperature range of the apatite fission-track partial annealing zone (PAZ) varies as a function of the kinetic characteristics of the apatite crystal as well as the cooling rate. These kinetic characteristics are controlled largely by the chemical composition of the apatites; fission-track etch pit diameter (Dpar) has been shown to be a proxy for quantifying these characteristics. Since some annealing models explicitly use Dpar as a kinetic indicator, the question of whether Dpar varies according to either operator variability or etching conditions has serious implications. The influence of etching conditions includes both minor variations in temperature and etch time as well as larger variations between the three popular concentrations of nitric acid that are presently in use within the fission-track community: 5 and 5.5 M HNO3 at 21 °C for 20 s, and 1.6 M (7 vol.%) HNO3 at 21 °C for 45 or 50 s. We have conducted a systematic study of Dpar in two widely used apatite age standards: Durango and Fish Canyon Tuff (FCT). Samples were analyzed by at least 2 operators in 2 laboratories; over 15,000 etch pits were measured. Operator imprecision is small compared to other effects both within and between operators. The measured Dpar parameters depend primarily on etching conditions including concentration of etchant, and the time and temperature of etching. Dpar size increases linearly with time but non-linearly with temperature. Differences between Dpar size of Durango and FCT are significantly smaller for the 1.6 M etch compared to the other two etches, implying that the weak etch has less resolving power for Dpar measurements. In addition, when etching conditions are changed, confined track lengths for different apatite compositions and Dpar values co-vary in a complex fashion. Therefore, the 5 or 5.5 M etch is recommended for studies that employ Dpar as a proxy for chemistry; the 1.6 M etch and other weak etches are not recommended. We propose a linear correction for Dpar based on measuring Dpar in two widely available AFT age standards, Durango and FCT, and cross-plotting the results with those obtained by R. Donelick. The slope of the resulting curve, which passes through the origin, provides a correction factor. This system yields reasonably good corrected values for 3 test samples using the 5 M and 5.5 M HNO3 etches.