On step-etching and ambient temperature annealing of apatite fission tracks

Apatite fission track dating and thermal history inverse modeling is a useful method to reveal and understand the thermal evolution of rocks over geological time scales at temperatures typically ≲ 110 °C. Spontaneous decay of 238U creates fossil tracks in uranium-bearing minerals through the time. The fossil tracks reflect the temperatures they subsequently experience as shortening (called annealing from here on). Tracks must be first etched using a given duration, temperature and etchant molarity, to allow them to be observed under optical microscopes. Through understanding the annealing kinetics and etching characteristics of fossil tracks, the shortening of lengths can be quantitatively interpreted using inverse models. However, since any fossil track may have experienced various temperatures through geological times, the unknown rate of annealing must be benchmarked to a known unannealed proxy. Induced tracks are generated by thermal neutron irradiations of 235U in nuclear reactors. From the moment of registration in the crystal lattice until etching, the induced tracks are exposed only to ambient temperatures, which are generally accepted as unannealed tracks. The annealing and length reduction rates of the induced tracks acquired at laboratory annealing temperature and time scales are extrapolated to geological time scales to estimate the degree of length reduction for a measured mean track length of an unannealed induced track data set that is etched under certain conditions. The extrapolations lie on the assumption that annealing through laboratory and geological time scales and etching properties of induced and fossil tracks are identical. Approaching with a scientific judgement, I have been focusing on the reasoning of conventional experimental procedures and fundamental assumptions. In the three paragraphs below corresponding to three chapters of this dissertation, I explain how the core ideas were produced, why the experiments were carried out, how they were designed and their major results and conclusions. The mean unannealed induced track lengths carry critical importance for fission track thermochronology, as they are proxies that determine the annealing rates of all shortened tracks. Experiencing room temperatures (~21 °C) results in shortening of induced tracks, as reported in previous studies. Track length measurements on induced tracks that are irradiated over ~2 to ~44 years ago, and have been experiencing room temperatures since then, result in gradual decrease (up to ~0.22 µm) of mean track lengths with increasing time over long time scales in three different apatite species. Recent inter-lab calibration studies show that different labs report different track length results from the same samples, even when they use the same etching protocols. We divided our samples into two aliquots and etched them with two most common standard etching protocols (5.0 M HNO3, 20s, 21 °C and 5.5 M HNO3, 20s, 21 °C). Eliminating all other possible experimental and instrumental differences, two analysts measured the exact same track lengths on captured images from both sets of samples with two different etching procedures. The results show that the analysts have less agreement on the tracks length measurements that etched with 5.0 M HNO3 due to their ambiguous track ends, indicating a level of under-etching. The analysts (Murat Tamer at University of Texas at Austin and Ling Chung at University of Melbourne) reject each other’s tracks by ~%14 from 3109 tracks. An additional 5s etching on both of samples show that the increase of 5.0M HNO3 track lengths is much higher than the tracks etched with 5.5M HNO3 protocol, supporting the contention that the former were more likely to be under-etched. Finally a new track etch model is proposed with variable track etching velocities. The finding of ambient-temperature annealing over decadal times led to the idea to improve experimental designs and redo a previous study that monitors ambient-temperature annealing at short time scales, such as etching the tracks right after thermal neutron irradiation from a scale from seconds to months. Improvements included more aliquots of samples, monitored etching procedure quality using a standard protocol, extended data range to as early as 37s through Cf irradiation prior neutron irradiation, and selection of apatite species. These new results are combined with the ambient-temperature annealing results from Chapter 1 and led to interesting discoveries. The tracks anneal at a much faster rate in the early stages of annealing (around up to few weeks) after which the rates decrease to a much slower rate. This indicates two different annealing mechanisms of fission tracks for early and late times. The degree of the annealing rate decrease varies with the apatite species. The laboratory annealing scales of induced tracks are one of the factors that determine the extrapolation quality of induced tracks to the geological time scales. The data of Chapter 1 extends the induced track annealing scales to the ambient-temperatures 21 °C in temperature and to ~44 years. Chapter 2 extended the time scales down to 37s. The new ambient-temperature data set is combined with literature high-temperature annealing data to provide new annealing equations. The extended limits (37s – 32 years in time and 21 °C in temperature) of the annealing window and the combined data set provided an improvement on the annealing equation. The availability of advanced modern fission track measurement instruments such as automatized stage connected to a microscope that is operated by an in-house software and understanding of fundamentals of fission tracks allowed me to study the mean track length increase with increasing etching time for my Master’s thesis at Technische Univesitaet and Bergakademie Freiberg, much of which was later published. The thesis showed that the length increase is a continuous progress along the track length and does not stop at 20s for both fossil and induced tracks, if the length measurements are carried out on the exact same track lengths in each etch step by recording their coordination by a motorized stage operated with software. These results contradicted previous step-etch results and the concept of maximum etchable length and leaves the 20s etching duration of the standard etching protocol in suspicion. With an improved design such as earlier (10s) etching time and data evaluation criteria of the step-etch experiments employed in my master thesis, the study in Chapter 3 contributed the lacking step-etching data of annealed induced tracks to compare with the unannealed induced and fossil track data to see similarities. Through etch-anneal-etch experiments, the bulk etching velocity of tracks are established in the given apatite species and the first quantitative track etch model is established. The results indicate that for both unannealed induced and fossil tracks the current standard etching procedure provides ~1.0 µm lower lengths than the modelled maximum etchable lengths. The other interesting discovery is that if the track etching is monitored from 10s, it reduces the variability of the effective etch times of individual tracks and provides more similar populations of lengths. An earlier start of etching at 10s rather than 20s directly affects the model results as well. In conclusion, the etching duration of 20s causes two major problems by insufficient etching and containing tracks with different populations of track etch times. This leads us to suggest the necessity of a new standard etch model, which can be proposed with some series of future experiments.