The alpha-recoil-track dating method: investigation of its basic principles and further development
Final Report Abstract
Our aim is to develop a geological dating method based on counting etched recoil tracks in mica. Recoil tracks are damage clusters created by the recoiling daughter atoms, following α-disintegration of uranium-, actinium- and thorium-series isotopes. The basic principle is the same as that of other dating methods: the sample age is calculated by dividing the number of tracks produced over time by their production rate (time = number of tracks / tracks per time unit). The track production rate is calculated from the uranium and thorium concentrations of the mica. The number of tracks is found by etching the mica surface in hydrofluoric acid and counting the etch pits with a high-contrast (Nomarski) optical microscope. A specific problem of the recoil track method lies in the fact that the tracks are small compared to the etch rate of the surface. The consequence is that the recoil track density increases with etch time, because track below the surface become exposed and etched. In the current model, the increase is linear and proportional to the surface etch rate and the number of tracks per volume. Our main experiments are designed to calculate the number of tracks from the measured slope of a line fitted to the track counts at consecutive etch times and the surface etch rate. This revealed a number of complications. First, our results indicate that the surface etch rate of mica is not constant, but increases from a low to a constant higher value. This observation is borne out by simple computer simulations of the etch process. Second, samples of the same mica, but with different track densities, etch at a different rate. Because the etch rate increases with the track density, this is thought to result from radiation damage, which creates surface defects where the etchant attacks the surface preferentially. These are likely point defects (lattice vacancies and interstitial atoms), produced by bombardment by the alpha particles emitted during the α-disintegration process that also produces the recoil tracks. Between six and eight alpha particles are produced for each recoil track, and each of them is capable of knocking about one hundred mica atoms out of their regular positions. Our etching simulations support the idea that even small (point) defects can enhance the surface etch rate. Third, some results suggest that there is a delay (etch induction time) before recoil tracks begin to etch. The simplest of several possible causes could be that tracks that intersect the unetched surface, from being truncated and exposed to the external environment, do not present the same disordered structure as those confined within the volume of the mica. In a second comprehensive experiment, we investigated the effect of elevated temperatures on the recoil track densities and on the surface etch rate. Based on similar experiments on damage tracks created by uranium fission (fission tracks), we expected to observe a gradual repair of the tracks over a certain temperature interval, and a corresponding gradual lowering of the recoil track densities. Despite difficulties in interpreting the results, this assumption appears to be contradicted by the data. This suggests a fundamental difference between fission tracks and recoil tracks. Fission tracks are elongate damaged regions, a thousand times longer than wide, and are repaired (annealed) from their endpoints towards the center. This shortens their etchable lengths so that the fraction that intersects the surface, and is etched, gradually decreases in proportion with their length. Recoil tracks, in contrast, are more or less globular damage clusters with no specific length. In this case, at a temperature sufficient for damage repair, recoil tracks anneal abruptly. The experiment also showed that the etch rate of the surface is not affected by temperature. We tracked the size (diameter) of each individual recoil track etch pit over several consecutive etch steps. Our expectation, based on earlier work on phlogopite mica and on computer simulations, was that, in contrast to the current mainstream ideas, that the tracks grow to a maximum size, and then possibly shrink. Although our experiment failed to confirm this, it does not disprove our model in our opinion. Rather, due to the low etch rates of muscovite mica, the slowing down of etch pit growth is suspected to occur at longer etch times than we were able to achieve. Before then, the growth of the saucer shaped etch pits causes their walls to become less steep, causing them to grow fainter to the point of invisibility in Nomarski differential interference contrast images. Our attempt to data the mica did not succeed due to problems determining its low uranium and thorium concentrations. We nevertheless believe, that this first attempt to date muscovite with the recoil track method, in spite of its problems, established important facts and set practical etch time and temperature limits, which should make future investigations more efficient, possible successful.
Publications
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2017. An investigation of the α-recoil-track dating method. Abstract volume of the Goldschmidt Conference, Paris, France, 13–18 august 2017
Wauschkuhn B., Jonckheere R., Ratschbacher L.
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2018. Alpha-recoil-track dating of low uranium muscovite. Abstract volume of the 16th International Conference on Thermochronology, Quedlinburg, Germany, 16–21 september 2019
Wauschkuhn B., Jonckheere R., Ratschbacher L.
