Untersuchung der Spaltspuren-Ätzung in Apatit und ihre Bedeutung für die Spaltspuren-Datierung und Modellierung
Mineralogie, Petrologie und Geochemie
Zusammenfassung der Projektergebnisse
We outlined a model of fission-track etching, based on established principles of crystal growth and dissolution, that explains their composite polygonal forms and their varied appearances in different apatite faces. From experiments on confined tracks, we calculated the corresponding etch rates for a common etchant, which allows us to predict the track dimensions as well as their geometries. This model supplants an earlier one that has underlain restrictive dating practises. We implemented our model and etch rates in a computer code, which identified several hitherto unknown biases related to track etching. We reported effects due to etch time; others related to the track densities, lengths, ion irradiation, and observation factors are being further investigated. Step-etching shows that track length continues to increase with etch time at a rate greater than the apatite etch rate well past the point where common protocols assume them to be etched to their ends. The length increase of fossil tracks differs from that of induced tracks, with shorter track showing less length increase than longer ones, which permits to establish a rough order of formation. Experiments have further shown that the track counts depend on the etch time and observation conditions. Transmitted-light counts under optimal conditions underestimate the true track densities by a large margin, in agreement with estimates based on length measurements that as much as one in ten surface tracks are not counted in apatite prism faces and mica detectors. This demonstrates the importance of polishing, etching and microscopic observation for fissiontrack dating and modelling. There is no such thing as a track count or a track length, independent of the conditions under which it was obtained. In contrast to common opinion, neither the fission constant λF or neutron fluence ϕ, but the track counts and measurements, are the real concerns of the fission-track method. The standard-based Z- and ζ-methods, introduced to resolve the socalled ϕλF-problem, in fact mitigate the systematic errors related to the track counts at a cost of precision. Our experiments and simulations reveal that track length measurements are also subject to etching and observation effects, with consequences for the thermal histories that are hard to estimate. Our model and measured etch rates make it possible to determine the effective etch times of individual confined tracks. This sheds light on known (orientation) and unknown (dip angle, …) biases. Modelling tracks within a suitable etch time window will better constrain thermal histories. This approach is being tested on samples from the Kontinentale Tiefbohrung and the Naab mountains. Our work overturns a fundamental assumption that shapes the current practice of fission track dating. The prism face is not a privileged orientation for track counts or length measurements, and faster etching faces are not useless. Their c-axis angles can be inferred from the azimuths and sizes of the track openings, and the surface etch rates from their size distributions. These estimates are imprecise at this stage but they will improve with a better understanding of track etching in apatite. Measuring confined tracks in non-prism faces with known angles to the caxis affords more data and a more even distribution of c-axis angles. Counting non-prism faces multiplies the grains available for dating, with obvious practical and statistical advantages. In provenance studies it could improve the resolution and increase the number of identifiable age components. Understanding etching and observation effects allows deconstructing the elusive Z- and ζ-factors into their physical (λF, ϕ, …) and experimental (G, Q, R) components. Our results show that, with appropriate accounting for these effects, the standardless and standard-based ages are consistent. This is however limited to the dated samples, and it is not certain that it extends to samples with complex thermal histories. The earlier etch model implied that the counting efficiencies depended on the surface etch rates, and nothing else. Since it has been replaced, this assurance is no longer given. Our work shows that the efficiencies depend on the polishing, etching and observation conditions. Their probable dependence on the track length distribution must now be investigated. The apatite fission track method is used in hydrocarbon exploration. While we aim to achieve significant methodological improvements, we do not anticipate taking part in their economical exploitation.
Projektbezogene Publikationen (Auswahl)
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(2017). Single-track length measurements of step-etched fission tracks in Durango apatite: "Vorsprung durch Technik" American Mineralogist (special collection: apatite: a common mineral, uncommonly versatile) 102, 987-996
Jonckheere R., Tamer M.T., Wauschkuhn B., Wauschkuhn F., and Ratschbacher L.
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(2019). On growth and form of etched fission tracks in apatite: A kinetic approach. American Mineralogist 104, 569-579
Jonckheere R., Wauschkuhn B., and Ratschbacher L.
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(2020). Some geometrical properties of fission-track-surface intersections in apatite. American Mineralogist 105, 1355-1364
Jonckheere R., Aslanian C., Wauschkuhn B. and Ratschbacher L.
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(2021). A quantitative description of fission-track etching in apatite. American Mineralogist 106, 518-526
Aslanian C., Jonckheere R., Wauschkuhn B., and Ratschbacher L.
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Fission-track etching in apatite: A model and some implications. American Mineralogist 107
Jonckheere R., Aslanian C., Wauschkuhn B. and Ratschbacher L.
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Short communication concerning experimental factors affecting fission-track counts in apatite. Geochronology 4, 1-11
Aslanian C., Jonckheere R., Wauschkuhn B., and Ratschbacher L.
