Ling Chung, Samuel C Boone, Andrew Gleadow, Malcolm McMillan and Barry Kohn
Abstract:
Training for fission tack analysis is normally a very slow and labour-intensive process. The development of the digital fission-track training module aims to deliver a guided training routine to the analyst’s computer, equipping researchers with equivalent level of confidence and skill to produce reliable and reproducible External Detector Method (EDM) and LA-ICP-MS/Digital Fission Track (LAFT) analyses using an image-based protocol. Originally, there are five stages of hand-on practice involved in comprehensive training conducted by Melbourne Thermochronology Research Group (MTRG). Chiefly utilising the MTRG-developed Fission Track Studio (FTS), a cross-platform dual software suite that is specialized for microscope control and image acquisition (TrackWorks) and image analysis (FastTracks), trainees are led to complete a series of carefully designed exercises to acquire the various skills in fission track dating. (1) Preparing grain mount for Durango (DUR) apatite provided. (2) Practicing grain selection and track-counting (track identification and region of interest, ROI, determination) by analysing seven sets of pre-imaged mica, DUR and Fish Canyon Tuff (FCT) apatites using FastTracks. (3) Challenging ‘unknowns’ by using the full capacity of FTS, i.e. capturing images of more than 25 selected grains each from the DUR mount and 20 well-characterized apatite reference mounts from the MTRG collection using TrackWorks and performing semi-automated analysis on grain and length images using FastTracks. (4) Acquiring single grain/spot uranium concentration using LA-ICP-MS. (5) Practicing age calculation. Reproducibility of data obtained from exercise (2) to (5), including single-grain track density, 3D confined track length measurements and average Dpar values, are constantly reviewed throughout the course.
Experience learned from this laboratory-based routine is a powerful tool for efficiently and comfortably examining data quality and evaluating source(s) of analytical discrepancy between analysts. The digital training module is a streamlined version of the laboratory-based practice. It allows trainees remote access to exercises for steps (2) to (5) above through analysing a series of selected grain and length image sets using FastTracks. Steps (3) and (4) require specialized laboratory equipment but can be practised by providing pre-captured images of 6 ‘unknown’ samples with a mixture of many suitable and unsuitable grains from which an appropriate selection of grains for analysis can be made. Similarly, provision of LA-ICP-MS data for the grains enables calculation of fission track ages from the analysed grains. In addition, a sub-module allows EDM users calculating their own user-specific zeta-calibration through analysis of images of apatite-mica sample pairs and co-irradiated external detectors from standard glasses. Together with proposed analytical solutions and a list of recommended reading list, trainees are able to evaluate their progress by comparing their data with solution files on a grain-by-grain and track-by-track basis. Images of two samples used in previous inter-laboratory studies (Ketcham et al., 2015 & Ketcham et al., 2018) are part of the exercise for data quality evaluation. As the training package is cloud-stored, researchers, who do not have access to suitable facility to conduct full suite of analysis or cannot physically travel for training due to various restrictions, can become trained without face-to-face tutelage or specialised equipment. In collaboration with two international laboratories, the module is being tested on both experienced conventional fission track analysts and untrained students and augmented for improved content and usability.
Development of the digital training module enables a new coordinated digital fission track analysis stream, whereby researchers can outsource sample preparation and image capture to laboratories equipped with suitable equipment. Captured image stacks and parent isotope concentrations, in the case of the LA-ICP-MS technique, would then be returned electronically to the newly trained researcher for digital fission track analysis and interpretation. This advance will enhance the accessibility and affordability of this powerful technique and make digital fission track analysis achievable for geoscientists globally.
References
Ketcham, R. A., Carter, A., & Hurford, A. J. (2015). Inter‐laboratory comparison of fission track confined length and etch figure measurements in apatite. American Mineralogist, 100 (7), 1452– 1468. [ Ссылка ]
Ketcham, R. A., van der Beek, P., Barbarand, J., Bernet, M., & Gautheron, C. (2018). Reproducibility of thermal history reconstruction from apatite fission‐track and (U‐Th)/He data. Geochemistry, Geophysics, Geosystems, 19, 2411– 2436. [ Ссылка ]
