Abstract We report on new image-analysis techniques that, for the first time, provide a practical solution to the problem of fully automated counting of fission tracks in natural minerals, a long-desired goal in fission-track dating. Specific challenges to be overcome have been the discrimination of fission tracks from non-track defects, polishing scratches, etc.; resolving multiple track overlaps; and reliable identification of small tracks amongst a similarly sized background of surface defects, fluid inclusions, etc. Most previous attempts at automated image analysis have failed in one or more of these tasks. The central component of our system is called ‘coincidence mapping’ and utilizes two images of the same tracks obtained in transmitted and reflected light. The complementary nature of the information in these two images allows a powerful discrimination of true fission tracks from most non-track features. The much smaller average track size in the reflected light image allows the resolution of most track overlaps apparent in transmitted light. The discrimination is achieved by segmenting the two images using a custom-developed thresholding routine and extracting the coincidence of features in the two binary images. The analysis is computationally efficient and takes only a few seconds to complete the processing of images that may contain up to many hundreds of tracks. Preliminary indications are that error rates are about the same as, or better than, those achieved by a human operator using normal counting conditions in transmitted light. The performance is even better at high track densities (>10 7 cm −2 ) giving the potential for measuring track densities up to an order of magnitude greater than a human operator can count. Automated counting should significantly increase the speed and consistency of analysis and improve data quality in fission-track dating through better counting statistics, increased objectivity and measurement of additional track description parameters that are not currently determined.

Abstract We assess the proposal of Hendriks & Redfield ( Earth and Planetary Science Letters , 236 , 443–458, 2005) that cross-over of the predicted apatite fission track (AFT)>(U–Th–Sm)/He (AHe) age relationship in the southeastern Fennoscandian shield in southern Finland reflects α-radiation-enhanced annealing (REA) of fission tracks at low temperatures and that more robust estimates of the denudation history are recorded through reproducible AHe data. New AHe results from southern Finland showing variable dispersion of single-grain ages may be biased by different factors operating within grains, which tend to give a greater weighting towards older age outliers. AHe ages from mafic rocks show the least dispersion and tend to be consistently lower than their coexisting AFT ages. In general, it is at the younger end of the single-grain variation range from such lithologies where most meaningful AHe ages can be found. AHe data from multigrain aliquots are, therefore, of limited value for evaluating thermal histories in southern Finland, especially when compared against coexisting AFT data as supporting evidence for REA. New, large datasets from the southern Canadian and Western Australian shields show the relationship between AFT age, single-grain age or mean track length as a function of U content (determined by the external detector method). These do not display the moderately strong inverse correlations previously reported from southern Finland in support of REA. Rather, the trends are inconsistent and generally exhibit weak positive or negative correlations. This is also the case for plots from both shields, as well as those from southern Finland, where AFT parameters are plotted against effective U concentration [eU] [based on U and Th content determined by inductively coupled plasma-mass spectroscopy (ICP-MS)], which weights decay of the parents more accurately in terms of their α-productivity. Further, samples from southern Finland yield values of chi-square χ 2 >5%, indicating that there is no significant effect of the range of uranium content between grains within samples on the AFT ages, and that they are all consistent with a single population. The oldest AFT ages in southern Finland apatites (amongst the oldest recorded from anywhere) are found in gabbros, which also have the highest Cl content of all samples studied. We suggest, that it is Cl content rather than REA that has influenced the annealing history of the apatites, which have experienced a history including reburial into the partial annealing zone by Caledonian Foreland basin sedimentation. The study of apatite from low U and Th rocks, with relatively low levels of α-radiation damage may provide the most practical approach for producing reliable results for AFT and AHe thermochronometry studies in cratonic environments.

Abstract Apatite fission-track (AFT) data from rocks above and below Lewis thrust fault lying in the footwall and hanging wall of Flathead normal fault record different thermal-history components, depending on individual structural and stratigraphic positions. Apatite fission-track temperature-history models (THMs) indicate that rapid cooling of the Lewis thrust sheet began at about 75 Ma. This cooling coincided with major displacement on the Lewis thrust. Subsequently, folding of the Lewis thrust sheet by underlying thrust duplex culminations formed the Akamina syncline, and a fossil AFT partial annealing zone was superimposed on the syncline. Apatite fission-track data from east of the Flathead graben record a subsequent cooling event during the middle Eocene onward that was coeval with extensional displacement on the Flathead fault and with accompanying uplift and erosion of its footwall. Apatite fission-track data from lower Oligocene sediments in the Flathead graben preserved the temperature history of the sediment source regions in the Lewis thrust sheet without significant subsequent annealing. A set of similar THMs that are consistent with the regional structural history can account for observed variations in AFT parameters at various levels, which are exposed in the Lewis thrust sheet and are penetrated below the thrust sheet by deep wells. From the onset of displacement on the Lewis thrust until the early Oligocene, paleogeothermal gradients in the thrust sheet (8.6–12°C/km) were lower than present values (~17°C/km). The changes in geothermal gradients are attributed to advective heat transfer by tectonically induced, topographically driven, deeply penetrating meteoric water flow. This is a complicated heat-transfer mechanism that can affect organic maturation history and petroleum systems in overthrust belts.