Annealing Of Tracks Research Articles

Unexpectedly narrower track diameter at site of fission event

Fission events in minerals create a damage track, which provides critical information for events that occur over a wide range of time scales, ranging from nuclear fission events (10−20 to 10−12 seconds) to track formation (10−17 to 10−9 seconds) and geological processes (millions of years). However, the site where fission occurs has never been identified along a fission track, as it is long (∼20 μm), thin (diameter <10 nm), and randomly oriented in the mineral matrix. Here, we, for the first time, observe an unexpectedly narrower track diameter (∼1 nm), rather than the conventionally assumed largest diameter, near the fission-event site by a novel technique that confines fission events to the sample surface. The low-charge states of fission fragments produce the narrower track nearest the fission site due to the instantaneous partitioning of negatively charged electrons between two positively charged fission fragments, roughly scaling to their proton numbers. Because a narrower fission track is easier to anneal, the nanostructure nearest the fission site provides spatial information necessary for understanding the atomic-scale mechanisms of track formation and track annealing, with significant implications for fission track thermochronology.

Annealing of swift heavy ion tracks in amorphous silicon dioxide

The annealing kinetics of the high energy ion damage in amorphous silicon dioxide (a-SiO2) are still not well understood, despite the material's widespread application in material science, physics, geology, and biology. This study investigates how annealing temperature, duration, and ambient environment affect the recovery of irradiation damage produced along the trajectory of swift heavy ions in a-SiO2. The track-annealing kinetics and the changing ion track morphology were investigated using synchrotron-based small-angle X-ray scattering (SAXS) and etching methods. We found that track annealing proceeds quicker near the sample surface demonstrated by a changing track etch rate as a function of depth. Measurements of ion tracks using SAXS show only small changes in the radial density distribution profile of the ion tracks. Activation energy of the annealing process at different sample depths was determined and the effect of the capping layer during the annealing process was also studied. Combination of oxygen diffusion and stress relaxation may contribute to the observed behaviour of preferential and anisotropic healing of the ion track. The results add to the fundamental understanding of ion track damage recovery and may have direct implications for materials for radioactive waste storage and solid state nanopores.

Thermal annealing of fission and ion tracks in epidote

Thermal annealing of fission and ion tracks in epidote

Simulating sedimentary burial cycles – Part 2: Elemental-based multikinetic apatite fission-track interpretation and modelling techniques illustrated using examples from northern Yukon

Abstract. Compositionally dependent apatite fission track (AFT) annealing is a common but underappreciated cause for AFT age dispersion in sedimentary samples. We present an interpretation and modelling strategy for samples with variable apatite composition that exploits multikinetic AFT annealing to obtain thermal histories that can provide more detail and better resolution compared to conventional methods. We illustrate our method using a Permian and a Devonian sample from northern Yukon, Canada, both with complicated geological histories and long residence times in the AFT partial annealing zone. Effective Cl values (eCl; converted from rmr0 values) derived from detailed apatite elemental data are used to define AFT statistical kinetic populations with significantly different total annealing temperatures (∼110–185 ∘C) and ages that agree closely with the results of age mixture modelling. These AFT populations are well resolved using eCl values but exhibit significant overlap with respect to the conventional parameters of Cl content or Dpar. Elemental analyses and measured Dpar for Phanerozoic samples from Yukon and the Northwest Territories confirm that Dpar has low precision and that Cl content alone cannot account for the compositional and associated kinetic variability observed in natural samples. An inverse multikinetic AFT model, AFTINV, is used to obtain thermal-history information by simultaneously modelling multiple kinetic populations as distinct thermochronometers with different temperature sensitivities. A nondirected Monte Carlo scheme generates a set of statistically acceptable solutions at the 0.05 significance level and then these solutions are updated to the 0.5 level using a controlled random search (CRS) learning algorithm. The smoother, closer-fitting CRS solutions allow for a more consistent assessment of the eCl values and thermal-history styles that are needed to satisfy the AFT data. The high-quality Devonian sample (39 single-grain ages and 202 track lengths) has two kinetic populations that require three cycles of heating and cooling (each subsequent event of lower intensity) to obtain close-fitting solutions. The younger and more westerly Permian sample with three kinetic populations only records the latter two heating events. These results are compatible with known stratigraphic and thermal maturity constraints, and the QTQt software produces similar results. Model results for these and other samples suggest that elemental-derived eCl values are accurate within the range 0–0.25 apfu (atoms per formula unit, with rmr0 values of 0.73–0.84), which encompasses most of the data from annealing experiments. Outside of this range, eCl values for more exotic compositions may require adjustment relative to better-constrained apatite compositions when trying to fit multiple kinetic populations. Our results for natural and synthetic samples suggest that an element-based multikinetic approach has great potential to dramatically increase the temperature range and resolution of thermal histories relative to conventional AFT thermochronology.

Modeling fission spikes in nuclear fuel using a multigroup model of electronic energy transport

In fission based nuclear reactors the fuel is subject to an intense neutron environment that drives the fission chain reaction. Due to this process fission fragments are created with energies reaching 1 MeV/amu that lose energy primarily through inelastic interactions with the electronic structure producing electronic excitations. Subsequently, these excitations thermalize through electron-phonon interactions resulting in the formation of a high temperature thermal spike and associated pressure spike. This process promotes atomic mobility that is expected to evolve lattice defects, including the annealing of latent ion tracks. In this work, a multigroup model for electron energy transport is developed and applied to molecular dynamics simulations in the LAMMPS code to examine fission energy deposition and fission effects in nuclear fuel. This technique utilizes MCNP Monte Carlo electron transport calculations to determine the initial injection of fission energy. To provide a more predictive approach than semi-empirical two-temperature models, the electron-phonon interactions are defined to include multiphonon energy transfer as a function of atomic and electron temperature, and are evaluated from electronic structure calculations using the VASP density functional theory code and PHONON lattice dynamics code. Application of this model to fission energy deposition in uranium dioxide predicts ion track formation and fission enhanced atomic mobility behavior within reasonable agreement of experimental trends. Furthermore, simulations of fission fragment interactions with latent ion tracks demonstrate an annealing effect due to this enhanced mobility.

Post-ore change and preservation of the late Paleozoic Tuwu porphyry Cu deposit in Eastern Tianshan, NW China: Constraints from geology and apatite fission track thermochronology

The late Paleozoic Tuwu porphyry Cu deposit is located in the Dananhu–Tousuquan arc in Eastern Tianshan, northwest China. In this study, we present new apatite fission track data of the intermediate-felsic intrusions (i.e., diorite porphyry, tonalite porphyry, quartz porphyry, and quartz albite porphyry) from Tuwu, together with inverse thermal modeling and previous study, to reveal exhumation and cooling history. Apatite fission track length analyses indicate that the track annealing type belongs to a transition from the undisturbed basement distribution to the mixed distribution. As a whole, two phases of cooling were recognized from inverse thermal modeling including a protracted slow cooling period with an average cooling rate of 0.32 ℃/Ma during late Cretaceous to Miocene and moderate slow cooling with a cooling rate of 1.89 ℃/Ma from Miocene to the present day. The first phase of cooling can be mainly attributed to a tectonically quiet and arid to semiarid climate whereas the later phases of cooling related to structural activation occurred at ~25–15 Ma in response to a distant effect of the India-Eurasia convergence. In addition, the exhumation rates of Tuwu intrusions from the two stages are 10.7 m/Ma and 63 m/Ma, respectively. The calculated total exhumation thickness at Tuwu is 1.2–2.3 km since late Cretaceous. Based on the integration of stratigraphic sequence, tectonic setting, paleogeographic, geochronology, and low temperature thermochronology study, we suggest that the thick deposition of coal-bearing stratum during the early and middle Jurassic, the extended duration slow cooling and exhumation controlled by arid to semiarid climate, and the differential lifting movement of fault blocks are key factors for preservation of the early Carboniferous Tuwu Cu deposit.

Simulating sedimentary burial cycles – Part 1: Investigating the role of apatite fission track annealing kinetics using synthetic data

Abstract. Age dispersion is a common feature of apatite fission track (AFT) and apatite (U–Th) / He (AHe) thermochronological data, and it can be attributed to multiple factors. One underappreciated and underreported cause for dispersion is variability in apatite composition and its influence on thermal annealing of fission tracks. Using synthetic data we investigate how multikinetic AFT annealing behaviour, defined using the rmr0 parameter, can be exploited to recover more accurate, higher-resolution thermal histories than are possible using conventional interpretation and modelling approaches. Our forward model simulation spans a 2 Gyr time interval with two separate heating and cooling cycles and was used to generate synthetic AFT and AHe data for three different apatite populations with significantly different annealing kinetics. The synthetic data were then used as input for inverse modelling in the Bayesian QTQt software to recover thermal-history information under various scenarios. Results show that essential features of the dual peak thermal history are captured using the multikinetic AFT data alone, with or without imposed constraints. Best results are achieved when the multikinetic AFT data are combined with the AHe data and geologic constraint boxes are included. In contrast, a more conventional monokinetic interpretation that ignores multikinetic AFT behaviour reproduces all the input data but yields incorrect thermal-history solutions. Under these conditions, incorporation of constraints can be misleading and fail to improve model results. In general, a close fit between observed and modelled parameters is no guarantee of a robust thermal-history solution if data are incorrectly interpreted. For the case of overdispersed AFT data, it is strongly recommended that elemental data be acquired to investigate if multikinetic annealing is the cause of the AFT apparent age scatter. Elemental analyses can also be similarly useful for broadly assessing AHe data. A future companion paper (Issler et al., 2021) will explore multikinetic AFT methodology and application to detrital apatite samples from Yukon, Canada.

A new phenomenological model for annealing of fission tracks in apatite: laboratory data fitting and geological benchmarking

Rana track annealing model 2007, based on an Arrhenius approach, is significantly modified, including new assumptions, fitting parameters, and analysis, to yield a new phenomenological model for annealing of fission tracks in apatite. Construction and optimization of the new six parameter model have been described in detail. The model shows a hybrid behavior. In Arrhenius plot, for higher temperatures (laboratory data included), iso-annealing lines are curved while, for low temperatures (geological time scale), iso-annealing lines become parallel straight lines. This is a new feature for any such model and has produced promising results. C-axis projected lengths from laboratory experiments on Durango apatite are employed to find model parameters. KTB borehole fission-track data are added to build the temperature function that modifies the original equation. Partial annealing zone (PAZ), closure temperature ( $$T_{\mathrm{C}}$$ ) and total annealing temperature ( $$T_{\mathrm{A}}$$ ) were calculated and compared to geological benchmarks. The predictions of the present model agreed well with low and high temperature benchmarks. PAZ, $$T_{\mathrm{C}}$$ and $$T_{\mathrm{A}}$$ predictions were also compared to the Fanning Curvilinear Arrhenius model predictions, resulting in good agreement. The present model is flexible enough to be applied to other fission-track systems, like zircon, muscovite and titanite.

Implementation of an Alpha Damage Annealing Model for Zircon (U‐Th)/He Thermochronology With Comparison to a Zircon Fission Track Annealing Model

AbstractRadiation damage exerts a fundamental control on He diffusion in zircon, which manifests as correlations between (U‐Th)/He date and effective uranium concentration. These correlations can be exploited with modeling to explore long‐term thermal histories. This manuscript focuses on one such model, the zircon radiation damage accumulation and annealing model (ZRDAAM) of Guenthner et al. (2013), https://doi.org/10.2475/03.2013.01, by integrating newly defined alpha damage annealing kinetics measured by Ginster et al. (2019), https://doi.org/10.1016/j.gca.2019.01.033, into ZRDAAM. I explore several consequences of this alpha damage annealing model as it relates to (U‐Th)/He date‐effective uranium (eU) correlations, using representative time‐temperature paths and previously published results. Comparison between the current version of ZRDAAM, which uses fission track annealing, and the new annealing model demonstrates that, for thermal histories with prolonged periods at low temperatures (&lt;50°C), alpha dose annealing kinetics yield slightly younger model dates at low to moderate eU concentrations, older dates at moderate to high eU, and substantially younger dates at the highest eU concentrations. The absolute eU concentrations over which the differences are observed varies for a given thermal history, so these ranges should be interpreted as relative or proportional. Younger model dates at high eU in most thermal histories result from lower amounts of annealing that occur with the Ginster et al. (2019) alpha dose annealing kinetics. This annealing model comparison illustrates that the choice of annealing kinetics has the greatest influence over model output for thermal histories involving either prolonged time periods in the 200–300°C temperature window, or a late‐stage reheating event.

Addendum to “Annealing of fission fragment tracks in CR-39 nuclear track detector — Phenomenological model” [Nucl. Instrum. Methods A 940 (2019) 40–44

Addendum to “Annealing of fission fragment tracks in CR-39 nuclear track detector — Phenomenological model” [Nucl. Instrum. Methods A 940 (2019) 40–44

Some geometrical properties of fission-track-surface intersections in apatite

Abstract Parallel fission-track-surface intersections identify the grains in an etched apatite mount that have been polished parallel to their prism faces and mark the orientations of their c-axes. Their lengths (Dpar) are a practical kinetic parameter that is indicative of the track annealing rate of apatite. Little is known, however, about their geometrical properties in non-prism faces. We present a model calculation of the frequency distributions of the orientations, lengths, and widths of track-surface intersections in non-prism faces. The current model does not include the effects of surface etching or measurement imprecision. However, as far as it goes, it is consistent with measurements in apatite surfaces up to 30° to the c-axis. Regardless of the model, we submit that the statistical properties of the fission-track-surface intersections have practical uses. The distribution of their orientations is characteristic of the orientation of the etched surface relative to the c-axis. The distribution of their lengths presents a possible tool for investigating track etching, in particular for evaluating the tracks added and lost through surface etching. The distribution of their widths is a potential kinetic parameter independent of surface orientation and less susceptible to the factors, such as the sampling method and surface etch rate, that produce conflicting Dpar values.

The along-track etching structure of fission tracks in apatite: Observations and implications

Interpretation of apatite fission-track length data rests on the assumptions that etching fully reveals latent tracks, and that laboratory annealing of induced tracks constitutes a good proxy for geological annealing of spontaneous tracks. Recent work using improved step-etching procedures questions both of these assumptions. Step etching experiments interrogating the etching structure of tracks near and beyond their tips have shown that tracks are often not etched to their full extent of latent track damage in a standard single-step, 20-s etch, and that the etching structure in this outer region differs between fossil and induced tracks. In this study, we develop a more complete picture of track revelation for fossil and unannealed and annealed induced tracks in Durango apatite. Using first steps as early as 10 s after the beginning of etching, we discern differences in etching structure between various track types, and document a gradual fall-off in etching velocity from the track interior toward its tips. When effective etching time is constrained by selecting earlier-etching confined tracks for later analysis, mean lengths are longer, but length dispersion is not diminished. Although some proportion of unannealed tracks are under-etched after 20 s of etching, with increased levels of annealing the probability of an induced track being fully etched increases. We use an etch-anneal-etch procedure, consisting of partially etching tracks, annealing the apatite grains, and then continuing etching, to establish bulk etching rates at track tips. Bulk etching is effectively isotropic, as is etching velocity in the central portions of tracks, away from their tips. We use our data to compare two end-member possibilities for along-track etching structure, which provides a first step toward a deeper understanding of annealing and development of a more robust standard etching protocol.

Is Low‐Temperature Fission‐Track Annealing in Apatite a Thermally Controlled Process?

AbstractWe report a new series of experiments to explore the phenomenon of low‐temperature annealing of fission tracks in apatite that feature a number of improvements over previous work. Grain mounts were preirradiated using 252Cf to increase confined track detection and allow briefer thermal neutron irradiation. We coirradiated and etched four apatite varieties (Durango, Fish Canyon, Renfrew, and Tioga) over five time steps equally spaced from 3.66 to 15 ln(s). A length standard was coetched with all experiments to ensure that subtle differences are within detection limits. Finally, we used a standard etching protocol, allowing the data to be comodeled with extensive high‐temperature data sets and recent analyses of induced tracks that underwent ambient‐temperature annealing over year‐to‐decade time scales. Ambient‐temperature annealing occurs at two different rates, with faster annealing at early stages that decreases to a slower rate that converges with empirical fanning linear or curvilinear models. The nature of this decrease varies among the apatite species examined, but no patterns could be determined. The fitted models make geological time‐scale predictions consistent with those based on high‐temperature data only and also make predictions consistent with reasonable inferred low‐temperature histories for all four apatite varieties. The empirical fanning curvilinear equation encompasses low‐temperature annealing at month‐to‐decade time scales, but low‐temperature annealing at shorter time scales may occur by a distinct mechanism. We consider but rule out annealing by radiation from short‐lived activated isotopes. We also reconsider the notion of the initial track length, and the appropriate length for normalizing confined track length measurements.

Annealing of ion tracks in apatite under pressure characterized in situ by small angle x-ray scattering

Fission track thermochronology is routinely used to investigate the thermal history of sedimentary basins, as well as tectonic uplift and denudation rates. While the effect of temperature on fission track annealing has been studied extensively to calibrate the application of the technique, the effect of pressure during annealing is generally considered to be negligible. However, a previous study suggested elevated pressure results in a significantly different annealing behaviour that was previously unknown. Here, we present a method to study track annealing in situ under high pressure by using synchrotron-based small angle x-ray scattering (SAXS). To simulate fission tracks in a controlled environment, ion tracks were created in apatite from Durango, Mexico using 2 GeV Au or Bi ions provided by an ion accelerator facility. Samples were annealed at 250 °C at approximately 1 GPa pressure using diamond anvil cells (DACs) with heating capabilities. Additional in situ annealing experiments at ambient pressure and temperatures between 320 and 390 °C were performed for comparison. At elevated pressure a significantly accelerated annealing rate of the tracks was observed compared with annealing at ambient pressure. However, when extrapolated to geologically relevant temperatures and pressures, the effects become very small. The measurement methodology presented provides a new avenue to study materials behaviour in extreme environments.

Summary of a comprehensive data bank for nuclear track research using CR-39 detectors

This data bank is the result of systematic experiments on nuclear track etching, fission fragment track annealing, and radiation detection & measurement. Data highlights from a comprehensive nuclear track data bank, and related physics and chemistry are described here briefly and clearly. Uncertainties in the reported data are also presented. All the data presented were collected using the most sensitive nuclear track detector CR-39, or Poly Allyl Diglycol Carbonate (PADC), over two decades. Closely related data sets from the published literature are also reviewed. Necessary details of the reviewed data and their citations are clearly given in the data bank. Aim here is to present tabulated experimental raw data with essentially needed calculations. All the necessary information related to the data bank is described clearly. Study presented here is useful for research students and scientists who possess the rudimentary knowledge of nuclear track detection. Single tabulation of the data, instead of plots, is aimed at making easy access of the data to users.

Annealing of fission fragment tracks in CR-39 nuclear track detector — Phenomenological model

Abstract In this short paper, a new model for annealing of latent nuclear tracks is developed. Physical phenomenological picture or foundation of the model is presented using experimental spectra of 252Cf fission fragment tracks annealed at 75–175 °C for 5–30 min. Here, annealing of nuclear tracks refers to the reduction of track lengths during annealing or thermal treatment. A new annealing parameter (t/T2) is identified in the model, which is useful in understanding and analysis of annealing of nuclear tracks. In definition of the annealing parameter above, the quantities t and T are annealing time and temperature, respectively. The model allows deduction of the activation energy of annealing simultaneously from track annealing data, collected over all the annealing conditions (t, T). This model provides a quick methodology to study kinetics of nuclear track annealing. The presented model is validated using the experimental results of annealing of fission tracks in CR-39 detector. The determined value of activation energy of annealing is 0.204 ± 0.012 eV. Presented results and discussion can be extended to swift heavy ion (SHI) tracks.

Standardless fission-track ages of the IUGS age standards

Standardless fission-track ages for four zircon, two titanite and three apatite age standards, most of them recommended by the Fission Track Working Group of the IUGS Subcommission on Geochronology, were redetermined. The age calculations used the decay constant for spontaneous fission of 238U (λf = (8.5 ± 0.1) 10−17 a−1) recommended by the IUPAC, the ratio of etchable to latent track length in both the dated minerals (zircon, titanite and apatite) and mica detectors and the 187 track counts with the external-detector and population methods. We found the resulting standardless fission-track ages are in line with results from more recent high precision geochronology including zircon U-Pb dating. These results confirm that standardless fission-track dating of zircon, titanite and apatite with the external-detector method is feasible, as well as dating of apatite with the population method, and that ζ-calibration can be based on the zircon U-Pb ages of the current standard samples. On the other hand, there is no room for an age correction for partial annealing of the fossil tracks based on their mean confined-track lengths of the apatite, titanite and zircon age standards.

Thermotectonic history of the Maastrichtian reservoir in Campos Basin

Abstract The present study of the Carapebus Formation on well samples refines our understanding of the thermotectonic history of the Maastrichtian oil reservoir in Campos Basin. Our investigations are based on the thermal analysis of 55 cuttings samples, of which 45 have also been analyzed with apatite fission track (AFT) dating, in addition to eleven cores analyzed for vitrinite reflectance (VR) and spore color. The apatites are partially annealed and the AFT central ages does not provide precise information on exhumation and thermotectonic events that occurred in the source terrain, whereas the youngest AFT ages provide a minimum age of 45.9 ± 5.5 Ma due to significant track annealing in the rocks. The VR (0.37–0.48 %Ro), spore color and Thermal Alteration Index (TAI) data correspond to the immature zone of hydrocarbons. The thermal history integrated with seismic profiles demonstrate that two phases of heating happened to reservoir, separated by a cooling event (65–37 Ma). The first heating phase started after deposition of the Carapebus Formation until 65 Ma and the second during late Eocene and reflect the subsidence at the basin. The cooling event shows a strong correlation with subaerial unconformity related in the basin and an important thermal uplift coeval with the last volcanic pulses of the Trindade mantle plume in the basin. During the uplift phase, the generation of hydrocarbon in reservoir was ceased and no more generation occurred until the Recent.

Observations on three-dimensional measurement of confined fission track lengths in apatite using digital imagery

We report the results of a comparative study to explore the usefulness of 3D measurements of confined fission track lengths (TINTs) relative to confined track length measurements (dips ≤10°), and evaluate their suitability for thermal history modeling. Confined fission track lengths were measured in 10 annealed Fish Canyon Tuff apatites containing synthetic mixtures of different length components, and two Durango apatites containing spontaneous fission tracks. Measurements were primarily carried out using a digital image-based microscope system, and they were compared to those from a regular optical drawing tube-digitizing tablet setup, and a confocal laser scanning microscope. The results indicate that 3D measurements of confined track lengths are closely comparable to conventional track measurements, and the mean track lengths of inclined (dips > 10°) and (dips ≤10°) confined tracks from the one sample are equivalent within the measurement uncertainty. A strong dip-bias was observed, so that almost all the confined tracks measured were dipping at < 30°, and the great majority (∼70%) were dipping at ≤10°, thereby qualifying as horizontal confined tracks. Our results suggest that a useful increase of more than 40% in sample size can be achieved from including dip-and refraction-corrected 3D track length measurements. Some evidence was seen for a small bias in favor of shorter tracks at higher dip angles but this has very little influence on the mean lengths or length distributions up to the practical limit of dips (∼30°) observed in these measurements. Results obtained using the same measurement system by a single analyst over time and between six different observers in the one laboratory show good reproducibility. These results also agree well with conventional confined track length measurements in the same samples in the second laboratory involved. We conclude that 3D measurements of confined track lengths, including both and inclined tracks, are suitable for use in current fission track annealing models derived from experiments using confined tracks.

A helium-based model for the effects of radiation damage annealing on helium diffusion kinetics in apatite

Widely used to study surface processes and the development of topography through geologic time, (U–Th)/He thermochronometry in apatite depends on a quantitative description of the kinetics of 4He diffusion across a range of temperatures, timescales, and geologic scenarios. Empirical observations demonstrate that He diffusivity in apatite is not solely a function of temperature, but also depends on damage to the crystal structure from radioactive decay processes. Commonly-used models accounting for the influence of thermal annealing of radiation damage on He diffusivity assume the net effects evolve in proportion to the rate of fission track annealing, although the majority of radiation damage results from α-recoil. While existing models adequately quantify the net effects of damage annealing in many geologic scenarios, experimental work suggests different annealing rates for the two damage types. Here, we introduce an alpha-damage annealing model (ADAM) that is independent of fission track annealing kinetics, and directly quantifies the influence of thermal annealing on He diffusivity in apatite. We present an empirical fit to diffusion kinetics data and incorporate this fit into a model that tracks the competing effects of radiation damage accumulation and annealing on He diffusivity in apatite through geologic time. Using time–temperature paths to illustrate differences between models, we highlight the influence of damage annealing on data interpretation. In certain, but not all, geologic scenarios, the interpretation of low-temperature thermochronometric data can be strongly influenced by which model of radiation damage annealing is assumed. In particular, geologic scenarios involving 1–2 km of sedimentary burial are especially sensitive to the assumed rate of annealing and its influence on He diffusivity. In cases such as basement rocks in Grand Canyon and the Canadian Shield, (U–Th)/He ages predicted from the ADAM can differ by hundreds of Ma from those predicted by other models for a given thermal path involving extended residence between ∼40–80 °C.