Annealing Of Fission Tracks Research Articles

Exothermic reactions and 39Ar–40Ar thermochronology: Hydration leads to younger apparent ages

Abstract Retrogression and hydration commonly affect large swaths of Earth’s crust, causing variable degrees of chloritization, sericitization, and/or serpentinization. Hydration is a strongly exothermic process that partially opens isotopic systems, thereby distorting the recorded apparent ages and cooling histories of reworked terranes. Using a simple one-dimensional numerical model involving heat released at variable temperatures to simulate exothermic hydration, we track the cooling histories for rocks that exhume from depth. The calculated cooling paths are used to quantify apparent 39Ar–40Ar ages in muscovite, biotite, and feldspar considering 40Ar production and diffusive Ar loss. For fluid incorporation relative to chloritization of ~10%, ~50 kJ of latent heat are released per kilogram of rock. For this scenario and exhumation rates between 1 mm·yr–1 and 4 mm·yr–1, muscovite grains ≤100 µm in diameter yield apparent ages that are younger by up to 10%, but always exceeding the typical uncertainty of Ar dating. Biotite and feldspar display a similar distortion, even for large grains of ~1 mm in size. The relative younging effect increases to >30% with enthalpy released, exhumation rate, and decreasing grain size, with younging reaching a maximum for hydration at approximately the nominal closure temperature of the respective thermochronometers. Using published data sets (from Sifnos, Greece, and Tian Shan, China), we suggest that rejuvenation of apparent mica ages is consistent with diffusive Ar loss due to exothermic hydration during exhumation. Our method applies to any thermally activated process, like element exchange in mineral thermometers or fission-track annealing, provided heat is released close to the characteristic closure temperature. This extends to processes beyond hydration, such as shear heating or localized magma emplacement, making our results pertinent for diverse thermochronometers and temperature-sensitive methods across a broad range of conditions.

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.

Arrhenius activation energy and transitivity in fission-track annealing equations

Fission-track annealing models aim to extrapolate laboratory annealing kinetics to the geological timescale for application to geological studies. Model trends empirically capture the mechanisms of track length reduction. To facilitate the interpretation of the fission-track annealing trends, a formalism, based on quantities already in use for the study of physicochemical processes, is developed and allows for the calculation of rate constants, Arrhenius activation energies, and transitivity functions for the fission-track annealing models. These quantities are then obtained for the parallel Arrhenius, parallel curvilinear, fanning Arrhenius, and fanning curvilinear models, and fitted with Durango apatite data. Parallel models showed to be consistent with a single activation energy mechanism and a reaction-order model of order ≈ − 4. However, the fanning curvilinear model is the one that results in better fits laboratory data and predictions in better agreement with geological evidence. Fanning models seem to describe a more complex picture, with concurrent recombination mechanisms presenting activation energies varying with time and temperature, and the reaction-order model seems not to be the most appropriate. It is apparent from the transitivity analysis that the dominant mechanisms described by the fanning models are classical (not quantum) energy barrier transitions.

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 (<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.

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.

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.

A natural long‐term annealing experiment for the zircon fission track system in the Songpan‐Garzê flysch, China

AbstractApplication of the zircon fission track (FT) method to derive reliable cooling histories requires that rocks have undergone temperatures sufficient to reach full resetting prior to cooling. Zircons commonly show significant variation in accumulated radiation damage and therefore FT annealing behaviour. Twenty‐eight samples of mainly anchizonal to lowermost greenschist facies Triassic sandstones from the Songpan‐Garzê flysch, China, were evaluated for their FT annealing status. Literature data suggest a heating period in the range of 100 myr duration. Our results define a temperature range for partial FT annealing of 270–350°C (based on illite crystallinity data), higher than the proposed range for radiation‐damaged zircons. We suggest that the long residence at high temperature has led to annealing of relevant parts of radiation damage along with FT annealing, so that even for long durations of FT annealing, full resetting requires temperatures in the range of greenschist facies conditions typical for zero‐damage zircons.

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.

Helium trapping in apatite damage: Insights from (U-Th-Sm)/He dating of different granitoid lithologies

Apatite (U-Th-Sm)/He (AHe) thermochronometry is widely used to constrain thermal histories and rates of tectonic, exhumation, and erosion processes. However, data interpretation is often challenging, especially when the thermal history includes extended residence time in the He partial retention zone (HePRZ), with highly dispersed dates revealing the complexity of diffusion processes in natural systems. This study investigates chemical and physical factors that may have impacted He diffusion in apatite over long timescales in a context of protracted residence in the HePRZ. Nine samples from the Ploumanac'h pluton and North Tregor (Armorican Massif, France) were collected in granitoids, differing in petrography and chemisty. This area was chosen because these samples underwent a similar thermal history since ~300Ma. We report new (U-Th-Sm)/He dates, along with apatite fission-track (AFT) data, as well as lithological and chemical characterization. The results show dispersed (U-Th-Sm)/He dates, ranging from 87±7 to 291±23Ma, whereas central AFT dates vary from 142±6 to 199±9Ma. Current predictive models for He diffusion and fission-track annealing in apatite could not reproduce the two datasets together. However, this apparent discrepancy gives insight into the parameters influencing He diffusion at geological timescales. The data confirm that radiation damage enhances He trapping, as the AHe dates are positively correlated to effective uranium (eU) concentration. The He age dispersion for constant eU content cannot be explained just by variations in grain size or chemical composition. To explore the potential influence of recoil damage trapping behavior and annealing kinetics on AHe dates, we tested a new diffusion model from Gerin et al. (2017). Given the expected model of the thermal history provided by AFT inversion, we investigated the influence of the trapping energy on AHe dates. The AHe date variations can be explained only if the trapping energy evolves from one crystal to another, increasing with the amount of damage. For a given trapping energy, minor variations in the recoil-damage annealing rate can consistently explain most of the remaining dispersion of the AHe dates.

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.

Radon emanation coefficients of several minerals: How they vary with physical and mineralogical properties

The escape rates of radon gas from rocks and minerals are of great relevance to many branches of geosciences, and it is, thus, important to understand the physical and mineralogical properties that control radon emanation rates. Mechanisms of radon loss from minerals have direct bearing on the reliability of U-Pb and U-Th-He geochronology. Fourteen minerals from three different mineral groups and with localities spanning three continents were selected for this study. The radon emanation coefficients (REC) for each mineral were measured as a function of grain size, temperature, 238 U and 232 Th activities, total absorbed α-dose, density, and mineral melting temperature. The measured 238 U and 232 Th activities ranged from 0.01 to 6487 Bq/g and from below detection limit to 776 Bq/g, respectively. The REC values for unheated, pulverized samples ranged from 0.083 to 7.0%, which is comparable to previously reported ranges (except for zircon). An inverse correlation between grain size and REC was observed. Full annealing of fission tracks resulted in an overall decrease in REC values, suggesting that nuclear tracks could possibly act as conduits for radon release. While activity, α dose, density, and melting temperatures are not strongly correlated with REC values, it was observed that minerals with high melting points (≥1400 °C) have lower REC values, most likely due to inhibition of radon release by compact crystal-lattice structures. This is the first attempt, to our knowledge, to correlate REC values with melting temperature, and this study reports six minerals for which no REC values have been previously reported.

Fission track dating of faulting events accommodating plastic deformation of biotites

AbstractWe performed fission track (FT) analyses to constrain the timing of fault slip events associated with plastic deformation of biotite grains formed at high temperature, higher than 200–300°C. Apatite FT ages of 49.6–16.7 Ma were obtained from Late Cretaceous granites near the fault, with younger ages near a basaltic dike that intruded at ~19 Ma. By comparing the distribution of apatite FT ages using forward thermal and FT annealing calculations, we interpreted the younger ages as being explained by basaltic intrusion. We concluded that the plastic deformation of the biotite grains could have occurred associated with the following events: (1) primal cooling of the granitic body at 68–50 Ma and/or (2) basaltic intrusion at ~19 Ma. A combination of detailed observation of microscale deformation structures and dense FT measurements successfully constrained timing of fault events distributed in bedrock without covering layers.

Mechanisms of radon loss from zircon: Microstructural controls on emanation and diffusion

Understanding how radon escapes from minerals is important for many fields in Earth science, yet few studies have focused on the mechanisms for radon escape. We measured radon emanation rate and radon loss upon heating for crushed aliquots of three large zircon crystals from three localities: Mud Tank (Australia), Bancroft (Canada), and Malawi (Africa). Our study, in conjunction with published data, shows that the room temperature radon emanation coefficient (REC) varies over 5 orders of magnitude in zircon. For low U zircon, Mud Tank, there are variations in REC that appear to be related to annealing at different temperatures, possibly due to annealing of fission tracks, however, all REC values for Mud Tank zircon are within error of one another. Bancroft and Malawi zircons have higher U content and do not show any systematic relationship of REC to annealing temperature. Results from Mud Tank zircon suggest that partial annealing of fission tracks decreases REC, but when all fission tracks are annealed REC reaches a maximum. REC in zircons with high U content, Bancroft and Malawi, is slightly higher than in zircon with lower U, although results are within error. Results of measurements of radon loss upon heating suggest that radon diffusion is slow, ∼30% of the radon is lost during heating at 975°C for 48h. Samples heated a second time yield less fractional radon loss, ∼10%, suggesting that diffusion parameters are changed during heating at temperatures ⩾975°C, which is likely the result of annealing of radiation damage. Diffusion parameters calculated from the fractional loss experiments reflect diffusion in highly radiation damaged or metamict zircons. Our results indicate that internal microstructures in zircon, such as fission tracks and alpha-radiation damage, influence radon escape for diffusion and recoil mechanisms, and hence if these effects can be further characterized, measurements of 222Rn escape have the potential to be useful for probing the internal structure of zircon, and other U-bearing minerals.

Thermochronologic constraints on evolution of the Linglong Metamorphic Core Complex and implications for gold mineralization: A case study from the Xiadian gold deposit, Jiaodong Peninsula, eastern China

The NNE-trending Linglong Metamorphic Core Complex hosts the majority of the gold deposits in the Jiaodong Peninsula of eastern China. Many of the deposits are hosted by the 163–155Ma Linglong granite in the footwall of the Linglong detachment fault. Argon thermochronology suggests that the granite had cooled to 400°C by 143±1.5Ma possibly as a result of normal movement on the detachment. Nine zircon fission track (ZFT) ages from samples collected along a NW–SE transect perpendicular to the central part of the Linglong detachment fault at the −652m level in the Xiadian deposit constrain the subsequent thermal evolution of Linglong Metamorphic Core Complex, which overlapped the period of major gold deposition.The ZFT ages vary from 136.9±3.3Ma (1σ) to 130.1±2.2Ma (1σ). The unaltered Linglong granite in the footwall and amphibolite in the hangingwall have similar ages at 136.9±3.3Ma (1σ) and 135.0±3.0Ma (1σ), whereas ages for the disseminated- and stockwork-style ores appear to be younger at ca. 131–130Ma, although there is an overlap of ages when considering the 1σ precision. Interestingly, ZFT ages show no marked differences between the hangingwall and footwall of the Linglong detachment fault, although significant movement along the fault occurred.The results are best interpreted to indicate that the Linglong granite was emplaced at ~160Ma, and cooled to 240±50°C at ~135Ma, as recorded by unaltered rocks in the footwall. Hydrothermal alteration along the Linglong detachment fault led to annealing of zircon fission tracks and the consistent younger ages of ~131Ma. Quartz aggregates associated with gold mineralization show evidence of recrystallization suggesting that the hydrothermal event was initiated at temperatures of at least 300–350°C, near the brittle–ductile transition, but cooled rapidly to ZFT closure temperatures within a country-rock environment that was already relatively cool. Therefore these ZFT ages suggest that the timing of mineralization at the Xiadian deposit was post-135Ma and likely very close to 131Ma. The mineralization and cooling occurred in the footwall of a major detachment fault under an extensional regime, possibly related to the progressive slab rollback of the paleo-Pacific plate, and controlled by the Linglong Metamorphic Core Complex.

Experimental evidence regarding the pressure dependence of fission track annealing in apatite

The main purposes of fission track thermochronology are unravelling the thermal histories of sedimentary basins, determining uplift and denudation rates, identifying the structural evolution of orogenic belts, determining sedimentary provenance, and dating volcanic rocks. The effect of temperature on fission tracks is well known and is used to determine the thermal history; however, the effect of pressure on the stability of tracks is still under debate. The present work aims to understand the role of pressure on the annealing kinetics of apatite fission tracks. The samples of Durango apatite used in our experiments were chosen for their international recognition as a calibration standard for geological dating. Neutron irradiation of the samples, after total annealing of their spontaneous tracks, produced induced tracks with homogeneous densities and lengths. The effect of pressure associated with temperature on fission track annealing was verified by experimental procedures using a hydraulic press of 1000 t with a toroidal chamber profile. The experiments consisted of a combination of applying 2 and 4 GPa with 20,150,190,235, and 290°C for 1 and 10 h. The annealing rate was analysed by measuring the lengths of the fission tracks after each experiment using optical microscopy. The results demonstrate that the annealing of apatite fission tracks has a pressure dependence for samples subjected to 2 and 4 GPa. However, when extrapolated to pressures of ⩽150 MPa, compatible with the normal geological context in which apatite fission track methodology is broadly used, this dependence becomes insignificant compared to the temperature effect.

Thermal history of the Maramureş area (Northern Romania) constrained by zircon fission track analysis: Cretaceous metamorphism and Late Cretaceous to Paleocene exhumation

Abstract This study presents zircon fission track data from the Bucovinian nappe stack (northern part of the Inner Eastern Carpathians, Rodna Mountains) and a neighbouring part of the Biharia nappe system (Preluca massif) in order to unravel the thermal history of the area and its structural evolution by integrating the fission track data with published data on the tectonic and sedimentary evolution of the area. The increase of metamorphic temperatures towards the SW detected by the zircon fission track data suggests SW-wards increasing tectonic overburden (up to at least 15 km) and hence top NE thrusting. Sub-greenschist facies conditions during the Alpine metamorphic overprint only caused partial annealing of fission tracks in zircon in the external main chain of the Central Eastern Carpathians. Full annealing of zircon points to at least 300 °C in the more internal elements (Rodna Mountains and Preluca massif). The zircon fission track central and single grain ages largely reflect Late Cretaceous cooling and exhumation. A combination of fission track data and stratigraphic constraints points to predominantly tectonic differential exhumation by some 7-11 km, connected to massive Late Cretaceous extension not yet detected in the area. Later events such as the latest Cretaceous (“Laramian”) juxtaposition of the nappe pile with the internal Moldavides, causing exhumation by erosion, re-burial by sedimentation and tectonic loading during the Cenozoic had no impact on the zircon fission track data; unfortunately it prevented a study of the low temperature part of the Late Cretaceous exhumation history.