Search

In order to further reveal the tectonic activity of the central and northern North China Craton (NCC) since late Paleozoic, the Datong coal-bearing basin was selected as the research object. The tectono-thermal events and uplifting cooling events of the basin were retrieved through zircon and apatite fission tracks and vitrinite reflectance measurements. The research shows that the Datong coal-bearing basin experienced three tectono-thermal events with ages of 245–207 Ma (middle–late Triassic), 179 ± 9 Ma (early Jurassic), and 140 Ma to 78 ± 11 Ma (middle–late Cretaceous), respectively. That just coincides with the lamprophyre activity, Kouquan fault activity, and Zuoyun basaltic andesite magmatic activity which surround the Datong coalfield. The basin also experienced three uplift events with the peak ages of 202 ± 18 Ma (late Triassic), 157 ± 7 Ma (late Jurassic), and 45 ± 3 Ma or 36 ± 3 Ma (middle Eocene), respectively. The Datong Permo-Carboniferous and Jurassic coal vitrinite reflectance proved that the average metamorphism temperature is 104–108 °C, even reaching 163–367 °C. The fission track results showed that the paleotemperature was even higher than 170–250 °C from 117 to 282 Ma and 80–120 °C from 20 to 68 Ma, in the Datong coal-bearing basin. The results show that the deep tectonic activities of the NCC were still active in the Mesozoic and even Cenozoic Paleogene. [ABSTRACT FROM AUTHOR]

Deep-seated structures can exhume deep crustal rocks (>20 km), transmitting the signal of geodynamic processes from the subduction zone to the interiors of the continents. The role of deep-seated structures can be analyzed with low-temperature thermochronological dating techniques. However, studies coupling low-temperature thermochronology with structural geological analyses of the deformational style are not common in the Northern Andes. In this contribution, we present new apatite (AFT) and zircon (ZFT) fission-track data coupled with meso- and microstructural analyses to reveal the deformational and exhumation history of the Santander Massif (SM; Northern Andes) and the related cortical Bucaramanga strike-slip fault (BF). Samples for thermochronological analyses were collected along an elevation profile with a significant elevation difference of 2.4 km across the western flank of the SM, crossing the BF. The time-temperature history modeling of ZFT data reveals phases of prolonged residence in the zircon partial annealing zone from ∼125 to 94 Ma and a cooling phase related to an exhumation episode at around 25 Ma based on samples collected near the BF. Inverse modeling of AFT data reveals structurally-controlled Pliocene exhumation rates of 0.6–0.7 km/Myr mediated by the action of secondary faults. A shift in the deformation style resulting from the oblique interaction of the SM and Mérida Andes domain is interpreted as the main driver of the Pliocene exhumation. This deformation phase is observed in the fault damage zone, where evidence of brittle-ductile deformation was exhumed. Finally, we discuss the geodynamic implications of our thermochronological and structural analyses, contrasting local and more regional competing hypotheses (Pamplona Indenter vs. slab break-off of the Caribbean plate), which may explain the tectonic evolution of the northern part of the Eastern Cordillera and the SM in the Colombian Northern Andes. • Oligocene Bucaramanga fault exhumation coincides with NW Panama block collision, influencing North Andes tectonic escape. • Active Miocene-Pliocene exhumation along the leading S–W edge of an indenting block linked to Caribbean slab break-off. • Meso- and microstructural data evidence crustal deformation and exhumation of different structural levels. [ABSTRACT FROM AUTHOR]

Keywords: Monazite; fission tracks; etching; FIB-SEM; etch-anneal-etch EN Monazite fission tracks etching FIB-SEM etch-anneal-etch 1065 1073 9 06/01/22 20220601 NES 220601 Introduction A series of experiments presented by [9] led to a first-order understanding of monazite fission track etching properties. [Extracted from the article]

This publication shows results of a comparison of three techniques for localising radioactive, and U-bearing particles in contrasted samples. Particles are localised by the means of three methods: (1) Fission Tracks (FT), (2) Imaging Plate (IP), and (3) real time autoradiography (BeaQuant®). These techniques were applied to various samples, including a sediment sampled in the vicinity of the Fukushima Dai-Ichi Nuclear Power Plant (FDNPP) and a sample made of pure U oxide particles. In addition, the efficiency of the combination of two methods (FT and IP) to localise specifically anthropogenic U-bearing particles was tested. [ABSTRACT FROM AUTHOR]

• Shrinkage of latent tracks during in situ ion irradiation is observed and quantified. • Rather than alpha-particles, alpha-recoils induce significant track-shrinkage. • Zircon is more sensitive than apatite to alpha-recoil-induced track-shrinkage. • The etchable lengths of latent fission tracks can be estimated as a function of α-dose. • Alpha-recoil-induced track shortening must be considered in natural zircon. The diffusion of defects (e.g., vacancies and interstitials) and elements used for dating (e.g., He and Pb) in a mineral structure is a thermal process: This is the primary assumption used to determine the age and thermal history of minerals. For instance, thermal history reconstruction, through the number and length distribution of tracks produced by spontaneous fission of 238U, is obtained by assuming a thermal event to be the only energy source for shortening of fission tracks. Here, we report a new, non-thermal energy source that induces additional shortening of fission tracks by the irradiation of alpha-recoils from the alpha-decay of 238U and 232Th. We simulate alpha-decay induced track-shortening by combining ion accelerator irradiations with transmission electron microscopy. This allows for the first observation of track-shrinkage during in situ ion irradiation. We show that rather than alpha-particles, alpha-recoils induce a significant shortening of fission tracks by nuclear-collisions. The shortening of track-length can be quantified as a function of alpha-decay event dose. However, apatite is less sensitive than zircon to this non-thermal process. The findings exemplify the interactions among different types of self-irradiation from alpha-particles, alpha-recoils and fission-fragment nuclei in single mineral grains and have important implications for the use of zircon and apatite for radiometric dating and thermochronology. [ABSTRACT FROM AUTHOR]

Application of the glass fission-track dating method to Chester Bluff tephra (CBt), exposed in loess deposits at Chester Bluff along the Yukon River in east-central Alaska, has clarified the age of the immediately underlying fossiliferous interglacial bed. Surprise Creek tephra (SZt), at site CRH47 in the northern Old Crow basin of the Yukon Territory, is a correlative of CBt so that the new age information on CBt can also be applied to the interglacial sediments below SZt. Two independent age determinations were obtained on CBt, 243 ±28 ka and 249 ±26 ka, giving a weighted mean age and error of 246 ±19 ka. Therefore, the closely associated interglacial bed belongs to the early part of Marine Oxygen Isotope Stage (MIS) 7. The stratigraphy and paleoenvironmental setting of SZt show that deposition of the tephra occurred soon after interglacial conditions, when the climate became colder, probably between MIS 7.5 and 7.4, that is, slightly younger than the mean fission-track age, but within the 1σ uncertainty. This result tightly constrains the age of the rich mammalian faunal assemblage found at and just below SZt at the CRH47 site. [ABSTRACT FROM AUTHOR]

Despite the conceptual elegance and simplicity of the External Detector Method (EDM) for fission track dating, an increasing number of laboratories are switching to LA-ICP-MS as a means of measuring the uranium content of apatite, zircon and sphene. LA-ICP-MS based fission track (LAFT) dating offers shorter turnaround times, removes the need to handle radioactive materials and hydrofluoric acid, and facilitates double-dating with the U-Pb method. This paper aims to bring the statistical treatment of LAFT data on an equal footing with the EDM by formulating four different analytical protocols, depending on the accuracy and reproducibility of the uranium measurements. Under the ‘absolute’ dating approach, the spontaneous track densities and uranium concentrations are directly plugged into the fundamental fission track age equation, assuming that both of these measured quantities have been determined accurately, and that the fission decay constant and equivalent isotopic track length are known. The ‘zeta calibration’ approach avoids making these assumptions by normalisation to a reference material of known age. Uranium zoning has a detrimental effect on the accuracy of LAFT ages. This effect can be removed by counting only those fission tracks located within the laser ablation pit. Alternatively, the uranium heterogeneity may be quantified by fitting multiple ablation spots in some or all the analysed grains, using a (lognormal) distributional assumption for the uranium concentration. LAFT dating is arguably less well suited than the EDM to young and U-poor samples that lack sufficient spontaneous fission tracks to reveal visual evidence for uranium zoning. Such samples occasionally contain no fission tracks at all, resulting in infinite analytical uncertainties. This paper introduces a pragmatic solution to this problem, in which the uranium measurement are converted to ‘virtual’ induced fission track counts to produce strictly positive ages with finite standard errors. With the age equation and zero-track strategy in place, LAFT ages can be subjected to more sophisticated statistical analysis. Using a logarithmic transformation, these ages can be visualised on radial plots and deconvolved into finite and continuous mixtures. The methods introduced in this paper have been implemented in a software package called IsoplotR that is available free of charge at http://isoplotr.london-geochron.com . [ABSTRACT FROM AUTHOR]

Synchrotron radiation techniques (μ-XRF and μ-XANES) were applied to the study of organic-rich phosphatized limestones of NW Greece (Epirus). The results revealed uranium accumulation in areas of the material containing, among others, carbonate apatite (francolite) and organic matter. The U L -edge of μ-XANES spectra showed that uranium was present in tetravalent form. U-bearing francolite crystals were separated from the rock and characterized by Raman spectroscopy and microprobe. The analysis of the crystals also indicated the presence of sodium and sulfur. The uranium presence in the crystals was also visualized, after neutron irradiation and etching, by the observation of the fission tracks. [ABSTRACT FROM AUTHOR]

HeFTy is a popular thermal history modelling program which is named after a brand of trash bags as a reminder of the ‘garbage in, garbage out’ principle. QTQt is an alternative program whose name refers to its ability to extract visually appealing (‘cute’) time–temperature paths from complex thermochronological datasets. This paper compares and contrasts the two programs and aims to explain the algorithmic underpinnings of these ‘black boxes’ with some simple examples. Both codes consist of ‘forward’ and ‘inverse’ modelling functionalities. The ‘forward model’ allows the user to predict the expected data distribution for any given thermal history. The ‘inverse model’ finds the thermal history that best matches some input data. HeFTy and QTQt are based on the same physical principles and their forward modelling functionalities are therefore nearly identical. In contrast, their inverse modelling algorithms are fundamentally different, with important consequences. HeFTy uses a ‘Frequentist’ approach, in which formalised statistical hypothesis tests assess the goodness-of-fit between the input data and the thermal model predictions. QTQt uses a Bayesian ‘Markov Chain Monte Carlo’ (MCMC) algorithm, in which a random walk through model space results in an assemblage of ‘most likely’ thermal histories. In principle, the main advantage of the Frequentist approach is that it contains a built-in quality control mechanism which detects bad data (‘garbage’) and protects the novice user against applying inappropriate models. In practice, however, this quality-control mechanism does not work for small or imprecise datasets due to an undesirable sensitivity of the Frequentist algorithm to sample size, which causes HeFTy to ‘break’ when datasets are sufficiently large or precise. QTQt does not suffer from this problem, as its performance improves with increasing sample size in the form of tighter credibility intervals. However, the robustness of the MCMC approach also carries a risk, as QTQt will accept physically impossible datasets and come up with ‘best fitting’ thermal histories for them. This can be dangerous in the hands of novice users. In conclusion, the name ‘HeFTy’ would have been more appropriate for QTQt, and vice versa. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, we will describe and indicate the performance of a new method based on the use of femtosecond laser ablation (fs-LA) coupled to a quadrupole-based inductively coupled plasma mass spectrometer (ICP-QMS) for analyzing the isotopic composition of micrometer-size uranium particles. The fs-LA device was equipped with a high frequency source (till 10kHz). We applied this method to 1–2μm diameter-uranium particles of known isotopic composition and we compared this technique with the two techniques currently used for uranium particle analysis: Secondary Ionization Mass Spectrometry (SIMS) and Fission Track Thermal Ionization Mass Spectrometry (FT-TIMS). By optimizing the experimental conditions, we achieved typical accuracy and reproducibility below 4% on 235U/238U for short transient signals of only 15s related to 10 to 200pg of uranium. The detection limit (at the 3 sigma level) was ~350ag for the 235U isotope, meaning that 235U/238U isotope ratios in natural uranium particles of ~220nm diameter can be measured. We also showed that the local contamination resulting from the side deposition of ablation debris at ~100μm from the ablation crater represented only a small percentage of the initial uranium signal of the ablated particle. Despite the use of single collector ICP-MS, we were able to demonstrate that fs-LA-ICP-MS is a promising alternative technique for determining uranium isotopic composition in particle analysis. [Copyright &y& Elsevier]

In this paper we describe the application of laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) coupling to particle analysis, i.e., the determination of the isotopic composition of micrometric uranium particles. The performances of this analysis technique are compared with those of the two reference particle analysis techniques: secondary ion mass spectrometry (SIMS) and fission track-thermo-ionization mass spectrometry (FT-TIMS), based on the measurement of the isotopic ratios of U/U in particles present in an inter-comparison particulate sample. The agreement of the results obtained using LA-ICP-MS with target values and with the results obtained using FT-TIMS and SIMS was good. Accuracy was equivalent to that of the other two techniques (±3 % deviation). However, relative experimental uncertainties present with LA-ICP-MS (7 %) were higher than those present with FT-TIMS (4.5 %) and SIMS (3 %). Furthermore, measurement yield of LA-ICP-MS coupling was close to that obtained with the same quadrupole ICP-MS for the measurement of a liquid sample (~10), but lower than that obtained with FT-TIMS and SIMS, respectively, by a factor of 10 and 20, although the particles analyzed using LA-ICP-MS were most likely smaller (diameter ~0.6 μm, containing 4-7 fg of U). Nevertheless, thanks to the brevity of the signals obtained, the detection capacity for low isotopic concentrations by LA-ICP-MS coupling is equivalent to that of FT-TIMS, although it remains well below that of SIMS (×15). However, with more sensitive double focusing ICP-MS, performances equivalent to those achieved using SIMS could be obtained. [ABSTRACT FROM AUTHOR]

Using artificially created swift, heavy ions of 129I with a typical fission-fragment mass and specific energies which are common during the spontaneous fission of 238U, the response in minerals commonly used in geochronological dating such as muscovite, zircon and apatite, has been evaluated by the means of Atomic Force Microscopy (AFM). The surface impact structures form hillocks with greater diameters but smaller heights for low-energetic ions and smaller diameters but greater heights for high-energetic ions. The observed hillock widths range from 22 ± 2 nm for low-energetic ions to 17 ± 1 nm for the highest energy in mica. Furthermore, the dimensions formed in apatite for low and high-energy ions range from 53 ± 2 nm to 27 ± 2 nm and from 64 ± 3 nm to 28 ± 1 nm in zircon. With increasing 129I particle energy hillock heights increase, but diameters decrease regardless of mineral orientation. The crystallographic orientation of apatite and zircon also seems to have no effect on hillock dimensions, which are identical for perpendicular and parallel to the c-axis oriented mineral grains. The results support the “Compound Spike” track formation model of Chadderton (2003), which combines the Coulomb ion explosion model with a thermal spike model. [ABSTRACT FROM AUTHOR]

Abstract: One of the purposes of this study is to give further constraints on the temperature range of the zircon partial annealing zone over a geological time scale using data from borehole zircon samples, which have experienced stable temperatures for ∼1 Ma. In this way, the extrapolation problem is explicitly addressed by fitting the zircon annealing models with geological timescale data. Several empirical model formulations have been proposed to perform these calibrations and have been compared in this work. The basic form proposed for annealing models is the Arrhenius-type model. There are other annealing models, that are based on the same general formulation. These empirical model equations have been preferred due to the great number of phenomena from track formation to chemical etching that are not well understood. However, there are two other models, which try to establish a direct correlation between their parameters and the related phenomena. To compare the response of the different annealing models, thermal indexes, such as closure temperature, total annealing temperature and the partial annealing zone, have been calculated and compared with field evidence. After comparing the different models, it was concluded that the fanning curvilinear models yield the best agreement between predicted index temperatures and field evidence. [Copyright &y& Elsevier]

The thermal recovery (annealing) of mineral structure modified by the passage of fission fragments has long been studied by the etching technique. In minerals like apatite and zircon, the annealing kinetics are fairly well constrained from the hour to the million-year timescale and have been described by empirical and semi-empirical equations. On the other hand, laboratory experiments, in which ion beams interact with minerals and synthetic ceramics, have shown that there is a threshold temperature beyond which thermal recovery impedes ion-induced amorphization. In this work, it is assumed that this behavior can be extended to the annealing of fission tracks in minerals. It is proposed that there is a threshold temperature, T, beyond which fission tracks are erased within a time t, which is independent of the current state of lattice deformation. This implies that iso-annealing curves should converge to a fanning point in the Arrhenius pseudo-space (ln t vs. 1/ T). Based on the proposed hypothesis, and laboratory and geological data, annealing equations are reevaluated. The geological timescale estimations of a model arising from this study are discussed through the calculation of partial annealing zone and closure temperature, and comparison with geological sample constraints found in literature. It is shown that the predictions given by this model are closer to field data on closure temperature and partial annealing zone than predictions given by previous models. [ABSTRACT FROM AUTHOR]

Abstract: The Apennine Range is a young convergent orogen that formed over a retreating subduction zone. Syn-orogenic metamorphic rocks are exposed in the mid-Tuscan ridge, Tuscan coast and Tuscan Archipelago and provide information about processes of continental subduction and exhumation associated with accretion and retreat. Apatite and zircon fission-track analyses were applied to some of these syn-orogenic metamorphic rocks. The new low-temperature thermochronological data helped to constrain the timing of the metamorphic peak and the age and modality of exhumation of these rocks that were structurally buried at 20 to 40km depth during the main tectonogenic phase of the Northern Appenine belt. In the Giglio and Elba islands and in the Monti dell''Uccellina, along the coast, minimum ages of partially reset zircon samples yielded a first time constraint on the Late-Oligocene age of the metamorphic peak. The ca. 27Ma obtained at the Monti dell''Uccellina is coeval to that found in the Alpi Apuane (located at the northern termination of the mid-Tuscan ridge). The data from the Tuscan Archipelago point to a slightly older age. In the Monti Pisani, south of the Alpi Apuane, and in the Argentario Promontary along the coast, south of the Monti dell''Uccellina, completely reset zircon samples, but composed by different populations, were behaving as multiple thermochronometers and confirm the already proposed isothermal nature of the exhumation occurred after the metamorphic peak. Zircon P1 peak indicated that at 13–10Ma rocks were already at shallow crustal levels (<9km). Zircon minimum ages and apatite ages point to a sharp increase in the exhumation rate at ~6–4Ma. Data indicate a common thermal history for the whole mid-Tuscan ridge and coastal outcrops. The 6–4Ma increase in exhumation rate can be attributed either to unroofing due to the extensional tectonics or to a contractional “out-of-sequence” or “re-activation” event occurring in the internal sector of Tuscany already affected by a regional extensional regime. [Copyright &y& Elsevier]

Abstract: The fission track technique is a sensitive detection method for particles which contain radio-nuclides like 235U or 239Pu. However, when the sample is a mixture of plutonium and uranium, discrimination between uranium particles and plutonium particles is difficult using this technique. In this study, we developed a method for detecting plutonium particles in a sample mixture of plutonium and uranium particles using alpha track and fission track techniques. The specific radioactivity (Bq/g) for alpha decay of plutonium is several orders of magnitude higher than that of uranium, indicating that the formation of the alpha track due to alpha decay of uranium can be disregarded under suitable conditions. While alpha tracks in addition to fission tracks were detected in a plutonium particle, only fission tracks were detected in a uranium particle, thereby making the alpha tracks an indicator for detecting particles containing plutonium. In addition, it was confirmed that there is a linear relationship between the numbers of alpha tracks produced by plutonium particles made of plutonium certified standard material and the ion intensities of the various plutonium isotopes measured by thermo-ionization mass spectrometry. Using this correlation, the accuracy in isotope ratios, signal intensity and measurement errors is presumable from the number of alpha tracks prior to the isotope ratio measurements by thermal ionization mass spectrometry. It is expected that this method will become an effective tool for plutonium particle analysis. The particles used in this study had sizes between 0.3 and 2.0μm. [Copyright &y& Elsevier]

(U-Th)/He and fission-track analyses of apatite along deep-seated tunnels crossing high-relief mountain ranges offer the opportunity to investigate climate-tectonic forcing on topographic evolution. In this study, the thermochronologic analysis along the Simplon tunnel (western-central Alps; Italy and Switzerland) constrains in detail the mechanisms controlling the topographic evolution of the Simplon Massif. Cooling rates vary from about 10°C/Ma at about 10 Ma to about 35°C/Ma in the last 3 Ma. Such increase in cooling rates corresponds to the inception of glacial cycles in the northern hemisphere. Age patterns show correlation with faults distribution until 2 Ma, suggesting that tectonics-controlled rocks exhumed up to that age. After 2 Ma thermo-chronometric data show that the Simplon area has experienced primarily erosional exhumation. All age patterns provided are not affected by topographic effects, thus indicating that present-day topography has been carved in the last 2 Ma, most likely controlled by glacial erosion. [ABSTRACT FROM AUTHOR]

The present-day topography of the Tian Shan range is considered to result from crustal shortening related to the ongoing India-Asia collision that started in the early Tertiary. In this study we report evidence for several episodes of localized tectonic activity which occurred prior to that major orogenic event. Apatite fission track analysis and (U-Th)/He dating on apatite and zircon indicate that inherited Paleozoic structures were reactivated in the late Paleozoic-early Mesozoic during a Cimmerian orogenic episode and also in the Late Cretaceous-Paleogene (around 65-60 Ma). These reactivations could have resulted from the accretion of the Kohistan-Dras arc or lithospheric extension in the Siberia-Mongolia zone. Activity resumed in the late Mesozoic prior to the major Tertiary orogenic phase. Finally, the ongoing deformation, which again reactivates inherited tectonic structures, tends to propagate inside the endoreic basins that were preserved in the range, leading to their progressive closure. This study demonstrates the importance of inherited structures in localizing the first increments of the deformation before it propagates into yet undeformed areas. [ABSTRACT FROM AUTHOR]

Copyright of Arabian Journal of Geosciences is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Abstract: Synthetic analogues of poor-silicated natural apatites have been doped with uranium. These minerals have been irradiated with a thermal neutron dose in the aim to induce the 235U fission and to obtain a fission track population. Thermal annealing experiments have been performed on the fission track population and allow us to compare the ability of the synthetic minerals to anneal such nuclear damages with their natural analogues. The thermal of the fission tracks in the synthetic minerals need more time and/or higher temperature to reach the same annealing rate as in the natural analogues. The alpha damage present in the natural analogues seems to enhance the thermal annealing of fission tracks. [ABSTRACT FROM AUTHOR]

We present new thermochronometric analyses of 4 samples from the Western Klamath mountains in California and Oregon, together with a re-evaluation of available geological constraint from a thermo-tectonic perspective. Early Cretaceous cooling of basement samples is seen to reflect significant exhumation by normal faulting, linked to the Separation Episode during which the Klamah region was rifted away from the formerly contiguous Sierra Nevada block. Syn-faulting sedimentation and subsequent Early Cretaceous re-heating of samples establishes the former continuity of scattered Hornbrook Formation remnants as a significant sedimentary basin spanning the Klamath region, with local maximum thickness of at least 5 km. This basin experienced significant inversion during early Cordilleran development around 110-120 Ma, with further unrooting and almost complete eversion in the Late Cretaceous, associated with Laramide uplift of the region. [ABSTRACT FROM AUTHOR]

Abstract: Recent data on He diffusion challenge the temperature sensitivity of apatite (U–Th)/He thermochronology: the damage induced by recoil of U and Th decay series during emission of α particles (α-recoil damage) has been proposed to modify He-diffusion properties through time. However, we propose that annealing of these irradiation defects may be an important phenomenon and may be significant in case of slowly-cooled or reheated basement rocks. To test this hypothesis, we developed a quantitative model including an explicit treatment of α-recoil damage, annealing, and their effect on He-diffusion kinetics, and calibrate it against literature data. Our model is based on two hypotheses: (1) helium is in equilibrium between an apatite crystal and its defects and (2) alpha-recoil damage annealing can be described analogously to fission-track annealing. This model has been embedded into a Monte Carlo simulation of helium production/ejection/diffusion and applied to data from the French Massif Central; a complex slowly-cooled terrain with burial reheating, where the thermal history has been constrained by previous fission-track (FT) data including FT length distributions. (U–Th)/He ages are close to the FT ages from the same samples and are generally reproducible among replicates, but some samples present He-age dispersion that is not correlated with crystal size. Our model reproduces the Massif Central data very well except for three samples where He ages are older than corresponding FT ages. We show that annealing of irradiation damage has an important impact on retentivity of helium and that the He content, [He] is only a rough approximation of the damage level. In particular our results show that independence of He ages on crystal sizes, in case of reheated samples, is a clear indication of the higher He retentivity induced by α-recoil defects and that an explicit treatment of defect annealing is required for a correct interpretation of (U–Th)/He ages in such a case. More generally a correlation with the crystal size can bring information on the thermal path only if the age of defects, well represented by the fission-track age, is available, due to the dependence of the partial retention zone on damages. Conversely, in case of rapid cooling or for samples having low U and Th contents, damage effects can be ignored without significant effects on He ages. [Copyright &y& Elsevier]

Nuclear tracks detectors have been applied to neutron dose determination at an isotopic neutron sources irradiation blocks. We have chosen the optimal conditions for tracks relevation in polycarbonate foils from uranium fission. The neutron doses have been theoretical calculated and experimentaly determinated. [ABSTRACT FROM AUTHOR]

Abstract: Thermoluminescence (TL), which is commonly used to date burnt flints from Palaeolithic sites, sometimes yields ages which are younger than those obtained by other methods. Many reasons could explain such discrepancies and one concerns the evaluation of the dose-rate originating from the sample itself, i.e., the internal dose-rate. In order to refine the parameters used in the internal dose-rate estimation, we investigated the uranium (U) distribution by the induced fission track method in three archaeological flint specimens. This approach shows that the U distribution is roughly uniform on a scale of a millimetre but not on a smaller scale (<100μm), because of the presence of quartz grains whose size is sample-dependent and because of relatively high local U concentrations. Therefore, the assumption of a homogeneous U distribution currently used to estimate the beta and alpha dose-rates may not be fully appropriate for all samples; in this case, this could lead to an underestimate of the TL age. [Copyright &y& Elsevier]

Abstract: The post-Variscan uplift of the western Anti-Atlas Precambrian core is studied by zircon fission track (ZFT) analysis of ten samples of granites and schists from the Kerdous and Ifni inliers. All samples yield Carboniferous ZFT ages ranging from 358±31Ma to 319±32Ma, with nine dates younger than 338±35Ma. The weighted mean age calculated for these nine samples is 328±30Ma. These results compare with the available K–Ar datings of white mica and biotite from the same rocks or from the overlying Ediacaran-Cambrian low-grade metasediments. The fact that different systems with distinct closure temperatures yield similar ages suggests the occurrence of a short Carboniferous thermal event followed by rapid cooling. Consistent with the regional geological framework, the thermal event is assigned to the Variscan folding, being followed by rapid exhumation and cooling related to the post-folding erosion. [Copyright &y& Elsevier]

Abstract: Using a new experimental approach, fission-track formation has been simulated, for the first time, under crustal conditions by exposing natural zircon, at a pressure of 7.5 kbar and a temperature of 250 °C, to a beam of relativistic heavy ions. The latent tracks were investigated using high-resolution transmission electron microscopy, and the diameters of several hundred tracks were measured. The mean values (±σ) of the track diameters were 5.2±0.5 nm and 5.4±0.4 nm for zircon at ambient and elevated pressure-temperature, respectively. Based on the number of measurements, this represents a statistically significant difference between the tracks at ambient vs. high-pressure/temperature conditions. The slightly larger size of the tracks at elevated pressure can be understood in terms of the increased efficiency of the damage process in a strained crystal lattice. This slight variation in track diameter (~0.2 nm) at high pressure probably will not affect the dimensions of etched tracks. [Copyright &y& Elsevier]

Abstract: The source of huge volumes of quartzofeldspathic detritus feeding Oligo-Miocene foredeep turbidites of the Apennines has long remained an unsolved geological enigma. In this article we reinterpret the available sandstone petrography, fission-track, stratigraphic, and geomorphological evidence to demonstrate that the Adriatic foredeep was dominantly sourced by a “Himalayan-type tectonic aneurysm” growing in the Central Alps since the late Oligocene: the Lepontine Dome. The Lepontine Dome is the envelope of distinct subdomes that formed stepwise from east to west during northwestward indentation of Adria beneath the axial Alps. Focused erosion of domal culminations cut by antecedent drainage started suddenly in the late Oligocene, with unroofing of recently-emplaced plutons and young amphibolite-facies gneisses. Exhumation was tectonically driven, and possibly accelerated by positive feedback between erosion and uplift of hot weak rocks from depth. The huge volumes of resulting detritus (Gonfolite–Macigno clastic wedge) were transported southward for hundreds of kilometers along the rapidly subsiding foredeep, formed since middle Eocene times in front of the Alpine retrobelt and nascent Apennine forebelt. Such massive sediment influx, following 15±2 Myr of starved-foredeep sedimentation, testifies to rapid formation of mountain relief in the Central Alps, synchronous to dextral activity along the Insubric Fault. The Lepontine Dome remains today as the only deeply-dissected region in the Alps, where medium-grade metamorphic rocks of the axial belt are extensively exposed. The relationships between geological structures and drainage patterns still mirror those formed originally at Oligocene times. River headwaters have not been significantly displaced since then, and their thalwegs constrain finite lateral displacement relative to the Southalpine retrowedge to a maximum of 10–20 km. Our reconstruction has wide implications, not only for a better understanding of the Tertiary evolution of the Alps–Apennines orogenic couple, but also for general relationships among tectonic activity, relief formation, drainage development, erosion distribution, and long-distance sediment transfer during continental collision and orogenic growth. [Copyright &y& Elsevier]

Abstract: Thin films of natural uranium and thorium deposited on muscovite were used as sources of neutron-induced fission fragments. Fragment energy loss in thin-source geometry is negligible. In this way, the observed fragment range results from the interaction of the fragment with the detector material. This characteristic enables the investigation of asymmetric fission and etching, through measurements of projected track length distributions in muscovite micas coupled with thin films. The means and standard deviations of the etchable length distributions of the heavy and light fission-fragment tracks were estimated by fitting a theoretical equation to the experimental data. The light fission-fragment accounts for ∼54% and the heavy fission-fragment for ∼46% of the etchable length of a full fission track. This average partition is the same for tracks from thermal-neutron-induced fission of 235U and fast-neutron-induced fission of 232Th. The mean etchable length of uranium fission tracks is ∼2.5% longer than that of thorium fission tracks. This difference is at the resolution limit of these measurements but correlates with the difference in the mean combined initial kinetic energies of the fission fragments. The mean etchable length of uranium fission tracks in muscovite is ∼5% shorter than their calculated latent track length, supporting earlier estimates of a length deficit of this magnitude. The length deficit and the standard deviation of the etchable length distribution of the light fission-fragment tracks are twice the equivalent values for the heavy fission-fragment tracks. This is interpreted in terms of a vt -profile (track etch rate) that depends on the mass of the track-forming particle. [Copyright &y& Elsevier]

Abstract: Combined ESR/U-series dating of tooth enamel is important for obtaining chronological constraints on archaeological sites. The precision and accuracy of this method depend on the uncertainties associated with the equivalent dose and the dose rate. The precision of the dose rate calculation, in turn, depends on a number of parameters such as the internal radionuclide concentrations and distributions in the enamel, dentine and cementum and in the surrounding sediments as well as on the cosmogenic dose rate. Furthermore, teeth are open systems for U and therefore constraints on the U-uptake model are needed. The combination of all the uncertainties results in uncertainties of on the age at 95% confidence level. This can, however, be improved by reducing the uncertainties on the internal radionuclide concentrations and distributions. We present a comparison of three methods for measuring the U concentrations in dental tissue of a 40ka old equid tooth. Fission tracks are used to map the U distribution with high spatial resolution. The U and Th concentrations and isotopic compositions were measured with high precision but poor spatial resolution using TIMS. It is demonstrated that in situ LA-ICPMS provides an effective method for precise measurements of the U concentration at high spatial resolution. This method also has high potential for isotopic U-series measurements using MC-ICPMS. The results of the U measurements are used to constrain the effective alpha- and beta-dose rates in enamel sampled for ESR analysis. The TIMS U–Th isotope concentrations and in particular the 230Th concentrations are used to constrain the U uptake for combined ESR/U-series dating. [Copyright &y& Elsevier]

The design, construction and application of a counting system using simple Automatic Micrometric Scanner (AMS) device for Fission Fragment Tracks (FFT) are described in detail. The system is used to detect the etched fission fragment tracks in thin plastic Solid State Track Recorders (SSTRs) (12 µm thickness). The track-detection mechanism is based on scanning concentric circles of specimen rather than the usual XV scanning methods. The circumference of the circle can be traversed to within ±2 µm and the reproducibility of the rotation is within 0.001°. The difference in the radii of the concentric circles is preset in the range of I to 99 999 µm. The system is capable of obtaining reliable results for the track densities up to 2 × 104 tracks/cm² and normal-incidence irradiations. The counting efficiency of the system is about 97% compared to visual methods employing a microscope equipped with CCD device, gas permeation technique and spark gas chambers for track densities up to the just-mentioned figure. [ABSTRACT FROM AUTHOR]

Abstract: The dating of the uplift onset of the Mongolian mountain ranges, the northernmost relief associated with the India-Eurasia convergence, is a fundamental issue to better understand the mechanisms of propagation of the Cenozoic transpressive deformation in Central Asia. Using apatite fission tracks we determined the timing and strain rates of the tectonics affecting the Gurvan Bogd system, in the Gobi-Altay, since the Middle Mesozoic to the Late Cenozoic. The region was firstly affected by a Lower–Middle Jurassic tectonic phase, characterized by a vertical crustal movement larger than 2 km. Then followed a protracted period without major crustal vertical movements until the last uplift phase. The peneplanation of the Jurassic relief produced an erosional surface that has undergone negligible denudation or sedimentation for more than 100 Ma. This same surface corresponds to the present summit plateaux of the massifs, standing about 2000 m above the surrounding region, which corresponds to the vertical movement produced by the ongoing uplift. Modelling of fission track data from the massifs of Ih Bogd and Baga Bogd shows that this uplift phase probably started at 5±3 Ma. The Gobi-Altay mountain range appears therefore as one of the youngest mountain ranges in Central Asia, which is consistent with the idea of a northward propagation of the transpressional deformation from the Himalayan front to the Siberian craton. The Cenozoic uplift rate of the massifs is estimated to be between 0.25 and 1 mm/yr, which is slightly higher than the upper Pleistocene vertical slip rates of the bordering faults. This suggests that thrust faults observed within the massifs would increase the uplift rate inside the massifs compared to the uplift rate determined at their boundaries. [Copyright &y& Elsevier]

Abstract: Apatite fission-track thermochronology has been based upon measurements in prismatic sections. Until now, all annealing data sets referred to this kind of measurement. However, prismatic sections are not always available, e.g. for detrital apatite grains, making the work impossible. An alternative would be to use grains in any orientation. This requires that a suitable annealing model be employed, i.e. one obtained from measurements in grains without preferential orientation. In this work, the angular distribution of confined fission tracks parallel to the surfaces of grains randomly oriented relative to the crystallographic c-axis is obtained. This distribution shows that track at higher angles relative to c-axis, when compared with the prismatic section distribution, are found. The procedure measurements in randomly oriented sections is presented along with empirical model equations. Model parameters have been obtained for the most common empirical equations and the partial annealing zone has been calculated for these equations. Results are compared with geological benchmarks; the fanning linear Arrhenius model yields the best agreement. This empirical model and those most used in literature have been applied to samples from the São Francisco Craton, Brazil, to show an instance in which the effects of grain orientation is important. Thermal histories of geological samples have been obtained through these equations. The samples tested presented moderate annealing and experienced a relatively simple thermal history. Under these conditions, taking into account the uncertainties in the model equations and confined track measurements, and using compatible systems, the presented model equations and the most used prismatic face ones constrained the same thermal history feature. [Copyright &y& Elsevier]

Abstract: Fission tracks have been used to estimate time–temperature (t–T) paths of mineral samples of geological interest. Several datasets on annealing rate have been presented, especially for apatite. However, there are still open problems related mainly to chemical composition of apatite and crystallographic orientation of the analyzed crystal sections. Therefore, new annealing data are desirable. However, there are practical limitations (cost and mainly time) on the number of experiments that can be carried out. In this work, a method for optimizing the choice of the experimental conditions of the future experiments is presented, the so-called D-optimal criterion. The D-optimization takes advantage of previous knowledge on the experimental space and the model to be fit, to maximize the information to be obtained from new experiments. It has proved, in other instances, to be suitable for highly non-linear models as the Arrhenius-type equations currently used to fit the fission-track data. A sample example is given, showing that this procedure produces good choices, leading to reliable data fitting. [Copyright &y& Elsevier]

Abstract: A model for the annealing kinetics of fission tracks is proposed based on a simplified track. The principal hypothesis is that the atoms displaced during the passage of the fission fragment have to transmit through a potential barrier to occupy the vacant sites left. The resulting model equation is applied to zircon data both from laboratory and geological sources. The model succeeded in fitting the data and the addition of the geological data during the fitting procedure improved the extrapolation of the model to the timescale of millions of years, which is the timescale of interest in geology applications. [Copyright &y& Elsevier]

Abstract: Fission tracks in glass samples are shortened when they experience thermal treatment. Consequently, the observable dimensions associated with track length also diminish. The dimension normally used to characterize track size is the mean diameter of the etched track at the surface, D. Another consequence of length shortening is the reduction of surface density, , implying a reduction of the fission-track age. In this work, a kinetic model is developed to describe the relationship between fission-track mean diameter shortening and surface fission-track density reduction in glass samples, based on geometrical and etching hypotheses. The result is a two-parameter equation given byin which h is the thickness of the layer removed during etching and n is a parameter related to the latent track geometry and etching reaction rate. The equation was compared with experimental data on Australite glass found in the literature and with fresh data on Macusanite glass presented in this work. The model fits the experimental curves quite well, showing that it describes the etching effects correctly (within its limitations). In addition, the model equation derived in this work can be very useful in age correction of partially annealed glass samples. [Copyright &y& Elsevier]

Abstract: The western cordilleras of the Northern Andes (north of 5°S) are constructed from allochthonous terranes floored by oceanic crust. We present 40Ar/39Ar and fission-track data from the Cordillera Occidental and Amotape Complex of Ecuador that probably constrain the time of terrane collision and post-accretionary tectonism in the western Andes. The data record cooling rates of 80–2 °C/my from temperatures of ∼540 °C, during 85 to 60 Ma, in a highly tectonised mélange (Pujilí unit) at the continent–ocean suture and in the northern Amotape Complex. The rates were highest during 85–80 Ma and decelerated towards 60 Ma. Cooling was a consequence of exhumation of the continental margin, which probably occurred in response to the accretion of the presently juxtaposing Pallatanga Terrane. The northern Amotape Complex and the Pujilí unit may have formed part of a single, regional scale, tectonic mélange that started to develop at ~85 Ma, part of which currently comprises the basement of the Interandean Depression. Cooling and rotation in the allochthonous, continental, Amotape Complex and along parts of the continent–ocean suture during 43–29 Ma, record the second accretionary phase, during which the Macuchi Island Arc system collided with the Pallatanga Terrane. Distinct periods of regional scale cooling in the Cordillera Occidental at ∼13 and ∼9 Ma were synchronous with exhumation in the Cordillera Real and were probably driven by the collision of the Carnegie Ridge with the Ecuador Trench. Finally, late Miocene–Pliocene reactivation of the Chimbo–Toachi Shear Zone was coincident with the formation of the oldest basins in the Interandean Depression and probably formed part of a transcurrent or thrust system that was responsible for the inception and subsequent growth of the valley since ∼6 Ma. [Copyright &y& Elsevier]

In contrast to Eocene Himalayan foreland basin sedimentary deposits in India and Pakistan, coeval sedimentary rocks of the Bhainskati Formation in Nepal contain scant petrographic evidence of orogenic input. Such data have been used as evidence to promote models of diachroneity of India-Asia collision. In this paper we document orogenic input into the Eocene foreland basin rocks of Nepal, from fission-truck analyses of detrital zircons. Our data provide evidence that significantly reduces the possible duration of any diachroneity of collision, and brings the interpretation of the sedimentary record into better agreement with ages of early thrusting and metamorphism in the orogen. We also use our detrital fission-track data to bolster previous age determinations of the overlying Dumri Formation, confirming the basin-wide occurrence of a major unconformity. Comparison of our data set with that from coeval along-strike rocks in India suggests that there is no evidence of diachroneity in early stages of the orogen's exhumation. [ABSTRACT FROM AUTHOR]

Abstract: Apatite (U–Th)/He and fission-track thermochronometers are combined to constrain models of denudation and escarpment development of the high elevation Eritrean margin along the southern Red Sea. (U–Th)/He ages have been determined for apatites from two coast-perpendicular transects that were previously used for apatite fission-track analysis [E. Abbate, M.L. Balestrieri, G. Bigazzi, Morphostructural development of the Eritrean rift flank (southern Red Sea) inferred from apatite fission track analysis, J. Geophys. Res., 107, B11, doi: 10.1029/2001JB001009, (2002)]. In central Eritrea near Asmara, He ages increase from c. 7 Ma at the coast to 152 Ma on the plateau along a transect. Further south, He ages from the margin border to the base of the escarpment span a narrower range (7 to 12 Ma). One sample from the top of a marginal high (2950 m) yields older ages (22–43 Ma). Forward modeling suggests that the He age distribution across the margin should allow scarp retreat and downwearing mechanisms to be distinguished in the case of a margin formed in the last twenty million years. The distribution of He ages suggests that the escarpment evolved by downwearing and that post-break up erosion of the escarpment was facilitated by in situ excavation rather than parallel retreat. This implies the existence of an inland drainage divide located seaward of the present day escarpment rim. We envisage that the inland divide formed during or immediately after continent break up, and that the eastern marginal belt represents a remnant of the apex structure. The short distance between the eastern marginal belt and the rim (c. 10 km) represents the total retreat of the escarpment. Comparison of the measured He ages from the coastal plain with those predicted from forward modelling indicates that the main phase of post-break up erosion started at about 15 Ma, closer to the initiation of seafloor spreading in the Red Sea than was thought previously. [Copyright &y& Elsevier]

Foreland basins record the regional isostatic compensation of mountain belts; during periods of crustal thickening, they subside, and when erosion unloads the mass of the mountains, the basins rebound and are eroded. In order to evaluate this mechanism for rebound, it is critical that the timing and magnitude of erosion are documented. We present data estimating the timing and magnitude of late orogenic or postorogenic erosion in the North Alpine Foreland Basin of Switzerland. Mineral cooling ages demonstrate that the basin underwent 1–3 km of erosion soon after 5 Ma. This erosion coincided with a decline in structural deformation in the Swiss Alps, and a doubling of sediment accumulation rates in surrounding depocenters. We propose that accelerated erosional unroofing of the Swiss Alps triggered isostatic rebound and erosion of the foreland basin after 5 Ma. A projection of the isostatic rebound of the basin into the mountains suggests that at least 6.5 km of erosion should have occurred in the high topography of the Aar Massif. Accelerated erosion in the Swiss Alps at that time is explained by an increase in atmospheric moisture driven by an intensification of the Atlantic Gulf Stream at 4.6 Ma. Consequently, we propose that the changing erosional capacity of the climate triggered late orogenic to postorogenic mass reduction and isostatic rebound of the Swiss Alps and their neighboring foreland basin. [ABSTRACT FROM AUTHOR]

One of the most important tasks of nuclear safeguards is to control the activities in nuclear facilities worldwide. These activities include the enrichment of 235U. To identify enrichments above the maximum authorised level in commercial enrichment plants (usually 5% 235U) swipe tests are performed and the 235U enrichment is determined in the sampled uranium oxide particles. A method is described that allows to measure the 235U content in small particles (around 1 μm in diameter) by the use of fission tracks. It is based on the fact that in uranium fuel, only 235U atoms are fissile with thermal neutrons and contribute to the amount of fission tracks counted for individual particles. Standard uranium oxide particles with a defined diameter and isotopic composition were prepared and used to calibrate the method. Results show that the detection efficiency has a non-linear dependence on the number of fission tracks per particle. The enrichment in 235U for single particles can be determined when good size measurements of the particles in a sample are available. [Copyright &y& Elsevier]