Confined Track Length Measurements Research Articles

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.

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.

Dating of altered mafic intrusions by applying a zircon fission track thermochronometer to baked country rock, and implications for the timing of volcanic activity during the opening of the Japan Sea

AbstractOnce a mafic intrusive rock has become altered, it is generally difficult to obtain a reliable intrusion age using conventional isotopic dating methods. To overcome this problem, this study used zircon fission track (ZFT) thermochronometry to determine the timing of crystallization of altered mafic intrusions. ZFT dating was carried out on samples of baked granite country rock adjacent to dolerite dikes (5–10 m thick) in the Takato area of central Japan. Three granite samples collected within 8 mm of a dike contact yielded consistent ZFT ages of 17–16 Ma, with confined track lengths indicative of the complete annealing of pre‐existing tracks by reheating due to dike intrusion. An older ZFT age was obtained for one granite sample collected within 20 mm of the contact, but confined track length measurements indicate that this is an incompletely reset age that lies between the ZFT age of the unbaked granitic country rocks (ca. 55 Ma) and the emplacement age of the dike. Petrographic examinations suggest that post‐intrusion hydrothermal activity did not influence the ZFT ages. We conclude that the 17–16 Ma ZFT age represents the emplacement age of the dikes. Our results show that ZFT dating of baked country rock is an effective tool for dating altered mafic intrusions, for which other dating techniques are not applicable. In the eastern part of Southwest Japan, dispersed volcanic activity occurred in the late Early to early Middle Miocene (18–15 Ma), and the volcanic belt extended into the forearc. This pulse of activity was possibly related to the injection of asthenospheric material into the trench‐side mantle wedge beneath the Japan arc. We also present young apatite fission track ages (ca. 4 Ma) that may reflect a Middle Miocene or later thermal event associated with local magmatic activity near the Takato area.

Fission-track dating of apatite from the Gory Sowie Massif, Polish Sudetes, NE Bohemian Massif: implications for post-Variscan denudation and uplift

Six samples from the Gory Sowie Massif gneisses in the West Sudetes, analyzed for the purpose of apatite fission- track dating (AFT) yielded ages ranging from 43 to 57 Ma. No regional variation in the results was observed and the samples form a rather uniform population. From confined-track length measurements, short values between 8.8 and 9.6 µm were obtained. Track length distribution in four of the six samples is bimodal. This suggests residence in temperatures corresponding to the par- tial annealing zone until relatively recent times and subsequent rapid cooling to ambient conditions. We propose a mid-Tertiary rise in the geothermal gradient as a possible factor responsible for reheating the Gory Sowie Massif after it had cooled to temper- atures around 60 - 40 uC in the Oligocene. According to reverse-modelling results, the cooling began in the Cretaceous and there- fore correlates with the event reported from elsewhere in the northern part of the Bohemian Massif. As Lower Carboniferous sed- imentary rocks overlie the gneisses, the overburden which was responsible for keeping temperatures above 100 uC until early Ter- tiary was probably the Carboniferous-Permian Variscan molasse. The amount of the Cretaceous/early Tertiary denudation can be estimated to be 4 - 8 km. The recent, rapid cooling phase began 7- 5 Ma ago and is interpreted as the combined result of decrease of geothermal gradient and/or increased tectonic activity in the Sudetes as evidenced by the presence of the Pliocene, coarse-clas- tic Gozdnica formation.

Correction factors for systematic errors related to the track counts in fission-track dating with the external detector method

Tracks from uranium fission are the basis of fission-track dating of minerals. The fission-track method has not attained the status of an independent dating method because of problems related to neutron fluence measurements and the decay constant. Fission-track ages are instead determined relative to the reference ages of standards using an empirical calibration factor ( ζ-method). Methodical advances during the last decade imply that these problems need not remain obstacles to establishing fission-track dating as an independent method. However, sources of systematic error related to the track density measurements need to be corrected before this can be the case. A correction factor (GQR), determined by experiment, is in good agreement with its calculated value. The ages of apatites of known age determined with the φ-method are also in good agreement with their reference ages. When applied to a set of samples of unknown age, the φ-method produces results that are consistent with those obtained with the Z- and ζ-methods. These results lend support to the method of neutron fluence measurement, to the recommended value of the decay constant and to the calculated and measured GQR-value. On the other hand, these results call into question the validity of age corrections based on confined track length measurements.

APATITE FROM AN OLIVINE PYROXENITE OF THE CANAÃ VALLEY, ESPÍRITO SANTO, BRAZIL: FISSION-TRACK AGE DETERMINATION AND CONFINED TRACK LENGTH MEASUREMENTS IN ORDER TO ESTABLISH AN INTERNAL STANDARD

We present the first results of fission-track age determinations and natural confined track length measurements in apatite crystals of an olivine pyroxenite from the Canaa Valley, Espirito Santo, Brazil. This apatite was chosen for its high abundance in the rock and also for its high uranium content. Confined track length measurements were performed by three different analysts, whereas fission-track ages corresponding to two different irradiations were determined by two analysts. Confined track length distributions as well as ages showed good reproducibility. The characteristics of this apatite (its abundance in the rock and the relatively large number of confined tracks) make it a very promising internal standard which experienced a more complex thermal history in comparison with the volcanic apatite international age standards. The fission-track age of around 95 Ma of the apatite sample from the olivine pyroxenite is consistent with the age interval 65-110 Ma previously measured by Fonseca and Poupeau (1984), Fonseca (1989) and Fonseca et al. (1992) in the Ribeira Belt. This age interval corresponds to the basaltic and alkaline volcanic activities related to the opening of the Atlantic Ocean.

APATITE FROM AN OLIVINE PYROXENITE OF THE CANAÃ VALLEY, ESPÍRITO SANTO, BRAZIL: FISSION-TRACK AGE DETERMINATION AND CONFINED TRACK LENGTH MEASUREMENTS IN ORDER TO ESTABLISH AN INTERNAL STANDARD

We present the first results of fission-track age determinations and natural confined track length measurements in apatite crystals of an olivine pyroxenite from the Canaã Valley, Espírito Santo, Brazil. This apatite was chosen for its high abundance in the rock and also for its high uranium content. Confined track length measurements were performed by three different analysts, whereas fission-track ages corresponding to two different irradiations were determined by two analysts. Confined track length distributions as well as ages showed good reproducibility. The characteristics of this apatite (its abundance in the rock and the relatively large number of confined tracks) make it a very promising internal standard which experienced a more complex thermal history in comparison with the volcanic apatite international age standards. The fission-track age of around 95 Ma of the apatite sample from the olivine pyroxenite is consistent with the age interval 65-110 Ma previously measured by Fonseca and Poupeau (1984), Fonseca (1989) and Fonseca et al. (1992) in the Ribeira Belt. This age interval corresponds to the basaltic and alkaline volcanic activities related to the opening of the Atlantic Ocean.

On the frequency distributions per unit area of the projected and etchable lengths of surface-intersecting fission tracks: influences of track revelation, observation and measurement

In addition to the statistical bounds discussed by Laslett et al. (1994) and Laslett and Galbraith (1996), thermal history analysis based on the projected and etchable length distributions of surface intersecting fission tracks is limited by systematic factors related to track revelation, observation and measurement. The effects of track revelation, in particular, distort these distributions in the length intervals of interest. An observation threshold poses a problem if it is described by a critical angle θ c , but not if it is described by other criteria proposed in the literature. Measurement imprecisions, predictably, blur the thermal history information contained in these distributions. Measurements of semi-confined tracks, added as a result of surface etching, are a more promising alternative to confined track length measurements for accessing the thermal history record in the fission track length distribution. On the other hand, measurements of the projected lengths of surface intersecting tracks offer the theoretical possibility of determining the true volumetric density N and true mean length m of an arbitrary population of fission tracks, thus allowing direct determination of the corrected age of samples with complex thermal histories. On a methodical level, knowledge of N and m allows to determine the efficiency with which fission tracks are counted under the optical microscope under exactly the same conditions as those under which fission track counts for routine dating purposes are performed.

Towards zircon fission-track thermochronology: Reference framework for confined track length measurements

Horizontal confined track (HCT) lengths and angles to the crystallographic c-axis were measured for both unannealed spontaneous and induced fission tracks (FT) in zircons to establish a reference framework of HCT length measurement. Induced track lengths were analysed on eleven zircon separates from various rock types, including four age standards, and spontaneous lengths on three zircon standards were analysed. In general, the observed HCT lengths in each sample are slightly anisotropic, with shorter tracks having greater angles to the c-axis. In addition, all samples show large anisotropy in the number of observed HCT's, particularly for induced tracks. As an observed mean HCT length is controlled by the combined influence of these two anisotropies, selective measurement of tracks that have specified azimuth angles to the c-axis (e.g., 60–90°) is recommended in annealing studies or thermochronological applications. Mean spontaneous and induced HCT lengths of 60–90° are approximately concordant for individual age standards, although slightly controlled by the difference in overetching effects due to the different etching criteria adopted. This indicates that natural track annealing at an ambient surface temperature at geological timescales is negligible in zircon, in contrast to the case in apatite. Mean lengths of induced HCT's do not differ significantly between the zircons analysed. Therefore the overall mean and standard deviation values of HCT lengths for 60–90° azimuth angles, i.e. 10.50 ± 0.04 (1 standard error) μm and 0.76 ± 0.03 μm for spontaneous tracks and 10.73 ± 0.04 and 0.63 ± 0.03 μm for induced tracks, respectively, offer reliable references for HCT length measurement in zircon FT thermochronology.

L'apatite de Durango (Mexique): Analyse d'un minéral standard pour la datation par traces de fission

Durango apatite is analyzed by four analysts using three different dating techniques. Both the annealing technique and the external detector technique are applied to basal and prismatic sections cut from single crystals. The population technique is applied to mounts of randomly oriented grains. Provided an individual determination of the geometry factor is performed, all ages are indistinguishable from the reference age of 31.4 ± 0.5 Ma. There is no noticeable influence of either the dating technique or the crystallographic orientation of the dated section on the FT age. Based on the projected lengths of surface tracks, spontaneous track length reduction varies from ∼4% to ∼10% (mean ∼6%), depending on the analyst. Two simple corrections are applied to eliminate the net effect of the missing short tracks on the mean projected length. After correction the amount of shortening of the spontaneous tracks is found to be slightly larger (mean ∼8%) but is still dependent on the analyst (∼4% to ∼12%). The mean is in fairly good agreement with the value obtained from confined track length measurements (∼9%). Finally, an isochronal (1 h) plateau annealing procedure was applied to a separate set of basal and prismatic sections cut from the same single crystals. Annealing temperatures up to 300°C do not affect the FT age. Erratic ages at 350°C are ascribed to the occurrence of unetchable gaps. It is argued that the absence of a distinct plateau up to temperatures where it could be expected from theoretical considerations indicates that the FT age of Durango apatite needs no correction.

Apatite fission-track age correction and thermal history analysis from projected track length distributions

The frequency distribution of fission-track lengths in an apatite reflects the mineral's thermal history since the time of its last crossing into the partial annealing zone (PAZ). Apatite thermal history analysis based on confined track length measurements is an already well-established tool in hydrocarbon exploration. In a less direct though more easily accessible way the thermal history is also reflected in the frequency distribution of the projected lengths of tracks intersecting the apatite surface. A mathematical description of the probability distribution of the projected length of surface tracks (PTLD, projected track length distribution) corresponding to different types of thermal history is presented and a new method for correcting thermally lowered fission-track ages is proposed. This correction method gives the time of entry into the PAZ at ∼120°C but also the time when the sample left the PAZ to enter the total track stability zone (at ∼60°C). An age function is subsequently defined which allows us to distinguish between different types of thermal history. Experimental verification shows that, although the proposed approach is less precise compared to existing methods based on projected track lengths, it is possible to distinguish between different thermal histories provided that a sufficient number of tracks are measured. On the other hand, the age function provides information relative to the samples' complete trajectory through the PAZ whereas existing methods are limited to temperatures below the closing temperature of the fission-track system.