Fission-track dating of Macusanite glasses with plateau and size correction methods

Two Macusanite pebbles (MB1 and MB2) were dated with the fission-track method. Six irradiations were carried out in different nuclear reactors: Pavia (Italy), IPEN-CNEN (Brazil) and IPEN-Lima (Peru). Measurements of the thorium and uranium induced-fission per target nucleus using natural thorium thin films and natural U-doped glasses calibrated against natural uranium thin films, together with a λ_F of 8.46×10⁻¹⁷ a⁻¹ were used to determine the ages. The apparent ages were corrected using the plateau and size correction methods. Track measurements were performed by different analysts, using different counting criteria. In addition, tracks were measured on samples which had been submitted to thermal treatment as well as on samples which had not been heated. Thermal treatments were carried out to erase the fossil tracks before neutron irradiation. No significant differences have been found in individual results, using the two Macusanite pebbles and the different nuclear reactors, age correction techniques, analysts, track-counting criteria, and thermal treatments before neutron irradiation. The great majority of the results (14/17) is compatible with the Ar–Ar ages of 5.12±0.11 and 5.10±0.11 Ma, Macusanite MB1 and MB2, respectively. However, the fission-track ages are systematically less (8%) than the Ar–Ar ages of the two Macusanite samples studied.     

Preparation of two uranium glass reference materials for fission-track dating of geological samples

The European Commission produced and certified uranium glass for fission-track dating in 1996 (Reference Material IRMM-540). This material is now out of stock and a project for the preparation of two new reference glasses is underway. The new glasses, containing nominally 15 and 55 mgkg⁻¹ uranium, were prepared from blended oxide powder, and cast using an improved method, which minimised micro-scale defects and optimised the yield. Glass rods were produced which were then cut into discs of 2 mm thickness and 15 mm diameter and subsequently polished.

Uranium homogeneity and fission-track stability are critical properties of the glass. An extensive study was carried out to verify the homogeneity by fission-track counting, investigating both within-disc and between disc effects, and to investigate potential track fading due to thermal annealing. No evidence of uranium heterogeneity was detected. The stability study, using an “isochronous” method, in which all measurements are made at the end of exposure to artificial ageing, is still underway.

The reference material will be certified for isotopic composition (natural), uranium mass fraction and neutron fluence (one disc out of each set of three will be irradiated, together with a mica foil, to induce fission tracks in a certified fluence). The reference materials will be released as IRMM-540R (15 mgkg⁻¹ U) and IRMM-541 (55 mgkg⁻¹ U).     

The use of apatite fission track thermochronology to constrain fault movements and sedimentary basin evolution in northeastern Brazil

An apatite fission track study of crystalline rocks underlying sedimentary basins in northeastern Brazil indicate that crustal blocks that occur on opposite sides of a geological fault experienced different thermal histories. Samples collected on the West block yielded corrected fission-track ages from 140 to 375 Ma, whereas samples collected on the East block yielded ages between 90 and 125 Ma. The thermal models suggest that each block experienced two cooling events separated by a heating event at different times. We concluded that the West block moved downward relative to the East block ca. 140 Ma ago, when sediments eroded from the East side were deposited on the West side. This process represents the early stage of sedimentary basin formation and the opening of the South Atlantic Ocean in the region. Downward and upward movements related to heating and cooling events of these crustal blocks at different periods until recent times are proposed.     

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.     

Fission-track analysis of meteorites: Dating of the Marjalahti pallasite

The results of the Marjalahti pallasite fission-track age determination are presented. Thorough examination of fossil tracks in the phosphate (whitlockite) crystals coupled with U-content determination in whitlockites can make it possible to estimate the contributions of all possible track sources to the total track density and to calculate a model fission-track age.

It is found that whitlockite crystals of the Marjalahti pallasite contain fossil tracks due to galactic cosmic rays (VH, VVH nuclei); fission of U and Th induced by cosmic rays; spontaneous fission of ²³⁸U; and spontaneous fission of extinct, short-lived ²⁴⁴Pu present in significant quantities in the early solar system.

A great track density attributed to the extinct ²⁴⁴Pu testifies to the high fission-track age. The model fission-track ages of (4.31±0.02)×10⁹ yr for the Marjalahti pallasite are calculated. Petrographic studies allow us to interpret the fission-track age as the time of the last shock/thermal event in the cosmic history of the pallasite.     

Cosmic history of some pallasites based on fossil track studies

Investigation of fossil charged-particle tracks in various mineral phases of extraterrestrial samples is a powerful method for research the early stages of the solar system. Examination of fossil charged-particle tracks in the phosphate and olivine crystals of 5 pallasites coupled with U content determination allowed the estimation of the contributions of all possible track sources to the total track density and calculation of a value of model fission-track age.

Phosphate crystals from all studied pallasites were established to contain fossil tracks due to galactic cosmic rays (VH, VVH nuclei); induced fission of U and Th by cosmic rays; spontaneous fission of ²³⁸U; spontaneous fission of extinct short-lived ²⁴⁴Pu nuclei presented in significant quantities in the early solar system. The discovery of a high track density attributed to the extinct ²⁴⁴Pu pointed obviously to the high value of the fission-track age. Model fission-track ages of (4.37±0.02) Gyr for the Marjalahti pallasite; (4.37±0.01) Gyr for the Omolon pallasite; (4.19±0.02) Gyr for the Bragin pallasite; (4.18±0.03) Gyr for the Krasnoyarsk pallasite; (4.21±0.02) Gyr for the Brenham pallasite were calculated. The comparison of represented data with petrographic analyses allowed us to interpret a value of fission-track age as the time of the last intensive shock/thermal event in the cosmic history of a pallasite.     

On methodical problems in estimating geological temperature and time from measurements of fission tracks in apatite

The results of apatite fission-track modelling are only as accurate as the method, and depend on the assumption that the processes involved in the annealing of fossil tracks over geological times are the same as those responsible for the annealing of induced fission tracks in laboratory experiments. This has hitherto been assumed rather than demonstrated. The present critical discussion identifies a number of methodical problems from an examination of the available data on age standards, borehole samples and samples studied in the framework of geological investigations. These problems are related to low- (<60°C) and high-temperature (>60°C) annealing on a geological timescale and to the procedures used for calculating temperature–time paths from the fission-track data. It is concluded that it is not established that the relationship between track length and track density and the appearance of unetchable gaps, observed in laboratory annealing experiments on induced tracks, can be extrapolated to the annealing of fossil tracks on a geological timescale. This in turn casts doubt on the central principle of equivalent time.

That such uncertainties still exist is in no small part due to an insufficient understanding of the formation, structure and properties of fission tracks at the atomic scale and to a lack of attention to the details of track revelation. The methodical implications of discrepancies between fission track results and the independent geological evidence are rarely considered. This presents a strong case for the re-involvement of track physicists in fundamental fission track research.     

Mesozoic cooling history of the “Bachu Uplift” in the Tarim Basin,China: Constraints from zircon fission-track thermochronology

We applied zircon fission-track analysis to outcrop and borehole samples to study the Mesozoic cooling history of the Bachu Uplift, the Central Uplift of the Tarim Basin. Zircon fission-track (ZFT) ages of 182 Ma – 249 Ma are younger than the sample depositional ages indicating substantial post burial thermal annealing and can effectively reveal cooling events in the Bachu Uplift. The strong correlation between single grain ZFT age and U content indicates that most of the zircon grains represent ages that have been partially annealed and so the age is not directly indicative of the time of cooling. The youngest ZFT age populations with modal peak ages of 151 ± 8 Ma (Well HT1 samples), 126 ± 6 Ma (Well T1 samples) and 192 ± 10 Ma (Xiaohaizi Reservoir profile samples) from the decomposition of the ZFT single-grain ages represent the onset of cooling events in the Bachu Uplift, which were related to the collisions of the Qiangtang Terrane and Lhasa Block with the southern margin of the Eurasia continent, respectively. This study provides new insights into the tectonic and sedimentary evolution of the Tarim Basin and even Central Asia by constraining the higher temperature (c. 250–180 °C) part of the basin thermal history.     

Unroofing history of the Sillai Patti granite gneiss, Pakistan: constraints from zircon fission-track dating

A group of alkaline igneous rocks is exposed in the north of the Peshawar Plain, extending from Tarbela in the east up to Loe–Shilman near the Pakistan–Afghanistan border in the west. The alkaline rocks consist mainly of granites, syenites, gabbros, ijolites and carbonatites. Granitic gneisses of Paleozoic age are exposed at the Malakand and further westward at Sillai Patti. However, the fission-track dating studies on zircon, based on the present work, indicate that the age of the Sillai Patti granite gneiss is less than the absolute age of granite gneisses. Therefore, the zircon fission-track age of 24.28±2.97 Ma of the Sillai Patti granite gneiss, represents a time of post-metamorphic denudation history of the area, when these rocks passed through the 210°C isotherm, corresponding to a depth of about 6.7 km inside the earth's crust from their present position if a paleogeothermal gradient of 30°C/km is assumed to have prevailed. Our average fission-track zircon age of 24.28±2.97 Ma is very similar to the average fission-track zircon age of 25.4±0.7 Ma of Mansehra granites. Average cooling rates of the Mansehra and Sillai Patti granite gneisses have been computed to be (8.00±0.22)°C/Ma and (8.00±0.98)°C/Ma, while the average denudation rates of the Mansehra and Sillai Patti granite gneisses have been computed to be (0.262±0.007) and (0.274±0.034) mm/yr, respectively, on the basis of zircon fission-track ages for the period between 25 Ma and the present time. This indicates that the two complexes have experienced similar average cooling/uplift-induced denudation histories during the last 25 Ma or so.     

The obsidian from Quiron (Salta Province, Argentina): a new reference glass for fission-track dating

In the course of a geochronological study of the volcanic activity in the Andean Cordillera in northern Argentina, we have found in the El Quevar volcanic complex a phenocryst poor obsidian (Quiron obsidian) showing an unusually high spontaneous track density. Defects which might produce “spurious” tracks are virtually absent. Application of fission-track dating using an absolute approach, based on the IRMM-540 standard glass for neutron fluence measurements, yielded an apparent age of and a plateau age of .A ⁴⁰Ar–³⁹Ar isochron age on biotite of was already available for the Quiron rhyolite. We determined further ⁴⁰Ar–³⁹Ar ages on several chips of the glass itself using two analytical approaches: total fusion with a focussed laser beam (LTFA) and a step-heating approach using a de-focussed laser beam (LSHA). We have obtained a weighted average of , an isochron age of and an integrated age of for LTF analyses, and a w.a. of , an iso.a. of and an int.a. of for LSH analyses (all age errors are 2σ).The Quiron obsidian is very easy to analyse for its high spontaneous track density and because microlites which might produce spurious tracks are very rare. Independent reference ⁴⁰Ar–³⁹Ar ages determined in different laboratories are available. For these reasons we believe that this glass may be very useful for testing fission-track system calibrations and apparent age correction procedures.Splits of obsidian Quiron will be distributed upon request to colleagues who intend to test it.     

Apatite fission-track data from upper Cretaceous formations in the Yuan'an Graben (China): Constraints on the timing of synsedimentary fault activity

Apatite fission-track signatures of upper Cretaceous Formations in the Yuan'an Graben are made to constraint on the timing of the Yuan'an and Tongchenghe synsedimentary fault activity. The apatite fission-track ages range from 102.0 ± 14.6 to 84.1 ± 3.7 Ma with P(χ²) >0.05; the mean confined track lengths of 14.18 ± 0.09 and 14.16 ± 0.08 μm with mean D_par values of 2.25 ± 0.02 and 2.26 ± 0.03 μm, respectively. These data are interpreted as dating their source-area exhumation, recording the exhumation and cooling of the footwall during major normal faulting. The results indicate that the timing of the Yuan'an and Tongchenghe synsedimentary fault activity occurred at 117–82 Ma and the intensive movement at 100–82 Ma; the onset time of extension in Jianghan Basin is ca. 117 Ma, which is related to the lithospheric extension associated with the subduction of the Pacific Plate beneath the Asian Plate.     

Cosmic history of the Bragin pallasite: evidence from the fission-track analysis

Chronology is rather a weak point in the investigation of pallasites, the stony-iron meteorites. No chronological data are known for the Bragin pallasite. Our attempt to reconstruct its cosmic history was based on the interpretation of fission-track analysis data. To apply this method only uranium-rich phosphates can be used. Extremely rare grains of stanfieldite were extracted from the silicate sawing residue and from the pallasite sample directly.

The researches pursued by us made it possible to find two populations of fossil tracks in stanfieldite grains. The tracks of these populations strongly differed both in size, shape and character of distribution. The first population, consisting of short (L2–6 μm instead of L8–12 μm for induced fission tracks), round-shaped tracks irregularly distributed, as we suppose, suffered an intense heating process, which caused a significant amount of partial annealing. The second population, consisting of longer (L8–12 μm), rhombic-shaped tracks homogeneously distributed, occurred after this thermal event. Only the second population track density was used for the fission-track age calculation.

After correction of the fossil track density, consisting of the second population tracks, for other possible track sources, the revealed tracks were unequivocally identified as those due to the spontaneous fission of ²⁴⁴Pu and ²³⁸U. The largest part of them was attributed to the spontaneous fission of ²⁴⁴Pu; ρ_Pu/ρ_U≈3. The model fission-track age of the studied pallasite turned out to be 4.20 Gyr. This value fix the time of the last shock/thermal event in the cosmic history of the Bragin pallasite, which had caused the partial annealing of tracks presented to that time and “fission-track clock” reset.     

Fission-track dating of South American natural glasses: an overview

Although many glass-bearing horizons can be found in South American volcanic complexes or sedimentary series, only a relatively few tephra and obsidian-bearing volcanic fields have been studied using the fission-track (FT) dating method. Among them, the volcanics located in the Sierra de Guamaní (east of Quito, Ecuador) were studied by several authors. Based upon their ages, obsidians group into three clusters: (1) very young obsidians, old, (2) intermediate-age obsidians, old, and (3) older obsidians, old. The FT method is also an efficient alternative technique for identification of the sources of prehistoric obsidian artefacts. Provenance studies carried out in South America have shown that the Sierra de Guamaní obsidian occurrences were important sources of raw material for tool making during pre-Columbian times. Glasses originated from these sources were identified in sites distributed over relatively wide areas of Ecuador and Colombia.

Only a few systematic studies on obsidians in other sectors were carried out. Nevertheless, very singular glasses have been recognised in South America, such as Macusanite (Peru) and obsidian Quiron (Argentina), which are being proposed as additional reference materials for FT dating.

Analyses of tephra beds interstratified with sedimentary deposits revealed the performance of FT dating in tephrochronological studies. A remarkable example is the famous deposit outcropping at Farola Monte Hermoso, near Bahia Blanca (Buenos Aires Province), described for the first time by the middle of the 19th century by Charles Darwin.

Considering the large number of volcanic glasses that were recognised in volcanic complexes and in sedimentary series, South America is a very promising region for the application of FT dating. The examples given above show that this technique may yield important results in different disciplinary fields.     

U and Th thin film neutron dosimetry for fission-track dating: application to the age standard Moldavite

Neutron dosimetry based on U and Th thin films was used for fission-track dating of the age standard Moldavite, the central European tektite, from the Middle Miocene deposit of Jankov (southern Bohemia, Czech Republic). Our fission-track age () agrees with a recent ⁴⁰Ar/³⁹Ar age, , based on several determinations on Moldavites from different sediments, including the Jankov deposit. This result indicates that the U and Th thin film neutron dosimetry represents a reliable alternative for an absolute approach in fission-track dating.     

The role of crystallographic angle in characterizing and modeling apatite fission-track length data

Recent experimental work on apatite fission-track length measurement has documented a number of factors that can exert a considerable influence on determinations of mean length. The principal source of length variation is anisotropy of annealing and etching behavior with respect to crystallographic angle. Procedural and environmental factors that influence which angular populations are preferentially sampled during measurement lead to variations in mean length that exceed standard statistical predictions. This paper evaluates the possibility of using c-axis projection to remove angular effects, and thus make length data more reproducible and informative.     

Signatures of structural recovery in colloidal glasses

Colloids near the glass concentration are often taken as models for molecular glasses. Yet, an important aspect of the dynamics of molecular glasses, structural recovery, has not been elucidated in colloids. We take advantage of a thermosensitive colloidal suspension to study the structural recovery after concentration jumps by using diffusing wave spectroscopy. The three classical aging signatures observed in molecular glasses are studied and the results are compared with those typical of molecular glasses. For the intrinsic isotherms, unlike molecular glasses, the colloid shows huge changes in relaxation time at equilibrium while the times required to reach the equilibrium state are nearly constant. For asymmetry of approach, we find a similar nonlinearity to that observed in the molecular glasses. For the memory experiment, while a memory effect is seen, the response is qualitatively different from that in molecular glasses.     

Effect of neutron-irradiation on optical properties of SiO2-Na2O-MgO-Al2O3 glasses

Silica based glasses are used as nuclear shielding materials. The effect of radiation on these glasses varies as per the constituents used in these glasses. Glasses of different composition of SiO₂-Na₂OMgO-Al₂O₃ were made by melt casting techniques. These glasses were irradiated with neutrons of different fluences. Optical absorption measurements of neutron-irradiated silica based glasses were performed at room temperature (RT) to detect and characterize the induced radiation damage in these materials. The absorption band found for neutron-irradiated glasses are induced by hole type color centers related to non-bridging oxygen ions (NBO) located in different surroundings of glass matrix. Decrease in the transmittance indicates the formation of color-center defects. Values for band gap energy and the width of the energy tail above the mobility gap have been measured before and after irradiation. The band gap energy has been found to decrease with increasing fluence while the Urbach energy shows an increase. The effects of the composition of the glasses on these parameters have been discussed in detail in this paper.      

Effect of some doping elements on the ionic conduction of mixed alkali-alumino-borosilicate glasses

Summary Some borosilicate glasses containing an equimolar ratio of different alkali types (K⁺ and Na⁺ ions) doped with 0.1 mole% R (R=Mn, Tb, Y, or Gd) were prepared. The results of d.c. electrical conductivity show that the absolute values of resistivities and the corresponding activation energies for the doped glasses are lower than those for glasses without doped ions. This behaviour is explained on the basis of the formation of elastic dipoles in such glasses and of the change of their fields owing to the existence of the doping ions. By thermal treatment of these glasses, it is found that the activation energies for the doped glasses are different from those obtained for glassess prepared without doping ions. This peculiarity is explained on the basis of the phase separation process in such glasses.     

Calibrated U-TH glasses for neutron dosimetry and determination of uranium and thorium concentration by the fission track method

We have prepared a series of standard uranium glasses. A primary standard containing 41 ppm of added U had its concentration and isotope ratio verified by mass spectrometry and by fission track techniques. Secondary standards were compared to the primary standard using the fission track technique. These glasses are useful for measuring both Th and U in solids. The U concentration in an unknown is determined by comparing the fission track density induced by thermal neutron irradiation with the corresponding density in the standard. Th is measured by a cyclotron irradiation using the same U standards. The glasses are also useful for neutron dosimetry. Investigators may obtain small samples of most of these standards from R. H. Brill of the Corning Museum of Glass. Although most of the glasses were fabricated from depleted uranium, one standard containing natural uranium is available. A standard Th glass with concentration of 41 ppm was also made.     

TiO2对磷酸钙玻璃结构影响的Raman光谱研究

利用Raman光谱、X射线衍射(XRD)和热分析(DSC)技术研究了包容TiO2的磷酸钙玻璃的结构特点.结果表明,TiO2添加量小于3 mol%时,与磷酸钙形成均质玻璃.添加量为6～12 mol%时,玻璃基体中生成了Ca2P2O7和CaTi4(PO4)6品相.随着TiO2的加入,玻璃体系中发生了偏磷酸盐向焦磷酸盐和正磷酸盐的结构转变.TiO2添加量小于3 mol%时,加入TiO2增强了玻璃结构的内聚力,使得玻璃转变温度逐渐提高,玻璃的热稳定性增强.