Variations of characteristics of fission tracks in muscovites by thermal effects

The applicability of the method of uranium fission tracks was studied in a number of sheets from a muscovite of about 600 m.y. Diagrams for fission track density (ρ) versus the chemical etching time (t_A), as well as the larger diameter of the transversal section (D) versus t_A curves were constructed, and the uranium concentration and uniformity of distribution were calculated. Some samples were heated at different constant temperatures for various time intervals, and the features presented by the heated and unheated tracks were analysed. The conclusions can be applied to corrections in age determination of muscovites that suffered natural annealing effects.     

Fission track ages of North American tektites

Fission track ages of North American tektites from Texas, Georgia and Martha's vineyard range widely between 0.8 my and 34.2 my with the tektites from Georgia giving the lowest ages. Size studies of the fission track etch pits reveal the thermal history of each individual tektite resulting in the partial loss of tracks. Therefore, the measured fission track ages can be understood as thermally lowered.

Based on annealing experiments, correction factors for the thermally lowered fission track ages are found. The weighted mean of the corrected fission track ages is 34.9 my for the bediasites and the Martha's Vineyard tektite. On the other hand, the corrected fission track ages of the Georgia tektites are much lower resulting from a more complex thermal history.

It may now be stated that North American tektites and Libyan Desert glasses cannot have had a simultaneous origin.     

The238U spontaneous fission decay constant re-determined by fission tracks

The decay constant(λ_f²³⁸) for the spontaneous fission of²³⁸U was re-determined by means of a man-made uranium glass of known age (126 yr). The spontaneous U fission tracks that had accumulated since the date of manufacture were counted on internal faces of the glass with an error of less than 1.7%. No thermal annealing of the spontaneous tracks was observed. The U content was determined by induced fission tracks. The value obtained forλ_f²³⁸ is(8.57 ± 0.42) × 10^?17yr^?1. Main sources of error are the date of glass melting and the determination of the thermal neutron dose.     

The registration-temperature dependence of heavy-ion track-etch rates and annealing sensitivity in crystals: implications for cosmic ray identification and fission track dating of meteorites

Etch rates and etchable lengths of cosmic ray tracks in meteoritic crystals have been used by several workers to derive the charge spectrum of ancient cosmic rays. This is done by comparing the fossil cosmic ray track record with fresh accelerator-produced calibration tracks. These calibration tracks are generally produced at room temperature, while meteorites spend a high proportion of their lifetimes orbiting at large distances from the Sun ( 3–5 AU) and are, consequently, at much lower temperatures (typically 100–150 K) during most of their cosmic ray exposure ages. We have irradiated crystals of apatite, olivine, enstatite and diopside held at 77, 293, 473 and 573 K, with 2 MeV/nucleon⁸¹Br ions, and then etched them. We find that their track etching properties are dependent upon the temperature of the mineral during registration. The track etch velocity generally increases with registration temperature up to 300 or 500 K (the upper limit depending upon the type of crystal). Our results also indicate that the annealing sensitivity of fission tracks in fluorapatite may be influenced by the registration temperature. This temperature dependence has important implications not only for cosmic ray particle identification but also for fission track dating of meteorites in view of the fact that the meteorite parent bodies were at elevated temperatures at the beginning af their life when²⁴⁴Pu fission tracks were being generated abundantly.     

磷灰石裂变径迹退火动力学模型研究进展综述

裂变径迹退火过程是非线性动力学过程，在一系列磷灰石裂变径迹室内退火实验基础上，研究者提出许多经验退火模型，其中扇型模型与实验数据拟合最好，多组分退火模型将扇型模型的应用扩展到复杂物源成分的磷灰石，理论物理退火模型建立在原子尺度动力机制上，并经由实验数据确定具物理意义的未知参数，目前还不如经验模型成功，室内退火实验表明除了温度、受热时间，退火行为与磷灰石的化学成分，径迹与结晶c轴的方位关系、由Dpar定量表征的颗粒溶蚀度等因素有密切关系。     

Characteristics of fission tracks in zircon: Applications to Geochronology and Cosmology

We describe a simple method for nearly isotropic revelation of fission tracks in zircon. The etchant is an equivolume mixture of 48% HF and 98% H₂SO₄; etching is carried out under pressure at 150° – 180°C. For fossil track densities above ～ 5 × 10⁶, the etching time is strongly anticorrelated with track density. The total etchable fission fragment track length (2 fragments) is 11 ± 0.5 μm. The length distribution is sharply peaked; the standard deviation in different samples is ± (0.7 – 0.8) μm. Thermal annealing studies indicate a lower activation energy for track fading than was previously repoted using a phosphoric acid etchant. A practical chemical dissolution method is described for quantitatively recovering zircon crystals from rock samples; this method should find application in fission track dating of even zircon poor rocks.     

Thermo-tectonic history of Ryoke Basement in Hohi volcanic zone, northeast Kyushu, Japan: Constraints from fission track thermochronology

Abstract Apatite and zircon fission track ages from Ryoke Belt basement in northeast Kyushu show late Cretaceous, middle to late Eocene, middle Miocene and Quaternary groupings. The basement cooled through 240 ± 25°C, the closure temperature for fission tracks in zircon, mainly during the interval 74-90 Ma as a result of uplift and denudation, the pattern being uniform across northeast Kyushu. In combination with published K-Ar ages and the Turonian-Santonian age of sedimentation in the Onogawa Basin, active suturing along the Median Tectonic Line from 100-80 Ma, at least, is inferred. Ryoke Belt rocks along the northern margin of Hohi volcanic zone (HVZ) cooled rapidly through ∼100°C to less than 50°C during the middle Eocene to Oligocene, associated with 2.5-3.5 km of denudation. The timing of this cooling follows peak heating in the Eocene-Oligocene part (Murotohanto subbelt) of the Shimanto Belt in Muroto Peninsula (Shikoku) inferred previously, and coincides with the 43 Ma change in convergence direction of the Pacific-Eurasian plate and the demise of the Kula-Pacific spreading centre. Ryoke Belt rocks along the southern margin of HVZ have weighted mean apatite fission track ages of 15.3 ± 3.1 Ma. These reset ages are attributed to an increase in geothermal gradient in the middle Miocene combined with rapid denudation and uplift of at least 1.4 km. These ages indicate that heating of the overriding plate associated with the middle Miocene start of subduction of hot Shikoku Basin lithosphere extended into the Ryoke Belt in northeast Kyushu. Pleistocene apatite fission track ages from Ryoke Belt granites at depth in the centre of HVZ are due to modern annealing in a geothermal environment.     

The stability of fission tracks in epidote and vesuvianite

The stability of fission tracks in epidote and vesuvianite (idiocrase) was studied at different temperatures for several periods of time. The tracks in epidote are almost as heat-resistant as those in garnet; in vesuvianite they are much less stable. These results are in line with the rule that tracks in minerals with high lattice energies need higher activation energies to fade than in minerals with low lattice energies.     

Fission tracks in a white inclusion of the Allende chondrite — Evidence for244Pu

Fossil fission tracks have been found in a coarse-grained white inclusion of the Allende chondrite. Tracks are present in excess of those produced by²³⁸U spontaneous fission and cosmic rays. The ratio of excess tracks to²³⁸U tracks is ～20, intermediate to ratios previously observed in meteorites but much lower than might be expected in light of the high initial²⁴⁴Pu/U ratio measured in these inclusions from Xe isotope ratios.     

Age and intensity of thermal events by fission track analysis: The Ries impact crater

A thermal event reduces the number of previously registered fission tracks in a mineral dependent upon the track retention properties of the individual mineral. Apatite, sphene and zircon have retention properties over a wide range of temperatures (from 100° to 550°C); apatite data reveal information at lowest temperatures while sphene and zircon data are useful for higher temperatures.Thermal events within this temperature range of 100°C to about 550°C are suitable for study with this technique. The age of the event is determined from samples in which the fission tracks are completely erased, while minerals containing partially removed (erased) tracks provide information on the temperatures occurring during the thermal event.As a test case, the analysis of the temperatures developed by the meteorite impact which produced the Ries crater at 14.7 m.y. ago is presented.     

Characteristics of tracks of ions of 14⩽Z⩽36 in common rock silicates

This paper concerns the identification of heavy ion tracks in minerals by measurements of track-etch rates and total etchable track lengths. With beams of Si, Cl, Ti, Fe, Zn and Kr at energies up to 10.35 MeV/nucleon we have irradiated nine minerals commonly used to study fossil cosmic ray tracks in meteorites and lunar samples. From our measurements of etched track length as a function of residual range, we have determined response curves for various minerals as a function of ionization rate, using the expression previously derived by Price, Fleischer and Moak. These curves increase smoothly with ionization rate instead of rising abruptly at some critical value as was previously thought. We have shown that the track etch rate concept accounts qualitatively for total etchable track length distributions, but that the positions of the peaks of different elements in these histograms occur at shorter lengths for fossil tracks than for fresh tracks. Our annealing data indicate that, at maximum lunar surface temperatures, tracks in olivine, orthopyroxenes and feldspars may be significantly shortened whereas tracks in clinopyroxenes will not be affected. We cite additional evidence that gradual rearrangement of radiation damage at ambient temperature makes the properties of fresh tracks and of ancient tracks different. It is thus not surprising that the histograms of fresh and fossil tracks do not match perfectly.     

Quantitative treatment of annealing of charged particle tracks in common minerals

A semi-empirical approach has been used to obtain an analytical expression for the annealing of charged particle tracks in minerals as a function of time and temperature. The new formalism allows quantitative evaluation of the extent of annealing in samples for any specified temperature and track production history. The validity of the present approach depends upon the applicability of the Arrhenius equation to the process of annealing of nuclear tracks in solids. Laboratory track annealing studies confirm the predictive aspects of the present approach and also the applicability of Arrhenius' equation to track annealing over a wide time-temperature range (5 seconds/1200 K to 5 × 10⁶ seconds/500 K). The present quantitative treatment of track annealing in rock minerals should permit an in-depth study of a variety of problems in meteoritics and earth sciences.     

Fission track tectonics: The tectonic interpretation of fission track apatite ages

Absolute uplift rates, regional uplift patterns, and time limits for uplift and fault movements can be studied with fission tracks in apatite. This is demonstrated for about 50 apatites from the Swiss and Italian Alps. Due to the relatively low thermal stability of tracks in apatite, the fission track ages of apatites from this area define the time when these rocks cooled down to temperatures to 125 ± 20°C.     

A crystal-chemical basis for Pb retention and fission-track annealing systematics in U-bearing minerals, with implications for geochronology

This study develops an empirical crystal-chemical framework for systematizing the kinetics of Pb loss and fission-track annealing in U-bearing minerals. Ionic porosity, Z (the fraction of a mineral's unit-cell volume not occupied by ions) potentially accounts for kinetic behavior by monitoring mean metal-oxygen bond length/strength. Various tests of a general kinetics-porosity relationship are presented, based upon diverse mineral data including: (1) Pb diffusion parameters; (2) measured closure temperatures (T_C) for fission-track annealing and (3) retentivities of both Pb and fission tracks, from apparent-age data. Every kinetic parameter (including T_C and mineral age for both the U/Pb and fission-track systems) is inversely correlated with Z within the sub-assemblage: zircon (Z ≈ 29%), titanite ( 34%) and apatite ( 38%). Assuming a diffusional closure model, Pb isotopic transport phenomena are described by a T_C-Z scale “calibrated” with field-based T_C data for titanite (≥ 680 ± 20°C) and apatite ( 500°C). Extrapolation of this scale yields T_C estimates for the following minerals: staurolite (T_C ≥ 1060°C, Z ≈ 25%); garnet (≥ 1010°C, 26.5%); zircon (≥900°C); monazite, xenotime, and epidote (≥ 750°C, 32%); and Ca-clinopyroxene (≥ 670 ± 30°C, 34 ± 1%, depending on composition). These empirical results imply that a (U/)Pb/Pb date for staurolite or garnet records the time of mineral growth, not post-growth isotopic closure, as also concluded in recent field studies. Because Z systematizes fission-track annealing, this recrystallization process, like volume-diffusion, must also be rate-limited by the strength of chemical bonds. The extent to which other recrystallization processes are likewise rate-limited is important to U/Pb geochronology because they potentially compete with diffusion as mechanisms for Pb-isotopic resetting in nature.     

Counting fission tracks in mica external detectors

The different ways in which minerals and external detectors respond to chemical etching are outlined. While it is prossible that the distribution of angles of inclination of fission tracks in some materials can be determined from the measurement of the aspect ratios of the etch pits this cannot be done using mica detectors. A method of measuring the distribution of angles of inclination in mica detectors based on the frequency distribution of areas of fission track entry holes is proposed. The results of two experiments are presented, and a means of correcting the number of tracks lost during etching is described.     

Fission track age and cooling rate of the Marjalahti pallasite

Nuclear tracks were studied in olivine and merrillite (phosphate previously called whitlockite) from the Marjalahti pallasite. The merrillite contains an important fission contribution due mainly to the spontaneous decay of now extinct²⁴⁴Pu. The U contents of 29 merrillite grains range from 60 to 140 ppb (median value: 85 ppb). Assuming a reasonable fractionation temperature of ～ 1750 K for the pre-pallasitic material, a lower limit of ～ 5 K/Myr is obtained for the cooling rate, in strong contrast with the previous metallographic result (～ 0.5 K/Myr). This disagreement, together with those observed in the case of mesosiderites, strengthens the need for a revision of the metallographic method of retracing the cooling histories of meteorites, as suggested by Narayan and Goldstein [31].     

用裂变径迹法研究断层活动年龄的初步探讨

裂变径迹年代测定是研究断层活动年龄的新方法,它基于磷灰石中裂变径迹的退火效应。本文报导了取自郯庐断裂带等地区12个磷灰石和一个榍石的裂变径迹年龄,结果表明,在断裂带中磷灰石裂变径迹的退火效应非常明显,效应随离断层带的距离增加而减弱。结合断层带的活动特点和原岩的年龄,对断层带中磷灰石裂变径迹年龄的解释及今后的研究方向作了探讨     

Uranium and boron distributions in some oceanic ultramafic rocks

From mica fission-track maps the serpentinized and weathered portions of four ultramafic rocks from oceanic ridge systems contained 0.5 to 2.4 ppm U compared to only 2.5 ppb in clinopyroxene, 0.6 ppb in chromite and less than 7.0 ppb in olivine. Orthopyroxene grains contained 0.4 ppb U which is three orders of magnitude lower than had previously been reported.Long thin tracks from (n, α) reactions with boron were recorded in cellulose nitrate plastic and were counted like fission tracks. The track density from boron was 2×10⁴ times higher than that from uranium fission alone. Boron in serpentine was variable on a 50-μm scale attaining 155 ppm concentrations. Orthopyroxene grains, in contrast, had maximum concentrations of 0.8 ppm.Most of the uranium and boron in the rocks is believed to have been introduced during serpentinization. From known crystal-melt partitioning ratios the uranium and boron distributions are consistent with the ultramafic rocks being cumulates or residues from partial melting events.     

Analyses of fission track etching and annealing phenomena in muscovite

Variation of fission track density with etching time has been analysed to standardize the etching conditions. The relations between the etching parameters (i.e., temperature, concentration and time) have been established. The differences in unheated samples and samples heated for different times at a constant temperature have been studied Graphs of thermal annealing vs. optimum etching time and of density reduction vs. length shrinkage have been constructed for applications in fission track geochronology.