The spontaneous fission rate of U-238 and fission track dating

The rate of spontaneous fission decay of uranium-238 (λ_f²³⁸) was determined in 4π-geometry by the fission track method. Uranium glasses of known age of manufacture were used. Spontaneous tracks have accumulated since the time of manufacture and induced tracks to determine the uranium content were produced by thermal neutron irradiations. Spontaneous tracks in all glasses were found to be partially annealed. By correcting for this annealing effect, a (λ_f²³⁸) = 8.7 ± 0.6 × 10^?17yr^?1 was obtained. Uncertainty in the neutron dose is the largest source of error.     

锆石的裂变径迹年龄测定技术

六十年代初发展起来的核裂变径迹测定年龄技术,已在地质年代学、宇宙年代学和考古工作等领域得到了广泛应用。裂变径迹测定年龄技术,是应用各种天然矿物或材料中微量杂质铀的自发裂变碎片,在绝缘固体介质中产生的辐射损伤痕迹(即裂变径迹)测定年龄的方法。该法的优点之一是应用范围广,在火成岩、沉积岩和变质岩中常可找到合适的测定对象。     

A determination of the decay constant for spontaneous fission of natural uranium using fission track accumulation

The decay constant for spontaneous fission of natural uranium has been determined by accumulating fission fragment tracks in Lexan plastic held adjacent to uranium metal for one year. The value, 6.8 ± 0.6. 10⁻¹⁷ yr⁻¹, is consistent with values most commonly used in fission track dating.     

On statistical estimation in fission track dating

Statistical methodology is described for obtaining point and interval estimates of age in fission track dating experiments, particularly for (a) the external detector method, which is reviewed briefly, and (b) the population method, which is considered in more detail. Some mathematical background to models, based on the underlying physical process, is given. This is used to suggest appropriate statistical methods, to assess them and to help understand sources of variation in the data. For the external detector method, a model is required for numbers of tracks given the amount of uranium; the usual Poisson model is derived as a consequence of independent thinning of a bivariate point process. For the population method, uranium heterogeneity does not necessarily invalidate the estimate but does represent a source of variation that must be allowed for in its error; thus an interval estimate of age might be relatively uninformative.     

玻璃包裹体裂变径迹在46O矿床中的应用

460矿床是火山岩型铀矿床。玻璃包裹体中裂变径迹研究表明:成矿岩体次流纹斑岩的原始铀含量为13.0ppm;裂变径迹测年龄法在强烈蚀变地区可以作为一种测年龄的有效辅助手段。     

On statistical models for fission track counts

The statistical basis for the usual analysis of fission track counts obtained by the external detector method is discussed and illustrated with examples. A consequence is that if any observed correlation between counts of spontaneous and induced tracks is due to heterogeneity in the density of uranium, then the model proposed by McGee and Johnson (1979)for assessing the experimental error is inappropriate and results based on it could be misleading. The same remark applies to the method proposed by Johnson, McGee, and Naeser (1979).     

Nuclear tracks,Sm isotopes and neutron capture effects in the Elephant Morraine shergottite

Nuclear track studies, uranium concentration measurements and Sm-isotope studies have been performed on both lithologies A and B of the Elephant Morraine shergottite, BETA 79001. Track studies show that EETA 79001 was a rather small object in space with a preatmospheric radius of 12 ± 2 cm, corresponding to a preatmospheric mass of 28 ± 13 kg. Phosphates have U-concentrations ranging from 0.3 to 1.3 ppm. There are occasional phosphates with excess fission tracks, possibly produced from neutron induced fission of U and Th, during the regolith exposure in the shergottite parent body (SPB). Sm-isotope studies, while not showing any clear cut excess in ¹⁵⁰Sm, enable us to derive meaningful upper limits to thermal neutron fluences of 2 to 3 × 10¹⁵ n/cm², during a possible regolith irradiation. These limits are consistent with the track data and also enable us to derive an upper limit to the neutron exposure age of EETA 79001 of 55 Myr in the SPB regolith.     

Fission track age of Stibiotantalite from Alto Ligonha,Mozambique

Fossil fission tracks in a stibiotantalite were revealed by etching and used for fission track dating. The fission track age of this specimen (2.3·10⁸ yr.) is much lower than Pb/Pb and Rb/Sr ages of other minerals from the same area. The explanation for this discrepancy could be the only moderate track retentivity of stibiotantalite which is similiar to muscovite.     

Fission-Track-Alter von Durango-Apatit,Mexiko

Spontaneous and induced fission tracks in apatite were revealed by etching with HNO₃ and used for fission track dating. The fission track age of specimens known as Durango-Apatite was found to be (36.3±3.5) · 10⁶ a.

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Die Untersuchungen wurden mit Mitteln des Österreichischen Fonds zur Förderung der wissenschaftlichen Forschung durchgeführt.

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Den Herrn Prof. Dr. M. Higatsberger und Dipl.-Ing. A. Burtscher von der Studiengesellschaft für Atomenergie in Seibersdorf verdanken wir die sorgfältige Durchführung der Bestrahlung der Proben.     

裂变径迹定年标准化的研究

用钴活化箔对SRM962标准铀玻璃进行了刻度,并比较了用钴活化范和用经过刻度的标准铀玻璃测量中子注量的定年方法,结果表明后者的定年结果优于前者,两者都有正系统偏差。利用磷灰石、结石和榍石的7种年龄标准样品和SRM962标准铀玻璃进行多次刻度,发现3种年龄标准矿物的Zeta常数基本一致,为327.4±7.8(±2σ).     

Distinction between the Youngest Toba Tuff and Oldest Toba Tuff from northern Sumatra based on the area density of spontaneous fission tracks in their glass shards

Determination of the area density of spontaneous fission tracks (ρ_s) in glass shards of Toba tephra is a reliable way to distinguish between the Youngest Toba Tuff (YTT) and the Oldest Toba Tuff (OTT). The ρ_s values for YTT, uncorrected for partial track fading, range from 70 to 181 tracks/cm² with a weighted mean of 108 ± 5 tracks/cm², based on 15 samples. Corrected ρ_s values for YTT are in the range of 77–140 tracks/cm² with a weighted mean of 113 ± 8 tracks/cm², within the range of uncorrected ρ_s values. No significant difference in ρ_s exists between YTT samples collected from marine and continental depositional settings. The uncorrected ρ_s for OTT is 1567 ± 114 tracks/cm² so that confusion with YTT is unlikely.     

Fission-track ages of four meteorites

A method for selective annealing of cosmic-ray tracks has been developed, permitting determination of fission-track ages in the presence of a large background of cosmic-ray tracks. The mesosiderite Bondoc contains 41 fission tracks/cm², of which about 75% are due to neutron-induced fission of U²³⁵ during cosmic-ray exposure. Its net fission-track age is 140 ± 40 Myr, nearly identical to its cosmic-ray exposure age of 150 Myr. The mesosiderite Mincy has a fission-track age of 1500 ± 400 Myr.Nakhla (nakhlite) contains an excess of apparent fission tracks, which may be either genuine fission tracks from Pu²⁴⁴ or etch pits mimicking fission tracks in length, thermal stability, random orientation, and other characteristics. On the assumption that they are fission tracks, the Pu²⁴⁴/U²³⁸ ratio at the onset of track retention in Nakhla was (3.1 ± 1.3) × 10^?3, nearly an order of magnitude lower than the initial solar system ratio. This may reflect a chemical fractionation of Pu and U, or a late impact or magmatic event. Different minerals of the Washougal howardite have different Pu²⁴⁴/U²³⁸ ratios, from (24 ± 7) × 10^?3 to (2.3 ± 0.7) × 10^?3. This may imply a succession of impacts over a period of time. Additionally, Pu and U may have been chemically fractionated from each other in this meteorite.Shocked meteorites show no consistent pattern in the retentivity of fission tracks and of fissiogenic or radiogenic noble gases. Some meteorites, e.g. Bondoc, Serra de Magé, and Mincy, retain gases more completely than tracks; others, e.g. Nakhla and Allende, retain them less completely.Uranium was determined in feldspar and/or pyroxene from 19 Ca-rich achondrites and mesosiderites. For most, only upper limits of 0.01–0.03 ppb were obtained. Apparently the uranium in these meteorites resides almost exclusively in minor phases, as in terrestrial and lunar rocks.     

Duluth Complex apatites: Age reference material for LA–ICP–MS‐based fission‐track dating

Anorthositic series apatites of the Duluth Complex, Minnesota, USA, have high spontaneous fission‐track densities of up to ~10⁷ cm^–2 and a homogeneous age of ~900 Ma, allowing high‐precision fission‐track dating based on LA–ICP–MS U analysis. Absolute fission‐track dating, track‐length measurement and chemical composition analysis were performed to evaluate a cooling history, which is essential for age reference materials. Preliminary inverse modelling for a sample with a shortened track‐length distribution yielded a monotonic cooling history from ~100°C at 925 Ma. The apatites incur an over‐etching problem when employing the commonly used etching protocol involving 5.5 M HNO₃.     

相山铀矿田铀多金属成矿时代与成矿热历史

相山铀矿田的铀多金属矿化主要可划分为碱性铀矿化、酸性铀矿化、铅锌银铜矿化和金矿化四种类型。通过沥青铀矿和矿化岩石U-Pb等时线、黄铁矿Rb-Sr等时线、绢云母~(40)Ar-~(39)Ar同位素年龄测定,结合铀多金属成矿特征研究,厘定了相山铀矿田铀多金属成矿时代,确定铀多金属矿化的成矿时序为:碱性铀矿化、铅锌银铜矿化、金矿化、酸性铀矿化。锆石裂变径迹研究表明,相山矿田铀多金属矿化样品的锆石裂变径迹峰值年龄与U-Pb、Rb-Sr和~(40)Ar-~(39)Ar同位素年龄一致性良好,裂变径迹年龄(峰值年龄)可以限定热液铀多金属成矿热事件时代。碱性铀成矿热事件的锆石裂变径迹峰值年龄为119. 8～125. 6Ma;金成矿热事件和铅锌银铜多金属成矿热事件的锆石裂变径迹峰值年龄为106. 1～113. 8Ma;酸性铀成矿热事件的锆石裂变径迹峰值年龄为86. 7～100. 0Ma;新发现一期锆石裂变径迹峰值年龄为66. 4～78. 6Ma的热事件,该期热事件可能为相山矿田最晚一期酸性铀成矿热事件。相山矿田66. 4～78. 6Ma的铀成矿热事件,与华南花岗岩型热液铀矿床的区域成矿热事件时代耦合,该发现对华南火山岩型铀矿成矿时代的重新认识,对火山岩型、花岗岩型铀矿床成矿统一性认识具有重要意义。     

裂变径迹法在研究造山带隆升过程中的应用介绍

介绍了４种类型的造山带隆升－冷却模式,不同模式对应不同的磷灰石裂变径年龄－－高程图特征;了不同构造造背景下的热历史特征一裂变径迹年龄序列。裂变径变宅遮挡同统计分布型式对应于不同的热历史,具有不同的年代学意义。     

Precision,accuracy and meaning of fission track ages

The precision, meaning, and accuracy of the fission track (ft) dating method are reviewed from an examination of the recent literature as well as previously unpublished data from the author's laboratory. It is concluded that forapparent ft ages (i.e. ages derived from the canonical age equation) a precision (2σ level) of the order of ±4% to ±5% can be reached provided that (i) uranium is sufficiently homogeneously distributed in the dated samples, at least locally; and (ii) a large enough number of tracks can be counted. Model ft ages,i.e. ages for which partial geological track annealing is taken into account, have variable degrees of precision. While model ages obtained with the track-size method seem, as evaluated from the literature, to have usually a limited precision of the order of ±30% (2σ), plateau ages usually have a precision better than ±5% at a 2σ confidence level. Because it provides an objective test on the accuracy of track identification, as well as some insight of the variability of closing temperatures between various samples of a given mineral phase, the Isochronal Plateau (icp) method, when applicable, will be preferred (Poupeauet al 1980a). However, for phases which could be damaged by heating at relatively high temperatures, as for example hydrated glass shards from tephra, an Isothermal Plateau (itp) approach is to be preferred. Due to uncertainties about the value of the²³⁸U spontaneous fission decay constant λ _f as well as difficulties inherent in the dosimetry of thermal neutrons in nuclear reactors, theft method of dating is not an independent one. Presently, it relies on the existence of geological standards (volcanic rocks) of known age, allowing to determine anoperational ‘λ _f ’ value (Naeseret al 1980). Accordingly, the accuracy of anft age is limited by the accuracy on the age of the standard. It should be better than ～5%. For volcanic, hypovolcanic rocks, and shallow intrusives, theft method dates the time of formation, provided they were not further reheated. More generally, the track method providescooling ages. Closing temperatures calculated from laboratory experiments vary from ?300°C to 100°C, according to minerals, for slow cooling rates (～1°C/m.y.). For apatites, recent geological calibrations (Naeseret al 1980; Gleadow and Duddy 1980) confirmed laboratory extrapolations. The association of theft method with other geochronometers is therefore critical to the study of the cooling history of old cratons as well as to the evaluation of uplift/erosion rates in recent belts.     

Testing inferences from palaeocurrents: application of zircon double‐dating to Miocene sediments from the Hengchun Peninsula,Taiwan

Detrital zircon provenance studies that combine low‐temperature fission‐track and high‐temperature U–Pb single‐grain age dating are powerful in constraining sediment provenance by documenting the tectonothermal evolution of the sediment source(s). We apply these techniques to Miocene (12–6 Ma) sandstones of the Hengchun Peninsula, southern Taiwan, which, based on diametrically opposite palaeocurrents, have a controversial provenance. U–Pb grain ages range from the Miocene (8 Ma) to Archaean (2.5 Ga). Cretaceous thermal cooling is recorded by detrital zircon fission tracks at Loshui and Lilongshan. Permian fission‐track grain ages account for <33% of zircon ages from Loshui, while at Lilongshan, Jurassic/Triassic grain ages account for 33–66% of all zircon fission‐track ages. Minor (<12%) Miocene age components are detected in both formations. These new data suggest that the primary sources of these Miocene sediments were similar. A simple model is proposed invoking sediment reworking in this complex tectonic setting. Terra Nova, 00, 000–000, 2010     

裂变径迹LA-ICP-MS/FT法原理、实验流程和应用

裂变径迹是一种重要的低温热年代学测年方法,目前被广泛应用于岩体冷却年龄测试及各种构造演化热历史等的研究.该方法主要包括传统的外探测器法和近年来新开发的激光-电感耦合等离子质谱法（LA-ICP-MS/FT）.由于传统的外探测器法一直受限于国内反应堆中子热化不充分、辐照周期长以及辐照带来的环境安全等问题,无法有效、快速地获得数据.相反, LA-ICP-MS/FT法的建立和应用不仅避免了因中子热化不充分、热中子分布不均带入的误差而提高了测年精度,而且还极大地提高了测试效率,缩短了解决科学问题的时间.通过对近年来新建立的裂变径迹LA-ICP-MS/FT法的测年基本原理和方法、实验步骤、常用年龄值和裂变径迹退火等的详细介绍,综述了该方法在相关应用方面的研究进展,并与传统的外探测器法进行了详细的对比.最后综述了LA-ICP-MS/FT法在造山带冷却隆升历史、沉积盆地分析、盆-山耦合系统分析、断裂活动、成矿时代和矿体保存等多方面的应用.     

Exhumation rates and age of metamorphism in the Sanbagawa belt: new constraints from zircon fission track analysis

Zircon fission track dating and track length analysis in the high‐grade part of the Asemigawa region of the Sanbagawa belt demonstrates a simple cooling history passing through the partial annealing zone at 63.2 ± 5.8 (2 σ) Ma. Combining this age with previous results of phengite and amphibole K–Ar and ⁴⁰Ar/³⁹Ar dating gives a cooling rate of between 6 and 13 °C Myr^?1, which can be converted to a maximum exhumation rate of 0.7 mm year^?1 using the known shape of the P–T path. This is an order of magnitude lower than the early part of the exhumation history. In contrast, zircon fission track analyses in the low‐grade Oboke region show that this area has undergone a complex thermal history probably related to post‐orogenic secondary reheating younger than c. 30 Ma. This event may correlate with the widespread igneous activity in south‐west Japan around 15 Ma. The age of subduction‐related metamorphism in the Oboke area is probably considerably older than the generally accepted range of 77–70 Ma.     

滇西金满铜矿床成矿年龄测定

采用石英流体包裹体Rb Sr等时线法和磷灰石裂变径迹法对滇西金满铜矿床的成矿时代进行测定。研究结果表明燕山晚期为金满矿床的初始成矿期 ,喜山期是主要成矿时期 ;流体包裹体的Rb Sr法和裂变径迹法测定热液矿床的成矿年龄具有一定的意义 ,但需进一步探讨 ;锶同位素数据为成矿流体来源提供了进一步的证据