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.     

The effect of weathering on fission track dating

Fission track ages have been determined on sphene, apatite, and zircon separated from fresh and highly weathered rock samples. Sphene and zircon ages are unaffected by even extreme weathering. Apatite from partially weathered granite gives a similarly unaffected age. Badly corroded apatites from a residual clay, however, do show a 17% reduction in apparent age, involving loss of both spontaneous fission tracks and uranium from the crystals.     

Uplift history and structure of the Transantarctic Mountains: new evidence from fission track dating of basement apatites in the Dry Valleys area, southern Victoria Land

Fission track analysis of apatites from basement rocks of the Wright Valley in southern Victoria Land provides information about the timing, the amount and hence the rate of uplift of the Transantarctic Mountains in this area. Apatite ages increase systematically with elevation, and a pronounced break in the age versus elevation profile has been recognised at about 800 m on Mt. Doorly near the mouth of Wright Valley. The apatite age of about 50 Ma at this point approximates the time at which uplift of the mountain range began. Samples lying above the break in slope lay within the apatite fission track annealing zone prior to uplift, during a Cretaceous to Early Cenozoic period of relative thermal and tectonic stability. At the lower elevations samples had a zero apatite fission track age before the onset of rapid uplift and have track length distributions indicating rapid cooling. Some 4.8–5.3 km of uplift are estimated to have occurred at an average rate of about 100 ± 5m/Ma since uplift began. From the total stratigraphic thickness known above the uplifted apatite annealing zone it can be estimated that the Late Cretaceous/Early Cenozoic thermal gradient in the area was about 25–30°C/km.The occurrence and pattern of differential uplift across the Transantarctic Mountains can be estimated from the vertical offsets of different apatite fission track age profiles sampled across the range. These show the structure of the mountain range to be that of a large tilt block, dipping gently to the west under the polar ice-cap and bounded by a major fault zone on its eastern side. Offset dolerite sills at Mt. Doorly show the mountain front to be step-faulted by 1000 m or more down to the McMurdo Sound coast from an axis of maximum uplift just inland from Mt. Doorly.     

Fission track geochronology of King Island,Bass Strait,Australia: Relationship to continental rifting

Fission track ages have been determined on sphene and apatite from the granitic rocks of King Island in Bass Strait, southeastern Australia. In all cases sphene and apatite ages are markedly discordant. Sphene ages compare very closely to earlier KAr measurements and indicate an emplacement age of about 350 m.y. for the east coast group of granites and their important scheelite mineralization. Apatite ages are all younger by about 80–200 m.y. suggesting that fission tracks were not fully retained in this mineral until the Cretaceous. During the Cretaceous King Island was at the edge of the developing Otway Rift Valley which resulted in the breakup of Australia and Antarctica. Uplift of the basement rocks along the rift margin with consequent rapid erosion allowed the apatites to cool below about 110°C and begin accumulating fission tracks for the first time. Differing degrees of uplift, at least partly fault controlled, have produced a regular pattern of apatite ages across the island. A relationship between apatite fission track ages and continental breakup may be a widespread phenomenon which could give valuable insight into the thermal and tectonic development of rifted continental margins.     

应用裂变径迹不同模式约束岩体冷却史的初步探讨—以滇西独龙江岩体为例

在利用磷灰石裂变径迹热年代学方法来约束岩体冷却史的应用中,由于地质条件和磷灰石退火性质的限制,表观年龄往往不能直接代表特定地质事件的时间.利用封闭径迹的长度直方图模式和高程-年龄模式可定性地反映出岩体冷却史是否遭受过干扰.平均径迹长度-年龄(或香蕉图)模式、径迹年龄谱模式以及反演模拟在某种程度上可用来限定地质过程中冷却启动的时间.然而,对于多阶段的冷却史,模式和模拟分析的不确定性仍较显著,对早期时限的定量揭示仍是研究和应用中的一个难点.只有结合多种方法、模式和模拟的运用并考虑地质背景才能较清晰地约束岩体的冷却历史.     

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.     

Early Cretaceous uplift and erosion of the northern Appalachian Basin, New York, based on apatite fission track analysis

The thermal history of outcropping Devonian sediments of the northern Appalachian Basin, New York, has been investigated using fission track analysis of detrital apatites from 57 sandstone samples. Based on lengths and apparent age measurements using fission tracks in apatite it is concluded that Lower Devonian sediments presently at the surface in the Catskill region were cooled rapidly from temperatures higher than about 110°C during Early Cretaceous times (120–140 Ma ago). In the western part of New York (Wellsville-Buffalo) data from late Devonian sediments are consistent with cooling at the same time as that identified for the Catskill region but from lower temperatures, in the range of approximately 80–110°C, the maximum temperature these sediments experienced since deposition. For a pre-uplift paleogeothermal gradient of 25–35°C/km, the confined track length data indicates uplift and erosion of 2–3 km for western New York and greater than 3–4 km for the Catskill region, a differential uplift pattern which is consistent with the historical stratigraphic data from the region. This conclusion is at variance with earlier interpretations put forth by others.Rapid broad scale uplift and erosion of the scale identified imply that large volumes of sediment could have been supplied from the northern Appalachian Basin during the Early Cretaceous. This timing for the dominant post-Devonian cooling phase in the basin is not accounted for by recent models of the tectonic evolution of the Appalachian Orogen but is compatible with the change from carbonate to siliciclastic deposition in the Atlantic coastal plain. It is suggested that this style of broad regional uplift without significant deformation is characteristic of a tectonic regime associated with, and subsequent to, continental rifting.Apatite fission track analysis is shown to be a basic tool in providing fundamental limits for thermal history assessment in regional tectonic problems.     

Fission track dating of zircon: Improved etching techniques

A simple new technique for the etching of fission tracks in zircon has been developed which overcomes the practical difficulties of previously described techniques. The etchant is a low melting-point binary eutectic of KOH and NaOH which is used at 200–220°C. As for previous etchants the etching time is strongly dependent on the spontaneous track density. The etched tracks have very similar characteristics to those produced by the earlier 100N NaOH etchant.     

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.     

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.     

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

Once 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.     

磷灰石裂变径迹分析新进展

综述了近十几年来磷灰石裂变径迹(AFT)分析在基础理论上的进展:AFT退火动力学的影响因素;AFT多元退火模型的建立;LA-ICP-MS裂变径迹定年及AFT分析自动化.总结了一些尚存分歧的研究问题:围压对AFT退火过程的影响,AFT年龄与磷灰石(U-Th)/He年龄的异常;特殊地质环境下AFT退火资料对退火模型的制约.最后展望了AFT分析的发展方向.     

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.     

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.     

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.     

孔雀河斜坡与库鲁克塔格隆起构造事件的裂变径迹证据

文章以塔里木盆地东北缘库鲁克塔格隆起与孔雀河斜坡盆山系统为主要研究对象,在该地区露头和钻井样品开展碎屑磷灰石、锆石裂变径迹研究,对库鲁克塔格构造演化中关键构造事件提供热年代学约束.锫石样品在加里东晚期-早海西期达到最大古地温,之后经历了长期的抬升降温过程,锆石最小峰值年龄记录了371～392 Ma 和328～305.7...     

Fission track ages on apatite of Bergell rocks from Central Alps and Bergell boulders in Oligocene sediments

Previous radiometric dating studies indicated that the Bergell region, in contrast to other regions of the Central Alps, experienced an early, rapid uplift, but with decreasing rate. Furthermore, there is also a geological record of the early uplift history of the Bergell granite by the existence of boulders which were derived from this granite and which occur in the Late Oligocene sediments of the Po plain.In this work the uplift history of the Bergell is studied in more detail by fission track dating of additional apatites from the Bergell region. Secondly, by determining apatite fission track ages the granitic boulders of the Po plain can be re-assigned to their original vertical position within the Bergell intrusive before erosion removed them in Late Oligocene time. A rather conservative estimate replaces them 6 km above the present morphology of the Bergell massif. Thus, the thickness of the Bergell granite must have been at least 8 km.Generally, fission track studies on boulders may become an important tool to study the vertical extent of mountain chains during the geological past.     

Zircon and apatite fission track analyses on mineralization ages and tectonic activities of Tuwu-Yandong porphyry copper deposit in northern Xinjiang, China

The mineralization ages reported in the past in the Tuwu-Yandong copper district not only are different,but also fall into the Hercynian epoch.This study has achieved 9 zircon and 7 apatite fission track analysis results.The zircon fission track ages range from 158 Ma to 289 Ma and the apatite ages are between 64 Ma and 140 Ma.The mineralization accords with the regional tectonics in the copper district.We consider that the zircon fission track age could reveal the mineralization age based on annealing zone temperature of 140―300℃ and retention temperature of ～250℃ for zircon fission track,and metallogenetic temperature of 120―350℃ in this ore district.Total three mineralization epochs have been identified,i.e.,289―276 Ma,232―200 Ma and 165―158 Ma,and indicate occurrence of the min-eralization in the Indosinian and Yanshan epochs.Corresponding to apatite fission track ages,the three tectonic-mineralizing epochs are 140―132 Ma,109―97 Ma and 64 Ma,which means age at about 100℃ after the mineralization.The three epochs lasted 146 Ma,108 Ma and about 100 Ma from ～250℃ to ～100℃ and trend decrease from early to late.It is shown by the fission track modeling that this district underwent three stages of geological thermal histories,stable in Cretaceous and cooling both before Cretaceous and after 20 Ma.