Fission Track Method Research Articles

Compositional effects on etching of fossil confined fission tracks in apatite

Abstract Fission-track analysis is a thermochronologic method for dating rocks and reconstructing their low-temperature thermal histories. We investigate the influence of the apatite composition on the etching of fossil confined fission tracks, and its consequences for the fission-track method. We conducted step-etch experiments with 5.5 M HNO3 at 21 °C on samples with etch pit diameters (Dpar) spanning most of the range for natural apatites (Panasqueira: 1.60 µm, Slyudyanka: 2.44 µm, Brazil: 3.92 µm, and Bamble: 4.60 µm) to determine their apatite etch rates vR (the rate at which each lattice plane is displaced parallel to itself) as a function of crystallographic orientation (ϕ’). Our measurements revealed significant differences between the four samples. We fitted three-parameter functions, vR = a(Dpar) ϕ’ eb(Dpar)ϕ’ + c, describing vR as a function of the angle to the apatite c-axis for our hexagonal samples (excluding Bamble) and Durango apatite. The parameters a and b both exhibit a linear correlation with Dpar, whereas the constant c is small (~0.1 µm.min-1) and its between-sample variation negligible at the resolution of our measurements. Bamble exhibits a different, bimodal relationship between vR and ϕ’, which we fitted with a sum of two sine functions. In all cases, including Bamble, there is a striking correlation between the angular frequencies of horizontal confined tracks and the magnitude of the apatite etch rate vR perpendicular to the track axes. This shows that the sample of confined tracks selected for measurement and modeling is to a much greater degree determined by the etching properties of the apatite sample than by geometric or subjective biases. The track etch rate vT is constant along most of the track length but varies from track to track. The mean vT correlates with Dpar, so that tracks etch to their full lengths in a shorter time in faster etching apatites. The mean rate of length increase between etch steps, vL, also correlates with Dpar. The length increments of individual tracks are however irregular. This points to an intermittent structure at the ends of the tracks.

Fission-track age calibration: Phi and Zeta, never the twain shall meet?

The fission-track method was intended to be an autonomous dating method requiring two track counts and a neutron fluence measurement. It became clear that it was beset with methodological problems, and neutron fluence measurements were abandoned in favour of age standards. The coming of mass-spectrometric uranium measurements eliminated the need for irradiations altogether, and stimulated renewed efforts to establish fission-track dating as an autonomous method. This ran into similar problems as before, and recent publications signal a return to age standards. The attempts at independent dating nevertheless revealed that experts agree on the recommended fission constant and the need to consider experimental factors related to the track counts. In spite of that, the renewed reliance on standards makes that fission-track ages are again not independent, but relative, ages with an uncertain meaning, and from their nature untestable.We report an attempt at standardless and standard-based dating of the Durango and Duluth apatites using neutron irradiation and the external detector method. We start from the assumption that neutron fluence measurements are the most precise and accurate achievable in fission-track dating, a consensus on the recommended fission constant, and on the need to consider experimental factors. One factor (ζ0) accounts for the combined experimental influences; ζ0 can be calculated from identifiable contributions related to track registration, etching and counting; ζ0 can also be determined by experiment and calculated from the ages of standards. This gives three separate, consistent, dimensionless calibration factors that, in contrast to Z and ζ, are independent of the neutron fluence or the uranium content of a reference glass; ζ0 remains a personal factor, however.The standardless fission-track ages of the Durango age standard are in agreement with its reference age and the standardless and standard-based ages of the Duluth apatites are consistent with each other. Furthermore, if ζ0 is used to correct conventional Z- (and by implication ζ-) values for experimental effects, the results are consistent with their definitions in terms of physical parameters. Although ζ0 is a personal factor, the small number of measurements to date is consistent with each other. In contrast, annealing corrections based on the fossil and induced confined fission track lengths overestimate the reference age of Durango and produce incompatible standardless and standard-based ages of the dated apatites. We argue that this points to unresolved issues with the relationship between the age and the fossil track length, and suggest a connection with etching.Plots of the single-grain ages against uranium content and radial plots of the data exhibit conspicuous structure. However statistical factors alone account for the nature and magnitude of the observed correlations. This implies that in the case of the Duluth apatites there is no identifiable effect of radiation damage on the single-grain ages despite their high accumulated α-doses and protracted thermal histories. Spurious correlations must be addressed before attempting to interpret the observed data structure in terms of radiation damage, apatite composition or age components.

Obsidian in Ethiopia: a Geoarchaeological perspective

In Ethiopia, obsidian is mainly found in the Afar Depression and the Main Ethiopian Rift. The compositional and petrological features of these obsidians vary. Some volcanic centers show that varying volcanic eruptions from a single center may not necessarily imply variable chemistry. Obsidian has been dated using K/Ar, Ar/Ar, and fission track methods to determine the time of eruption but there are also a few dates on artifacts by hydration dating to establish the time of tool manufacture. Many of the geological sources were utilized by prehistoric populations beginning at least since the Early Stone Age but obsidian became commonly used during the Middle Stone Age. Obsidian based stone tool use for scraping in Ethiopia persisted until recent times.

Short communication: Modeling competing effects of cooling rate, grain size, and radiation damage in low-temperature thermochronometers

Abstract. Low-temperature multi-thermochronometry, in which the (U-Th) / He and fission track methods are applied to minerals such as zircon and apatite, is a valuable approach for documenting rock cooling histories and relating them to geological processes. Here we explore the behaviors of two of the most commonly applied low-temperature thermochronometers, (U-Th) / He in zircon (ZHe) and apatite (AHe), and directly compare them against the apatite fission track (AFT) thermochronometer for different forward-modeled cooling scenarios. We consider the impacts that common variations in effective spherical radius (ESR) and effective uranium concentration (eU) may have on cooling ages and closure temperatures under a range of different cooling rates. This exercise highlights different scenarios under which typical age relationships between these thermochronometers (ZHe>AFT>AHe) are expected to collapse or invert (either partially or fully). We anticipate that these predictions and the associated software we provide will be a useful tool for teaching, planning low-temperature multi-thermochronometry studies, and for continued exploration of the relative behaviors of these thermochronometers in temperature–time space through forward models.

Thermochronometric Studies of Mountains in Island Arc Settings Based on Fission-track Method: A Review

Thermochronometric Studies of Mountains in Island Arc Settings Based on Fission-track Method: A Review

New fission-track ages of Australasian tektites define two age groups: discriminating between formation and reset ages

In this study tektites from across the Australasian strewn field (specifically, mainland Australia, Tasmania, Java, Malaysia) have been dated by the fission-track method. Twelve tektites from 9 localities define two age groups: one has a weighted mean age of 822 ± 20 ka based on 12 age determinations, and the other is 655 ± 20 ka based on 10 age determinations. The older group represents formation age of the tektites, and, given its error, is close to the precise 40Ar/39Ar age of the Australasian tektites at 788 ± 3 ka. Tektites of the younger group have a widespread distribution, occurring in Australia and southeast Asia. Their major- and trace-element composition is similar to the older tektites and their young age is thought to be due to heating subsequent to their original fall position, probably by wildfires that totally reset the fission-track clock. Soil surface temperatures, in the range of 500–800 °C, have been reported during experimental studies and would anneal all fission tracks in tektites within a few hours. The fission-track age record with its ~10% associated error on individual age determinations cannot precisely link thermal resetting to a particular elevated phase of wildfires. This task must await studies on high-resolution sedimentary sequences deposited around 650 ka. The view that these younger tektites were formed by a second impact event is discounted.

Thermochronology and exhumation history of the basement and sediments of the NNE border of the Paraná basin, Brazil

Combined zircon fission-track (FT) and U–Pb dating were used to obtain correlated cooling ages and tectonic processes in the eastern border of the Phanerozoic Paraná Basin, a large cratonic sedimentary basin in South America deposited over Precambrian basement, which is represented mainly by the Neoproterozoic Brasília Orogenic Belt. Zircon ages obtained for clastic sediments suggest that the provenance history of the Paraná Basin was derived chiefly from the proximal basement sources. Twelve zircon samples were collected, from which 194 grains were analyzed and simultaneously dated by the FT and U–Pb methods, and 146 zircon grains were additionally dated only through the fission-track method (FT). The standard sample of Fish Canyon Tuff (FCT) was also analyzed through FT and U–Pb, and the age results of 26.6 ± 1.1 Ma, and 28.33 ± 0.68 Ma, respectively, are consistent with the literature data. The fission-track ages, obtained for the Araxá (~330 Ma) and Canastra (~500 Ma) Groups, and Piumhi Massif (~700 Ma) pertaining to the Brasília Belt, in average increase towards the São Francisco Craton. This fission-track age pattern can be linked to the temperature and exhumation of each one of these units. The U–Pb detrital zircon ages obtained for the basement units range from 1.6 to 3.2 Ga and indicate different provenance sources in the evolution of the Brasília Belt. However, it is clear that the main sources of metasedimentary rocks are derived from the São Francisco-Congo Craton. For samples collected in the NNE border of the Paraná Basin, FT ages were obtained for 127 zircon grains with ages ranging between 100 and 600 Ma. Forty of these grains dated simultaneously with FT and U–Pb bear mean age populations of 0.6 and 1.0 Ga, and subordinately between 1.5 and 2.5 Ga. These results suggest that the units that represent the São Francisco Craton's Neoproterozoic passive margin were being formed with depositional sediments 1.0 Ga ago and their recycling was the main source for the zircon derivation recorded in the Paraná Basin sediments. An important methodological result of this work is that U–Pb radiometric dating was done in the same areas of single zircon crystals employed to determine the FT ages, which improved the concordance in the U–Pb ages. As such, U–Pb dating on zircon micro areas that have undergone leaching, metamictization or another type of lattice damage was avoided.

Zircon fission‐track and U–Pb double dating using femtosecond laser ablation–inductively coupled plasma–mass spectrometry: A technical note

AbstractWe present a new LA–ICP–MS system for zircon fission‐track (FT) and U–Pb double dating, whereby a femtosecond laser combined with galvanometric optics simultaneously ablates multiple spots to measure average surface U contents. The U contents of zircon measured by LA–ICP–MS and standardized with the NIST SRM610 glass are comparable to those measured by the induced FT method, and have smaller analytical errors. LA–ICP–MS FT dating of seven zircon samples including three IUGS age standards is as accurate as the external detector method, but can give a higher‐precision age depending on the counting statistics of the U content measurement. Double dating of the IUGS age standards gives FT and U–Pb ages that are in agreement. A chip of the Nancy 91500 zircon has a homogeneous U content of 84 ppm, suggesting the possibility of using this zircon as a matrix‐matched U‐standard for FT dating. When using the Nancy 91500 zircon as a U‐standard, a zeta calibration value of 42–43 year cm2 for LA–ICP–MS FT dating is obtained. While this value is strictly valid only for the particular session, it can serve as a reference for other studies.

Thermochronology and Exhumation History of the Northeastern Fennoscandian Shield Since 1.9 Ga: Evidence From 40Ar/39Ar and Apatite Fission Track Data From the Kola Peninsula

AbstractResults from thermochronological studies have multiple applications to various problems in tectonics and landform evolution. However, up to now a lack of thermochronological data from the northeastern Fennoscandian Shield has complicated the interpretation of tectonothermal evolution of the region. Here, we use both new and previously published multimineral 40Ar/39Ar data (amphibole, mica, and feldspar) on the various Precambrian magmatic and metamorphic complexes to reconstruct the thermal history of NE Fennoscandia within the Kola Peninsula area in the interval 1900–360 Ma. Using the apatite fission track method as well as a numerical model of the heating‐cooling process of northeastern Fennoscandia's upper crust, we have reconstructed its thermal evolution for the interval 360–0 Ma. According to our model, since Lapland‐Kola orogenesis (1930–1905 Ma) northeastern Fennoscandia experienced a quasi‐monotonous cooling with the average rate of ~0.15 °C/Myr, which is equal to an exhumation rate of ~1–2 m/Myr. New apatite fission track data and time‐temperature modeling reveal a “hidden” endogenous thermal event in the NE Fennoscandia that took place between 360 and 300 Ma. This we attribute to an elevated geothermal gradient due to Baltica's drift over the African large low shear‐wave velocity province in the lowest mantle and/or thermal blanketing by insulating Devonian‐Carboniferous sedimentary/volcanic cover. Our model is further supported by evidence of Late Devonian‐Carboniferous rifting in the East and South‐Western Barents Basin, as well as various 360–300 Ma magmatic events within SW Fennoscandia and the Baltic countries.

Sedimentary provenance of the Marília Formation (Bauru Basin), Southeast Brazil

The Marília Formation is a lithostratigraphic unit positioned at the top of the Bauru Group of Maastrichtian age. The present work presents a provenance study of the Marília Formation sandstones through the combined dating of the fission‐track method (FTM) and U–Pb, in which individual zircon grains are simultaneously dated by these two methods. Thirty‐one zircons were dated in the C5 sample and 26 in the C7 sample for a total of 57 grains. The data demonstrated a wide range of U–Pb ages, presenting maximum and minimum ages, respectively: 2,905 ± 11 and 128 ± 1 Ma for the C7 sample and 2,676 ± 10 and 455 ± 5 Ma for the C5 sample. Regarding the fission‐track (FT) ages, they have been grouped into three time intervals, which are associated to geological events relatively well recognized in the published literature: ages younger than 250 Ma (Wealdenian Reactivation); between 250 and 470 Ma (Palaeozoic orogenesis—Ocloyic, Precordilleran, Chanic, and Sanrafaelic); and older than 470 (Precambrian collisions—Brasiliano I, II, and III). There is a predominance of results of Precambrian age using the U–Pb dating, mainly in the Neopalaeoproterozoic, and with FT dating corresponding to the Ocloyic, Precordilleran, Chanic, and Sanrafaelic orogeneses. From the comparison of the results between the two dating methods, the age variations suggest that the zircons originated during the main south‐western orogenesis of Gondwana and were reworked during the Palaeozoic and Mesozoic geological episodes. The ages associated to the Upper Cretaceous are related to the tectonic and magmatic activity of the Alto Paranaiba Uplift.

A natural long‐term annealing experiment for the zircon fission track system in the Songpan‐Garzê flysch, China

AbstractApplication of the zircon fission track (FT) method to derive reliable cooling histories requires that rocks have undergone temperatures sufficient to reach full resetting prior to cooling. Zircons commonly show significant variation in accumulated radiation damage and therefore FT annealing behaviour. Twenty‐eight samples of mainly anchizonal to lowermost greenschist facies Triassic sandstones from the Songpan‐Garzê flysch, China, were evaluated for their FT annealing status. Literature data suggest a heating period in the range of 100 myr duration. Our results define a temperature range for partial FT annealing of 270–350°C (based on illite crystallinity data), higher than the proposed range for radiation‐damaged zircons. We suggest that the long residence at high temperature has led to annealing of relevant parts of radiation damage along with FT annealing, so that even for long durations of FT annealing, full resetting requires temperatures in the range of greenschist facies conditions typical for zero‐damage zircons.

Revealing granitic basement of Garba Hill, Muara Dua Region, South Sumatera based on landsat images, structure, and petrography

The basement that forms the three large basins on the island of Sumatra (North Sumatra basin, Central Sumatra Basin and South Sumatra basin) is very interesting to study. Previous studyers reported the basement phenomenon of South Sumatra basin is very exceptional because all of the basement boundaries are exposed on the surface. The highland of basement that forms the Garba Hill in Muara Dua, South Sumatra has been identified using Landsat Image band 432 and 753. Hue interpretation from Muara Dua’s area landsat image indicates resistance level of rock, water and vegetation.The blue hue on the image portrays river that flows in this area, like Komering River.The pink, gray and light green hues were identified as lack of resistance of bedrock lithology and interpreted as granitic rocks that had been weathered. The dark green to black hue in the form of original hill shape at an altitude of 798 m above sea level illustrates that the vegetations are thick with more resistance of bedrocks lithology, and based on field observation, crystalline rock in the form of granite was found. This interpretation has been supported by structural observation result and petrographic analysis of several samples of the crystalline rock. The basement alignment pattern observed in the SRTM image indicates trends of N320-350°E. This phenomenon is consistent with the NNW-SSE pattern resulted from structural study. This rock has a mineral composition consisting of quartz, orthoclase, plagioclase, feldspar and biotite. This set of minerals characterizes granitic magma. The specialty of basement in South Sumatera basin need to be studied in more detail using the fission track method to know the evolution of the basic thermal history of the bedrock.

Geochronology of the so-called South Bulgarian granitoids – a review (1836–2000)

This review treats the concepts concerning the age of granitoid magmatism in southern Bulgaria and considers the age assessments of the large granite intrusions in the metamorphic complexes known as “South Bulgarian granites”. In terms of the available contributions, this analysis encloses a long period, from 1836, the time of Ami Boué, the first geologist who visited Bulgarian land, to 2000. The review discusses both ideas and geochronological data on a large time-span, from the Archean to the “Tertiary”. The emphasis is on the progress of Bulgarian researchers’ views that had been made prior to the first pioneering attempts at radiological determinations (i.e., using He and Pb quantitative methods), and especially after the advent of modern equipment for radioisotope dating, based on various isotope systems (e.g., K-Ar, U-Th-Pb, Rb-Sr) and mathematical models of lead isotopes (Pb-Pb) following the basic model of “plumbotectonics” and the fission track method. The bulk of radioisotope data (more than 300 dates on feldspar and 500 dates on galena and other ore minerals) have been made by using the Pb-Pb method, which was introduced by Blagoy Amov and improved by the same worker via his “dynamic model of a continuous lead-isotope evolution” in the Earth’s crust. From 1969, when pegmatites that are genetically linked to granite intrusions of southern Bulgaria were dated as “Tertiary”, until 2000, all of the above-mentioned methods were tested. These methods confirmed the “Tertiary” ages of the granitoids of the Rhodope Massif and their difference in age from the Hercynian granitoids of the Srednogorie Zone. Owing to this, two groups of granitoids of dissimilar ages, previously referred to as “South Bulgarian granites”, were determined. This distinction was corroborated by varieties of mineralogical, petrological and geochemical studies, which were carried out by collaborators from the Department of Geochemistry of the Geological Institute. The “magical boundary”, defined by Acad. Strashimir Dimitrov and followed by the majority of Bulgarian geologists, that “the South Bulgarian granites are pre-Mesozoic in age, since fragments of them are present in the Permian–Triassic conglomerates of the Lozen Mountain” has been overcome, mainly due to the results of geochemical studies. Also, the absence of Archean and Proterozoic metamorphism in the Rhodope Massif has been revealed by U-Th-Pb and Pb/Pb radiogeochronological investigations. The analysis of available mineralogical, geochemical and radiogeochronological data, supported by the ages of the migmatites from Ardino area (63–32 Ma), allowed to assume that both migmatized gneisses and the South Bulgarian granites of the Rila and Rhodope Mountains, as well as the volcanic rocks and associated intrusions and ore mineralizations, were a product of a single and prolonged stage of the Alpine activation of the Rhodope crystalline complex that started during the Cretaceous.

A new approach for electron microprobe zircon fission track thermochronology

Fission track thermochronology (FTT) has been applied for decades to quantify rates and timing of processes in the shallow crust. The most widely used approach is the external detector method (EDM). In this conventional approach, an age is obtained by counting both the fossil (238U spontaneous fission) and the induced (235U induced fission) tracks using an optical microscope. The induced tracks are obtained through the irradiation of the sample with thermal neutrons in the nuclear reactor, which causes fission of 235U. Based on the studies carried out by Gombosi et al. (2014), we present an alternative method of dating zircons using electron probe microprobe analysis (EPMA) to measure uranium concentration [U]. The electron microprobe analysis fission track (EPMA-FT) method was applied to three samples of rapidly cooled zircons: the Fish Canyon Tuff, Poços de Caldas (syenite) and Serra Geral zircons. The analyses were performed using two approaches: 1) using the age equation described in Gombosi et al. (2014) and 2) using a new age equation calibration developed for this work. The results using the Gombosi et al. (2014) age equation were 26.7±4.1Ma, 80.6±12.8Ma and 130.9±20.1Ma, respectively, and the results using the age equation from this work were 27.8±1.9Ma, 83.8±7.7Ma and 136±12Ma, respectively. The uncertainty of the age is affected mainly by 238U concentration and ρS (the spontaneous fission track density) determinations. Other factors can affect the uncertainty of the age, but their contributions are smaller. For all samples, the yield ages found by the two methods are consistent and overlap within two standard deviations of published reference ages determined from other radiometric techniques (i.e., K/Ar, 40Ar/39Ar, and/or U/Pb) and traditional FTT by the EDM.

Revisiting the fission track method for the analysis of particles in safeguards environmental samples

This paper details an improved approach to environmental particle analysis for safeguards by means of a combination of an upgraded version of the so-called fission track method with state-of-the-art microscope and microprobe techniques. Improvements to the fission track method comprise a novel sample assembly, the automation of several of its steps and the extensive use of correlative microscopy. This is followed by an automated isolation of particles-of-interest by means of laser micro-dissection (LMD) and their collection onto a harvester for transfer to other micro-analytical instruments for further analysis. The samples examined in this contribution were analysed for their nuclear material signatures, in particular the presence of uranium isotopes. The length of a single analysis cycle herewith was reduced to 12 days.

Implementación del laboratorio de termocronología de baja temperatura: Intercomparación del Servicio Geológico Colombiano - Universidad Grenoble Alpes

This paper presents a historical description of the implementation process of the Low Temperature Thermochronology laboratory at the Colombian Geological Survey. The laboratory is focused on thermochronological dating of apatites and zircons with the fission track method. Here we explain the applied methodology, the results of the inter-calibration process performed in collaboration with the fission-track laboratory at ISTerre Joseph Fourier University and the internal inter-comparison of the SGC laboratory analysts. Furthermore, a viability study and irradiation conditions for fission track analysis in the nuclear reactor IAN-R1 is presented, in order to demonstrate the technical facilities, precision and reliability of the results obtained in the SGC laboratory to the scientific community.

裂变径迹法测年概述及其在造山带研究中的应用

裂变径迹法是上个世纪60年代兴起的低温热年代学方法。裂变径迹法早期以定年为主，随着裂变技术的发展，这种方法广泛应用于造山带的研究，包括：造山带隆升时代和隆升速率的研究、造山带热演化历史模拟研究。 Fission track method as a geological dating tool was firstly proposed in the early 1960s. In the past decades there has been a major expansion in application of fission track dating. As the development of fission track technique, it has been applied to the study of oregenic belt widely, including: to determine the uplifting time and rate of orogenic belt and to study the thermal history of orogenic belt.

A multi-geochronological study of the Hakusan volcano, central Japan

The Hakusan volcano, central Japan, is located in a region where two subducting plates (the Pacific Plate and the Philippine Sea Plate) overlap near the junction of four plates adjacent to the Japanese Islands (the Pacific Plate, the Philippine Sea Plate, the Eurasia Plate, and the North American Plate). The Hakusan volcano consists of products from four major volcanic episodes: Kagamuro, Ko-hakusan, and Shin-Hakusan I and II. To date the eruption events of the Hakusan volcano we applied thermoluminescence and fission track methods. 238U(234U)–230Th disequilibrium and 206Pb/238U methods were applied to date the zircon crystallization ages for estimating the magma residence time before the eruptions. The eruption ages we obtained are ca 250 ka for Kagamuro, ca 100 ka and ca 60 ka for Ko-Hakusan, ca 50 ka for Shin-Hakusan I, and <10 ka for Shin-Hakusan II. They are concordant with previous reports based on K–Ar dating. Some of the pyroclastic rocks, possibly originating from Shin-Hakusan II activities, are dated to be ca 36 ka or 50 ka, and belong to the Shin-Hakusan I activity. The zircon crystallization ages show several clusters prior to eruption. The magma residence time was estimated for each volcanic activity by comparing the major crystallization events and eruption ages, and we found a gradual decrease from ca. 500 ky for the Kagamuro activity to ca. 5 ky for the Shin-Hakusan II activity. This decrease in residence time may be responsible for the decrease in volume of erupted material estimated from the current topography of the region. The scale of volcanic activity, which was deduced from the number of crystallized zircons, is more or less constant throughout the Hakusan volcanic activity. Therefore, the decrease in magma residence time is most likely the result of stress field change. 白山火山の活動史を明らかにするために,年代学的研究を行った∘噴火年代を調べるために熱ルミネッセンス法およびフィッション▪トラック法を用いた∘結果はK–Ar法による既報と一致したが,いくつかの新データは活動期の時代区分の再考が必要となることを示した∘またジルコンの結晶晶出年代をU–Pb法,およびU–Th放射非平衡法により求めたところ,ジルコンの結晶晶出は活動期を通し概ね定常的に続いていた∘結晶晶出年代と噴火年代の比較により推定した噴火前の滞留期間と噴火規模には相関があった∘白山の噴火規模は時代に伴い小さくなっていることが知られているが,これは火山としての活動度の低下ではなく応力場の変化によるものと推定した∘

A refinement of the chronology of rift-related faulting in the Broadly Rifted Zone, southern Ethiopia, through apatite fission-track analysis

To reconstruct the timing of rift inception in the Broadly Rifted Zone in southern Ethiopia, we applied the fission-track method to basement rocks collected along the scarp of the main normal faults bounding (i) the Amaro Horst in the southern Main Ethiopian Rift and (ii) the Beto Basin in the Gofa Province. At the Amaro Horst, a vertical traverse along the major eastern scarp yielded pre-rift ages ranging between 121.4±15.3Ma and 69.5±7.2Ma, similarly to two other samples, one from the western scarp and one at the southern termination of the horst (103.4±24.5Ma and 65.5±4.2Ma, respectively). More interestingly, a second traverse at the Amaro northeastern terminus released rift-related ages spanning between 12.3±2.7 and 6.8±0.7Ma. In the Beto Basin, the ages determined along the base of the main (northwestern) fault scarp vary between 22.8±3.3Ma and 7.0±0.7Ma. We ascertain through thermal modeling that rift-related exhumation along the northwestern fault scarp of the Beto Basin started at 12±2Ma while in the eastern margin of the Amaro Horst faulting took place later than 10Ma, possibly at about 8Ma. These results suggest a reconsideration of previous models on timing of rift activation in the different sectors of the Ethiopian Rift. Extensional basin formation initiated more or less contemporaneously in the Gofa Province (~12Ma) and Northern Main Ethiopian Rift (~10–12Ma) at the time of a major reorganization of the Nubia–Somalia plate boundary (i.e., 11±2Ma). Afterwards, rift-related faulting involved the Southern MER (Amaro Horst) at ~8Ma, and only later rifting seemingly affected the Central MER (after ~7Ma).

Identification of uranium-bearing particles in swipe samples with fission track method

In this study, an advanced method has been developed to identify uranium-bearing particles through fission track and make the particles and fission track detector form a whole. This method can result in the one-to-one correspondence between the fission track and uranium-bearing particles. It is unnecessary to separate the particles from the detector and relocate the particles when they are picked up under microscope. This simple identification is helpful in reducing the loss of particles, which would be picked up and transferred to mass spectrometer for analysis.