Articles de revues : « 40Ar/39Ar method »

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Shi, Wenbei, Fei Wang, Lin Wu, Liekun Yang, Yinzhi Wang et Guanghai Shi. « Geologically Meaningful 40Ar/39Ar Ages of Altered Biotite from a Polyphase Deformed Shear Zone Obtained by in Vacuo Step-Heating Method : A Case Study of the Waziyü Detachment Fault, Northeast China ». Minerals 10, n^o 8 (22 juillet 2020) : 648. http://dx.doi.org/10.3390/min10080648.

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Discordant biotite 40Ar/39Ar age spectra are commonly reported in the literature. These can be caused by a number of processes related to in vacuo heating, homogenization of the argon distribution, and production of misleadingly flat age spectra. Problematic samples are typically derived from metamorphic belts; thermal overprinting and chloritization are two of the main known causes of disturbed age spectra. Biotite and muscovite of the Waziyü detachment fault, Yiwulüshan metamorphic core complex, Jinzhou, China, yield highly variable 40Ar/39Ar data that hinder reconstruction of their deformation history. We combined mineralogical studies with detailed 40Ar/39Ar dating of biotite, phengitic white mica, and K-feldspar augen from this fault. We infer that argon within the biotite was modified by hydrothermal fluids during fault activity and associated epidotization, chloritization, and muscovitization such that bulk sample step-heating, single grain total fusion, and in situ laser ablation of biotite produced mixed 40Ar/39Ar ages. However, detailed step-heating of biotite shows that this mineral records the ages of cooling and later alteration based on data from a coexisting rigid feldspar porphyroblast and neo-crystallized phengite that record two periods of fault activity at ~120–113 and 18–12 Ma. Our data reveal that the discordant biotite 40Ar/39Ar age spectra might represent a mixed age and that only detailed step-heating methods can extract meaningful geological details of the deformation history of a fault. Therefore, the mineral and the method must be carefully considered if metamorphic or deformed samples are dated.

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Renne, Paul R. « Progress and Challenges in K-Ar and40Ar/39Ar Geochronology ». Paleontological Society Papers 12 (octobre 2006) : 47–66. http://dx.doi.org/10.1017/s1089332600001340.

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K-Ar and more recently the40Ar/39Ar variant are well established dating methods. The40Ar/39Ar method requires irradiation with neutrons, posing some complications that are greatly outweighed by the benefits. The40Ar/39Ar method is particularly powerful due to the availability of internal reliability criteria, the ability to analyze single crystals, and the amenability of the analyses to automation.40Ar/39Ar dating has the capability for unsurpassed precision and is applicable to the broadest range of geologic environments and time scales of any radioisotope dating technique. For chronostratigraphic applications,40Ar/39Ar is most important in the Cenozoic, becoming progressively less useful into the early Phanerozoic due to alteration and loss of radiogenic argon. Precision and accuracy of40Ar/39Ar dating have been improved considerably in recent years, but an uncertainty of about 1% in the decay constant for40K, probably mainly in the electron capture decay branch, still limits accuracy at about this level. Inconsistent use of standards (neutron fluence monitors) and attribution of variable ages to standards is still a source of confusion, but straightforward recalculation procedures can overcome the underlying problems provided that appropriate standards are used.

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Rutte, Daniel, Paul R. Renne, Jonathan Morrell, Liqiang Qi, Mauricio Ayllon, Karl van Bibber, Jonathan Wilson et al. « Boutique neutrons advance 40Ar/39Ar geochronology ». Science Advances 5, n^o 9 (septembre 2019) : eaaw5526. http://dx.doi.org/10.1126/sciadv.aaw5526.

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We designed and tested a compact deuteron-deuteron fusion neutron generator for application to 40Ar/39Ar geochronology. The nearly monoenergetic neutrons produced for sample irradiation are anticipated to provide several advantages compared with conventional fission spectrum neutrons: Reduction of collateral nuclear reactions increases age accuracy and precision. Irradiation parameters within the neutron generator are more controllable compared with fission reactors. Confidence in the prediction of recoil energies is improved, and their likely reduction potentially broadens applicability of the dating method to fine-grained materials without vacuum encapsulation. Resolution of variation in the 39K(n,p)39Ar neutron capture cross section at 1.3 to 3.2 MeV and discovery of a strong resonance at ~2.4 MeV illuminate future pathways to improve the technique for 40Ar/39Ar dating.

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Alekseev, Daniil, et Alexey Travin. « Measurement of Temperature Distribution on Mineral Surface During Argon-Argon Laser Step Heating Dating ». Siberian Journal of Physics 8, n^o 1 (1 mars 2013) : 16–23. http://dx.doi.org/10.54362/1818-7919-2013-8-1-16-23.

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New pyrometric method of temperature distribution measurement on surface of natural mineral during laser 40Ar/39Ar dating has been devised. Peculiarity of this method is that it is relatively simple and does not require in availability of information about under test object absorption ability. It is very important during measurement of temperature distribution on surface of natural mineral. Proposed method includes forming of two digital pictures of sample in one frequency only, using of proposed method in conventional laser complexes intended for 40Ar/39Ar dating does not require large additional technical expense. Special program on the basis of platform Framework 4.0 has been devised for data processing automatization. Temperature field in several geological samples has been researched using method proposed. Data obtained indicates that temperature distribution in some of the samples during laser stepwise dating is sufficiently heterogeneous. Nonuniformity of temperature distribution can result in appearance do not taking into account inaccuracy in age value, that larger of analytical inaccuracy in few time. Thus, data obtained indicates that control of temperature distribution homogeneity during laser 40Ar/39Ar stepwise dating is necessary condition of accuracy dating result

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Andersen, Nathan L., Brian R. Jicha, Brad S. Singer et Wes Hildreth. « Incremental heating of Bishop Tuff sanidine reveals preeruptive radiogenic Ar and rapid remobilization from cold storage ». Proceedings of the National Academy of Sciences 114, n^o 47 (7 novembre 2017) : 12407–12. http://dx.doi.org/10.1073/pnas.1709581114.

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Accurate and precise ages of large silicic eruptions are critical to calibrating the geologic timescale and gauging the tempo of changes in climate, biologic evolution, and magmatic processes throughout Earth history. The conventional approach to dating these eruptive products using the 40Ar/39Ar method is to fuse dozens of individual feldspar crystals. However, dispersion of fusion dates is common and interpretation is complicated by increasingly precise data obtained via multicollector mass spectrometry. Incremental heating of 49 individual Bishop Tuff (BT) sanidine crystals produces 40Ar/39Ar dates with reduced dispersion, yet we find a 16-ky range of plateau dates that is not attributable to excess Ar. We interpret this dispersion to reflect cooling of the magma reservoir margins below ∼475 °C, accumulation of radiogenic Ar, and rapid preeruption remobilization. Accordingly, these data elucidate the recycling of subsolidus material into voluminous rhyolite magma reservoirs and the effect of preeruptive magmatic processes on the 40Ar/39Ar system. The youngest sanidine dates, likely the most representative of the BT eruption age, yield a weighted mean of 764.8 ± 0.3/0.6 ka (2σ analytical/full uncertainty) indicating eruption only ∼7 ky following the Matuyama−Brunhes magnetic polarity reversal. Single-crystal incremental heating provides leverage with which to interpret complex populations of 40Ar/39Ar sanidine and U-Pb zircon dates and a substantially improved capability to resolve the timing and causal relationship of events in the geologic record.

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Franz, Gerhard, Masafumi Sudo et Vladimir Khomenko. « 40Ar/39Ar dating of a hydrothermal pegmatitic buddingtonite–muscovite assemblage from Volyn, Ukraine ». European Journal of Mineralogy 34, n^o 1 (12 janvier 2022) : 7–18. http://dx.doi.org/10.5194/ejm-34-7-2022.

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Abstract. We determined 40Ar/39Ar ages of buddingtonite, occurring together with muscovite, with the laser-ablation method. This is the first attempt to date the NH4-feldspar buddingtonite, which is typical for sedimentary–diagenetic environments of sediments, rich in organic matter, or in hydrothermal environments, associated with volcanic geyser systems. The sample is a hydrothermal breccia, coming from the Paleoproterozoic pegmatite field of the Korosten Plutonic Complex, Volyn, Ukraine. A detailed characterization by optical methods, electron microprobe analyses, backscattered electron imaging, and IR analyses showed that the buddingtonite consists of euhedral-appearing platy crystals of tens of micrometers wide, 100 or more micrometers in length, which consist of fine-grained fibers of ≤ 1 µm thickness. The crystals are sector and growth zoned in terms of K–NH4–H3O content. The content of K allows for an age determination with the 40Ar/39Ar method, as well as in the accompanying muscovite, intimately intergrown with the buddingtonite. The determinations on muscovite yielded an age of 1491 ± 9 Ma, interpreted as the hydrothermal event forming the breccia. However, buddingtonite apparent ages yielded a range of 563 ± 14 Ma down to 383 ± 12 Ma, which are interpreted as reset ages due to Ar loss of the fibrous buddingtonite crystals during later heating. We conclude that buddingtonite is suited for 40Ar/39Ar age determinations as a supplementary method, together with other methods and minerals; however, it requires a detailed mineralogical characterization, and the ages will likely represent minimum ages.

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Stuart, F. M. « The exhumation history of orogenic belts from 40Ar/39Ar ages of detrital micas ». Mineralogical Magazine 66, n^o 1 (février 2002) : 121–35. http://dx.doi.org/10.1180/0026461026610017.

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AbstractThe exhumation history of mountain belts can be derived from radiometric dating of detrital mineral grains in proximal and distal post- and synorogenic sediments. The application of single-crystal dating techniques avoids the averaging effect that characterizes multi-grain and whole-rock techniques and allows the identification of populations of grains with distinct thermal histories. Of the major single crystal dating methods available, 40Ar/39Ar dating of detrital K-bearing minerals, in particular white mica, is perhaps the most versatile and widely applied technique. For a closure temperature of Ar of 350–400°C, muscovite 40Ar/39Ar ages record the time a rock mass passed through 8–10 km beneath actively eroding mountain belts. Detrital muscovit ages eroded from orogenic mountain belts have been used extensively to identify the provenance of sediments from source regions with distinct thermal histories, determine the history and rate of exhumation of the source region, and provide an upper limit on the sediment age. Here I review the principles of 40Ar/39Ar dating of detrital muscovite and illustrate the method with examples showing how the provenance and the thermal history of sediment source regions derived from such studies can be used to constrain the exhumation and tectonic history of orogenic belts.

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Yudin, Denis, Nikolay Murzintsev, Alexey Travin, Taisiya Alifirova, Egor Zhimulev et Sofya Novikova. « Studying the Stability of the K/Ar Isotopic System of Phlogopites in Conditions of High T, P : 40Ar/39Ar Dating, Laboratory Experiment, Numerical Simulation ». Minerals 11, n^o 2 (12 février 2021) : 192. http://dx.doi.org/10.3390/min11020192.

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Typically, 40Ar/39Ar dating of phlogopites from deep-seated xenoliths of kimberlite pipes produces estimates that suggest much older ages than those when these pipes were intruded. High-pressure (3 GPa) laboratory experiments enabled the authors to explore the behaviour of argon in the phlogopite structure under the conditions that correspond to the mantle, at the temperatures (from 700 to 1000 °С), far exceeding closure temperature of the K/Ar isotopic system. “Volume diffusion” remains foremost for describing the mobility of argon in phlogopite at high pressures. The mantle material age can be estimated through the dating of the phlogopites from deep-seated xenoliths of kimberlites, employing the 40Ar/39Ar method, subject to correction for a partial loss of radiogenic 40Ar when xenolith moves upwards to the Earth’s surface. The obtained data served as the basis for proposing the behaviour model of the K/Ar isotopic system of minerals in conditions of great depths (lower crust, mantle), and when transporting xenoliths in the kimberlite melt.

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Spikings, Richard A., et Daniil V. Popov. « Thermochronology of Alkali Feldspar and Muscovite at T > ; 150 °C Using the 40Ar/39Ar Method : A Review ». Minerals 11, n^o 9 (21 septembre 2021) : 1025. http://dx.doi.org/10.3390/min11091025.

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The 40Ar/39Ar method applied to K-feldspars and muscovite has been often used to construct continuous thermal history paths between ~150–600 °C, which are usually applied to structural and tectonic questions in many varied geological settings. However, other authors contest the use of 40Ar/39Ar thermochronology because they argue that the assumptions are rarely valid. Here we review and evaluate the key assumptions, which are that (i) 40Ar is dominantly redistributed in K-feldspars and muscovite by thermally-driven volume diffusion, and (ii) laboratory experiments (high temperatures and short time scales) can accurately recover intrinsic diffusion parameters that apply to geological settings (lower temperatures over longer time scales). Studies do not entirely negate the application of diffusion theory to recover thermal histories, although they reveal the paramount importance of first accounting for fluid interaction and secondary reaction products via a detailed textural study of single crystals. Furthermore, an expanding database of experimental evidence shows that laboratory step-heating can induce structural and textural changes, and thus extreme caution must be made when extrapolating laboratory derived rate loss constants to the geological past. We conclude with a set of recommendations that include minimum sample characterisation prior to degassing, an assessment of mineralogical transformations during degassing and the use of in situ dating.

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Karpenko, M. I., et V. V. Ivanenko. « A LASER VERSION OF THE 39Ar-40Ar METHOD OF GEOCHRONOLOGY ». International Geology Review 28, n^o 11 (novembre 1986) : 1357–62. http://dx.doi.org/10.1080/00206818609466371.

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Reid, Anthony, Marnie Forster, Wolfgang Preiss, Alicia Caruso, Stacey Curtis, Tom Wise, Davood Vasegh, Naina Goswami et Gordon Lister. « Complex ⁴⁰Ar ∕ ³⁹Ar age spectra from low-grade metamorphic rocks : resolving the input of detrital and metamorphic components in a case study from the Delamerian Orogen ». Geochronology 4, n^o 2 (20 juillet 2022) : 471–500. http://dx.doi.org/10.5194/gchron-4-471-2022.

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Abstract. In this study, we provide 40Ar / 39Ar geochronology data from a suite of variably deformed rocks from a region of low-grade metamorphism within the Cambro–Ordovician Delamerian Orogen, South Australia. Low-grade metamorphic rocks such as these can contain both detrital minerals and minerals newly grown or partly recrystallised during diagenesis and metamorphism. Hence, they typically yield complex 40Ar / 39Ar age spectra that can be difficult to interpret. Therefore, we have undertaken furnace step heating 40Ar / 39Ar geochronology to obtain age spectra with many steps to allow for application of the method of asymptotes and limits and recognition of the effects of mixing. The samples analysed range from siltstone and shale to phyllite and contain muscovite or phengite with minor microcline as determined by hyperspectral mineralogical characterisation. Whole rock 40Ar / 39Ar analyses were undertaken in most samples due to their very fine-grained nature. All samples are dominated by radiogenic 40Ar, and contain minimal evidence for atmospheric Ca- or Cl-derived argon. Chloritisation may have resulted in limited recoil, causing 39Ar argon loss in some samples, which is especially evident within the first few percent of gas released. Most of the age data, however, appear to have some geological significance. Viewed with respect to the known depositional ages of the stratigraphic units, the age spectra from this study do appear to record both detrital mineral ages and ages related to the varying influence of either cooling or deformation-induced recrystallisation. The shape of the age spectra and the degree of deformation in the phyllites suggest the younger ages may record recrystallisation of detrital minerals and/or new mica growth during deformation. Given that the younger limit of deformation recorded in the high-metamorphic-grade regions of the Delamerian Orogen is ca. 490 Ma, the ca. 470 to ca. 458 Ma ages obtained in this study suggest deformation in low-grade shear zones within the Delamerian Orogen may have persisted until ca. 20–32 million years after high-temperature ductile deformation in the high-grade regions of the orogen. We suggest that these younger ages for deformation could reflect reactivation of older structures formed both during rift basin formation and during the main peak of the Delamerian orogeny itself. The younger ca. 470 to ca. 458 Ma deformation may have been facilitated by far-field tectonic processes occurring along the eastern paleo-Pacific margin of Gondwana.

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Szopa, Krzysztof, Roman Włodyka et David Chew. « LA-ICP-MS U-Pb apatite dating of Lower Cretaceous rocks from teschenite-picrite association in the Silesian Unit (southern Poland) ». Geologica Carpathica 65, n^o 4 (1 août 2014) : 273–84. http://dx.doi.org/10.2478/geoca-2014-0018.

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Abstract The main products of volcanic activity in the teschenite-picrite association (TPA) are shallow, sub-volcanic intrusions, which predominate over extrusive volcanic rocks. They comprise a wide range of intrusive rocks which fall into two main groups: alkaline (teschenite, picrite, syenite, lamprophyre) and subalkaline (dolerite). Previous 40Ar/39Ar and 40K/40Ar dating of these rocks in the Polish Outer Western Carpathians, performed on kaersutite, sub-silicic diopside, phlogopite/biotite as well as on whole rock samples has yielded Early Cretaceous ages. Fluorapatite crystals were dated by the U-Pb LA-ICP-MS method to obtain the age of selected magmatic rocks (teschenite, lamprophyre) from the Cieszyn igneous province. Apatite-bearing samples from Boguszowice, Puńców and Lipowa yield U-Pb ages of 103± 20 Ma, 119.6 ± 3.2 Ma and 126.5 ± 8.8 Ma, respectively. The weighted average age for all three samples is 117.8 ± 7.3 Ma (MSWD = 2.7). The considerably smaller dispersion in the apatite ages compared to the published amphibole and biotite ages is probably caused by the U-Pb system in apatite being less susceptible to the effects of hydrothermal alternation than the 40Ar/39Ar or 40K/40Ar system in amphibole and/or biotite. Available data suggest that volcanic activity in the Silesian Basin took place from 128 to 103 Ma with the the main magmatic phase constrained to 128-120 Ma.

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Vetrov, Evgeny, Johan De Grave, Natalia Vetrova, Fedor Zhimulev, Simon Nachtergaele, Gerben Van Ranst et Polina Mikhailova. « Tectonic History of the South Tannuol Fault Zone (Tuva Region of the Northern Central Asian Orogenic Belt, Russia) : Constraints from Multi-Method Geochronology ». Minerals 10, n^o 1 (9 janvier 2020) : 56. http://dx.doi.org/10.3390/min10010056.

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In this study, we present zircon U/Pb, plagioclase and K-feldspar 40Ar/39Ar and apatite fission track (AFT) data along the South Tannuol Fault Zone (STFZ). Integrating geochronology and multi-method thermochronology places constraints on the formation and subsequent reactivation of the STFZ. Cambrian (~510 Ma) zircon U/Pb ages obtained for felsic volcanic rocks date the final stage of STFZ basement formation. Ordovician (~460–450 Ma) zircon U/Pb ages were obtained for felsic rocks along the structure, dating their emplacement and marking post-formational local magmatic activity along the STFZ. 40Ar/39Ar stepwise heating plateau-ages (~410–400 Ma, ~365 and ~340 Ma) reveal Early Devonian and Late Devonian–Mississippian intrusion and/or post-magmatic cooling episodes of mafic rocks in the basement. Permian (~290 Ma) zircon U/Pb age of mafic rocks documents for the first time Permian magmatism in the study area creating prerequisites for revising the spread of Permian large igneous provinces of Central Asia. The AFT dating and Thermal history modeling based on the AFT data reveals two intracontinental tectonic reactivation episodes of the STFZ: (1) a period of Cretaceous–Eocene (~100–40 Ma) reactivation and (2) the late Neogene (from ~10 Ma onwards) impulse after a period of tectonic stability during the Eocene–Miocene (~40–10 Ma).

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Harlavan, Y., et A. Sandler. « Steps toward dating early diagenetic K-feldspar by the 40Ar–39Ar method ». Sedimentary Geology 229, n^o 4 (août 2010) : 254–67. http://dx.doi.org/10.1016/j.sedgeo.2010.06.012.

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Hanes, J. A., D. York et C. M. Hall. « An 40Ar/39Ar geochronological and electron microprobe investigation of an Archean pyroxenite and its bearing on ancient atmospheric compositions ». Canadian Journal of Earth Sciences 22, n^o 7 (1 juillet 1985) : 947–58. http://dx.doi.org/10.1139/e85-100.

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A detailed 40Ar/39Ar step-heating geochronological study of pyroxene mineral separates from the Munro–Warden ultramafic sill in the Archean Abitibi Greenstone Belt of the Canadian Superior Province yields a precise primary age of 2703 ± 11 Ma (1σ). This is in excellent agreement with previous U–Pb zircon dates of 2700–2712 Ma for the enclosing felsic volcanics and a Sm–Nd age of 2622 ± 60 Ma for the sill. Electron microprobe analyses, coupled with the argon evolution patterns, demonstrate that deuteric amphibole is responsible for this primary age, whereas the volumetrically dominant diopside shows anomalously young ages. An isotope correlation plot reveals two components of contaminating argon with differing 40Ar/36Ar ratios in distinctly different sites. One component, trapped during the deuteric formation of amphibole, has an 40Ar/36Ar ratio that is significantly less (258 ± 3) than the present-day atmospheric value of 295.5. This may be an indication of the atmospheric ratio 2.7 Ga ago. The other component has a high 40Ar/36Ar ratio and was trapped during a later thermal event. The data demonstrate the potential of the 40Ar/39Arep-heating method for not only obtaining primary ages from low-potassium Archean ultramafic rocks, but also providing information on ancient atmospheric compositions.

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Goodwin, Mark B., et Alan L. Deino. « The first radiometric ages from the Judith River Formation (Upper Cretaceous), Hill County, Montana ». Canadian Journal of Earth Sciences 26, n^o 7 (1 juillet 1989) : 1384–91. http://dx.doi.org/10.1139/e89-118.

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A new isotopic age of ca. 78 Ma was obtained by the conventional K–Ar method and by laser-fusion 40Ar/39Ar analysis of biotite and sanidine crystals from two bentonites in the Judith River Formation of Kennedy Coulee, Hill County, Montana. The 40Ar/39Ar analyses yielded weighted mean ages of 78.2 ± 0.2 Ma (biotite) and 78.2 ± 0.2 Ma (sanidine) for sample 84MG8-3-4, and 79.5 ± 0.2 Ma (biotite) and 78.5 ± 0.2 Ma (sanidine) for sample 85MG7-16-1. These are the first isotopic age determinations from the Judith River Formation. The dated bentonites in Kennedy Coulee bracket two significant Judithian "age" mammal localities, UCMP V77083 and V81234.The radioisotopic age of ca. 78 Ma for the Hill County local faunas and correlations of other Judithian local faunas with marine units suggest that the duration of the Judithian was at least 5 Ma. Judithian local faunas are probably correlative with the late Campanian of the Western Interior. Similarities in the principal Judithian mammalian faunas from Alberta, Montana, and Wyoming can now be interpreted as documenting a low rate of evolutionary change rather than individual or populational variation of approximately contemporaneous local faunas.

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Shmakova, A., K. Kulikova, A. Travin et L. Bogatyrev. « New 40Ar/39Ar data of dolerites of the Kanin-Timan province of intraplate magmatism (Kanin peninsula) ». Vestnik of geosciences, n^o 6 (12 août 2022) : 3–9. http://dx.doi.org/10.19110/geov.2022.6.1.

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The Kanin-Timan region is located in the north-east of the European part of Russia and represents the western edge of the Timan-Pechora Plate. In the Devonian time, the Kanin-Timan region experienced rifting processes caused by the influence of a mantle plume. As a result of these processes, the Kanin-Timan dolerite complex was formed, which includes dolerite dikes and basalt sheets. The rocks of the complex have been studied throughout the study region. In the Middle Timan, the basalt plagioclase of the Upper Vorykva cover was dated by the 40Ar/39Ar method, the dating result indicates an age of 389 ± 6 Ma. Dolerites of the Kanin-Timan complex were also dated by the K-Ar method in the last century. As a result of dating, conflicting data on the absolute age from 378 to 288 Ma were obtained. To clarify the age of dolerites, 40Ar/39Ar dating was carried out. According to the isotope age data, the dolerites of the Kanin Peninsula are 419 ± 8 Ma, the age of the basalts of the Upper Vorykva cover is 389 ± 6 Ma. Thus, the magmatic activity that led to the formation of the Kanin-Timan complex in the Kanin-Timan region began earlier on the Kanin Peninsula, and then on the Timan. According to the research results, we believe that the Kanin-Timan complex began its formation in the Early Devonian with the peak during the Middle and Late Devonian time.

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Pálfy, J., P. L. Smith et J. K. Mortensen. « A U-Pb and 40Ar/39Ar time scale for the Jurassic ». Canadian Journal of Earth Sciences 37, n^o 6 (1 juin 2000) : 923–44. http://dx.doi.org/10.1139/e00-002.

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Published time scales provide discrepant age estimates for Jurassic stage boundaries and carry large uncertainties. The U-Pb or 40Ar/39Ar dating of volcaniclastic rocks with precisely known stratigraphic age is the preferred method to improve the calibration. A radiometric age database consisting of fifty U-Pb and 40Ar/39Ar ages was compiled to construct a revised Jurassic time scale. Accepted ages have a precision of ±5 Ma (2σ) or better and are confined to no more than two adjacent stages. The majority of these calibration points result from integrated bio- and geochronologic dating in the western North American Cordillera and have not been previously used in time scales. Direct dates are available only for the Triassic-Jurassic boundary and the initial boundary of the Crassicosta chron and the Callovian stage. The chronogram method was used to estimate all Early and early Middle Jurassic zone boundaries (attempted here for the first time), late Middle Jurassic substage boundaries, and Late Jurassic stage boundaries. Significant improvement is achieved for the Pliensbachian and Toarcian, where six consecutive zone boundaries are determined. The derived zonal durations are disparate, varying between 0.4 and 1.6 Ma. The latest Jurassic isotopic database remains too sparse, therefore chronogram estimates are improved using interpolation based on magnetochronology. The initial boundaries of Jurassic stages are proposed as follows: Berriasian (Jurassic-Cretaceous): 141.8+2.5&#150 1.8 Ma; Tithonian: 150.5+3.4&#150 2.8 Ma; Kimmeridgian: 154.7+3.8&#150 3.3 Ma; Oxfordian: 156.5+3.1&#150 5.1 Ma; Callovian: 160.4+1.1&#150 0.5 Ma; Bathonian: 166.0+3.8&#150 5.6 Ma; Bajocian: 174.0+1.2&#150 7.9 Ma; Aalenian: 178.0+1.0&#150 1.5 Ma; Toarcian: 183.6+1.7&#150 1.1 Ma; Pliensbachian: 191.5+1.9&#150 4.7 Ma; Sinemurian: 196.5+1.7&#150 5.7 Ma; Hettangian (Triassic-Jurassic): 199.6 ± 0.4 Ma.

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Villeneuve, Mike, Hamish A. Sandeman et William J. Davis. « A method for intercalibration of U-Th-Pb and 40Ar-39Ar ages in the Phanerozoic ». Geochimica et Cosmochimica Acta 64, n^o 23 (décembre 2000) : 4017–30. http://dx.doi.org/10.1016/s0016-7037(00)00484-1.

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Takigami, Y., et H. Sakai. « Accreted 1700Ma dolerite in the Siwalik Group, Central Nepal, dated by the 40Ar-39Ar method ». Chinese Science Bulletin 43, S1 (août 1998) : 126. http://dx.doi.org/10.1007/bf02891593.

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Jourdan, F., P. R. Renne et W. U. Reimold. « The problem of inherited 40Ar* in dating impact glass by the 40Ar/39Ar method : Evidence from the Tswaing impact crater (South Africa) ». Geochimica et Cosmochimica Acta 71, n^o 5 (mars 2007) : 1214–31. http://dx.doi.org/10.1016/j.gca.2006.11.013.

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Dallegge, Todd A., et Paul W. Layer. « Revised chronostratigraphy of the Kenai Group from 40Ar/39Ar dating of low-potassium bearing minerals, Cook Inlet Basin, Alaska ». Canadian Journal of Earth Sciences 41, n^o 10 (1 octobre 2004) : 1159–79. http://dx.doi.org/10.1139/e04-057.

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Thirty-seven tephra beds, primarily from coal partings in the Sterling and Beluga formations, were successfully dated by the 40Ar/39Ar method providing a new and revised understanding of the chronostratigraphy of late Tertiary strata within Cook Inlet Basin. Meticulous sample preparation, multiple analyses, and statistical evaluation of the data were required for these low-K, plagioclase- and hornblende-bearing tephras. Dating of subsurface core material provides the first subsurface-to-outcrop tie in Cook Inlet between well 212-24 in the Beluga River Unit and deposits in the Clam Gulch, Diamond Creek, and Fox Creek areas. The new 40Ar/39Ar chronostratigraphic framework place the age of upper part of the Kenai Group strata between 4.6 and 9.4 Ma and support the 8-Ma interpretation of the boundary between the Homerian and Clamgulchian paleobotanical stages. The 49-Ma age from core data in Pioneer Unit suggests the Tyonek Formation is older than previously thought or that these units belong to an older formation. The chronostratigraphic framework demonstrates significant offset on faults along the Kenai Peninsula, the presence of faults in slumps and vegetated areas, disconformities in the stratigraphic succession, and that parts of the Sterling and Beluga formations are time-equivalent strata representing lateral facies variations. Based on crosscutting relations and structural folding, the established chronohorizons indicate that much of the structural deformation in Cook Inlet is no older than early Pliocene in age. The repeated section, due to faulting and the coeval nature of the formations, could significantly affect previous resource assessments of coal and hydrocarbon distributions.

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Hanes, J. A., S. J. Clark et D. A. Archibald. « An 40Ar/39Ar geochronological study of the Elzevir batholith and its bearing on the tectonothermal history of the southwestern Grenville Province, Canada ». Canadian Journal of Earth Sciences 25, n^o 11 (1 novembre 1988) : 1834–45. http://dx.doi.org/10.1139/e88-173.

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Results of 40Ar/39Ar step heating for muscovite, potassium feldspar, and plagioclase from the ca. 1240 Ma Elzevir trondhjemite in the Central Metasedimentary Belt of the Canadian Grenville Province have been combined with earlier data and used to deduce a thermal history for the eastern Elzevir terrane following the Ottawan orogeny (1050–950 Ma). Muscovite yielded precise plateau dates of ca. 900 Ma, whereas the potassium feldspar displayed disturbed age spectra with ca. 730–760 Ma dates for broad "plateau" regions. This age difference could be explained by slow cooling or by thermal overprinting at 760–800 Ma that has updated the microcline. The plagioclase spectra provide evidence for a low-temperature hydrothermal event ca. 380 Ma ago that has generated sericite in saussurite and has caused partial argon loss from the microcline. This result agrees with the other plagioclase thermochronometry and paleomagnetism on the Cordova gabbro 25 km to the west but provides a lower estimate for the upper age bracket of this event. It is suggested that earlier models of protracted, post-700 Ma cooling of this part of the Grenville orogen may be a consequence of variable updating of plagioclase by this Devonian alteration event.Results of 40Ar/39Ar step heating for muscovite and microcline from the Bark Lake diorite in the Bancroft terrane are in agreement with earlier work in this area, and the cooling path of the Elzevir trondhjemite is seen to be indistinguishable, within the limitations of the method, from that of the Bark Lake diorite in the time period 1050–750 Ma.

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Ismail, Rafika, David Phillips et William D. Birch. « 40Ar/39Ar dating of alkali feldspar megacrysts from selected young volcanoes of the Newer Volcanic Province, Victoria ». Proceedings of the Royal Society of Victoria 125, n^o 2 (2013) : 59. http://dx.doi.org/10.1071/rs13019.

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The Newer Volcanic Province (NVP) in Victoria, with extension into south-eastern South Australia, represents the youngest chapter of Cenozoic volcanism in south-eastern Australia. However, most ages have been determined by the potassium–argon (K–Ar) method, and the age data are not comprehensive. In addition, few ages exist for the array of scoria cone volcanoes in the NVP. Seven alkali feldspar samples, mostly anorthoclase megacrysts, from volcanic centres in the NVP were used for 40Ar/39Ar dating in the present study. In geochronological order, with ages quoting 95% confidence limits, locations are Mount Franklin near Daylesford (0.110 ± 0.014 Ma), Red Rock near Alvie (0.116 ± 0.048 Ma), Lake Bullenmerri at Camperdown (0.116 ± 0.019 Ma), Ridge Road Quarry near Daylesford (2.01 ± 0.11 Ma) and Mount Kororoit near Diggers Rest (3.74 ± 0.26 Ma). Two samples from The Anakies, near Bacchus Marsh, produced discordant results suggesting a maximum age of ca. 1.9 Ma. The analyses and reported ages in the present study not only provide new geochronological data for the province, but also elucidate the difficulties in dating very young basalts using the 40Ar/39Ar dating method. These results are consistent with the erosion levels of the scoria volcanoes sampled, and indicate a major episode of explosive volcanic activity at ca. 100 ka. In contrast, the more eroded Mount Kororoit is considered to be ca. 3.7 Ma in age. The age of The Anakies is more equivocal owing to the indicated presence of excess argon and a maximum age of ca. 1.9 Ma is suggested for this locality. Given the latter results and lack of precision obtainable from the younger samples, the possibility remains that other samples contained extraneous argon and that the ages generated are thus maximum eruption ages. Analyses of additional samples from these and other localities will be required to further resolve this issue.

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Smirnov, V. N., K. S. Ivanov, A. V. Travin, A. V. Zakharov et Yu V. Erokhin. « 40Ar-39Ar dating of pegmatites from Murzinka-Adui region (Middle Urals) : Results and their geological interpretation ». LITHOSPHERE (Russia) 22, n^o 5 (4 novembre 2022) : 612–23. http://dx.doi.org/10.24930/1681-9004-2022-22-5-612-623.

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Research subject. The pegmatites of Murzinka-Adui region (Ethtern sector of the Middle Urals), represented by three types of veins miarolitic pegmatites with topaz and beryl (mines Mokrusha, Kazennitsa, Semenovskaia), miarolitic rubillite-lepidolite pegmatites (Lipovka veins ﬁeld) and rare metal pegmatites with Ta-Nb-Be mineralization (deposits Kvartal’noe and Lipovyi Log).Materials and methods. Isotope dating was carried out via the 40Ar-39Ar method by a mass spectrometer Micromass 5400 at the Institute of Geology and Mineralogy SB RAS for mica of diﬀerent composition (muscovites, biotites and lepidolites).Results. 40Ar/39Ar ages for mica from dated pegmatite veins of the topaz-beryl type coincide within the measurement accuracy: mine Kazennitsa – 252.3 Ma, Mokrusha – 253.7 and Semenovskaia – 250.3 Ma. The average age of the two series of mica from rubillite-lepidolite pegmatites was found to be 254.1 Ma. The average age of four samples of muscovites from rare metal Ta-Nb-Be pegmatites equals 252.6 Ma.Conclusions. The closure of the isotope system of micas in pegmatites of all three studied types (topaz-beryllium, rubillite-lepidolite and rare-metal with Ta-Nb-Be mineralization) occurred almost simultaneously during the timeframe of 254 to 250 Ma years ago. The obtained age values, however, cannot be identiﬁed with the time of formation of pegmatites. The analysis of the available data suggests that the closure of the micas K-Ar isotope system of the studied pegmatites as well as previously dated schists and blastomylonites of the Bazhenovo suture zone and granites of the Murzinka-Adui block marks an important episode in the geological history of the eastern margin of the Urals and the basement of the adjacent part of the West Siberian plate: the change of the transpression regime prevailing during the collision stage of the region’s development to the regime of limited postcollision extension.

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West, David P., et Daniel R. Lux. « Dating mylonitic deformation by the 40Ar-39Ar method : An example from the Norumbega Fault Zone, Maine ». Earth and Planetary Science Letters 120, n^o 3-4 (décembre 1993) : 221–37. http://dx.doi.org/10.1016/0012-821x(93)90241-z.

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Qiang Hu, Patrick E. Smith, Norman M. Evensen et Derek York. « Lasing in the Holocene : extending the 40Ar39Ar laser probe method into the 14C age range ». Earth and Planetary Science Letters 123, n^o 1-3 (mai 1994) : 331–36. http://dx.doi.org/10.1016/0012-821x(94)90277-1.

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Dunlop, David J. « Grenvillia and Laurentia — a Precambrian Wilson cycle ? » Canadian Journal of Earth Sciences 51, n^o 3 (mars 2014) : 187–96. http://dx.doi.org/10.1139/cjes-2013-0101.

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John Tuzo Wilson coined the term “plate” in plate tectonics. He is famous for inventing transform boundaries, hot spot tracks, and the Wilson cycle of ocean birth, growth, and decline. Less well remembered is his work in the 1950s on tectonic and radiometric age provinces of the Canadian Shield, as part of which he fathered U/Pb geochronology in Canada. This work gave strong support to the notion of continental growth through accretion of successively younger terranes onto an ancient cratonic core. The present paper reviews how paleomagnetism can trace the motions of continents to test Wilson’s ideas. Continental accretion often involves deep burial of one of the colliding elements through subduction or crustal underplating; such was the case with the Grenville orogen and its subprovinces in their Proterozoic accretion onto the Laurentian craton. The resulting heating and metamorphism erases most pre-collisional magnetic information but adds something new: the possibility of following the post-metamorphic uplift and cooling history, in time and space. The time element is provided by a new form of isotopic geochronology, thermochronometry, which provides dates for specific minerals together with the temperatures at which they became closed to isotopic migration. U/Pb dating of sphene is one method used; another is the 40Ar/39Ar variant of K/Ar dating applied to hornblende, micas, and feldspars, which have a wide range of Ar closure temperatures. The two specific Grenville studies described deal with parallel uplift histories determined by 40Ar/39Ar dating and by magnetics for the accreted terranes of the Central Metasedimentary Belt in Ontario and with the paleomagnetic detection of the post-1240 Ma closing of a small ocean between the Elsevir terrane and Laurentia during the Grenvillian orogeny.

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Zhou, Xin, Jianqing Ji, Jing Zhou, Lhamo Yungchen, Wuxun Quan et Jiyao Tu. « A New 40Ar/39Ar Analysis Method of Volcanoclastic Strata to Determine Eruption Periods—Example of Xintaimen, China ». Journal of Geology 129, n^o 1 (1 janvier 2021) : 63–76. http://dx.doi.org/10.1086/713685.

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Schneider, B., K. F. Kuiper, K. Mai, J. P. T. Foeken, F. M. Stuart et J. R. Wijbrans. « Fuerteventura – Assessment of a calibration site for cosmogenic 3He exposure dating with the 40Ar/39Ar incremental heating method ». Quaternary Geochronology 21 (juin 2014) : 58–69. http://dx.doi.org/10.1016/j.quageo.2013.09.005.

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Eberth, D. A., D. A. Russell, D. R. Braman et A. L. Deino. « The age of the dinosaur-bearing sediments at Tebch, Inner Mongolia, People's Republic of China ». Canadian Journal of Earth Sciences 30, n^o 10 (1 octobre 1993) : 2101–6. http://dx.doi.org/10.1139/e93-182.

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The coarsening-upward clastic sequence at Tebch, Inner Mongolia, China, containing the dinosaur Psittacosaurus mongoliensis, comprises the remains of a fluviolacustrine system deposited in an extensional tectonic setting.The presence of Asteropollis sp. cf. Asteropollis trichotomosulcatus (Singh) Singh, 1983 in conjunction with the absence of tricolpate pollen indicates a Barremian or possibly early Aptian age. The overlying Tebch basalt, dated by the 40Ar/39Ar laser step-heating method, yields a mid-Aptian age of 110 ± 0.52 Ma.This chronology supports the argument that the Juifotang Formation of the Jehe Group in western Liaoning is Early Cretaceous, with the Jurassic–Cretaceous boundary very low within, or below, the base of the Jehe Group. Our data also suggest that the Berriasian–Hauterivian (Neocomian) age suggested for the one known Russian locality that has yielded P. mongoliensis (Shestakovskaya Svita at Gorno-Altayaskaya, Avtonomnaya Oblast) may be excessively old.

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Crick, Laura, Andrea Burke, William Hutchison, Mika Kohno, Kathryn A. Moore, Joel Savarino, Emily A. Doyle et al. « New insights into the ∼ 74 ka Toba eruption from sulfur isotopes of polar ice cores ». Climate of the Past 17, n^o 5 (18 octobre 2021) : 2119–37. http://dx.doi.org/10.5194/cp-17-2119-2021.

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Abstract. The ∼74 ka Toba eruption was one of the largest volcanic events of the Quaternary. There is much interest in determining the impact of such a large event, particularly on the climate and hominid populations at the time. Although the Toba eruption has been identified in both land and marine archives as the Youngest Toba Tuff, its precise place in the ice core record is ambiguous. Several volcanic sulfate signals have been identified in both Antarctic and Greenland ice cores and span the Toba eruption 40Ar/39Ar age uncertainty. Here, we measure sulfur isotope compositions in Antarctic ice samples from the Dome C (EDC) and Dronning Maud Land (EDML) ice cores at high temporal resolution across 11 of these potential Toba sulfate peaks to identify candidates with sulfur mass-independent fractionation (S-MIF), indicative of an eruption whose plume reached altitudes at or above the stratospheric ozone layer. Using this method, we identify several candidate sulfate peaks that contain stratospheric sulfur. We further narrow down potential candidates based on the isotope signatures by identifying sulfate peaks that are due to a volcanic event at tropical latitudes. In one of these sulfate peaks at 73.67 ka, we find the largest ever reported magnitude of S-MIF in volcanic sulfate in polar ice, with a Δ33S value of −4.75 ‰. As there is a positive correlation between the magnitude of the S-MIF signal recorded in ice cores and eruptive plume height, this could be a likely candidate for the Toba super-eruption, with a plume top height in excess of 45 km. These results support the 73.7±0.3 ka (1σ) 40Ar/39Ar age estimate for the eruption, with ice core ages of our candidates with the largest magnitude S-MIF at 73.67 and 73.74 ka. Finally, since these candidate eruptions occurred on the transition into Greenland Stadial 20, the relative timing suggests that Toba was not the trigger for the large Northern Hemisphere cooling at this time although we cannot rule out an amplifying effect.

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Gurbanov, A. G., D. G. Koshchug, V. M. Gazeev, A. B. Leksin, S. V. Vyatkin et A. Y. Dokuchaev. « The Elbrus volcanic center : new features of EPR dating of rockes ». Moscow University Bulletin. Series 4. Geology, n^o 3 (15 décembre 2022) : 61–69. http://dx.doi.org/10.33623/0579-9406-2022-3-61-69.

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The restoration of the evolution of the Elbrus Volcanic Center (EVC) is of great importance for predicting possible eruptions. Some stages of its development fall on the time interval, which is difficult to measure by conventional radioisotope techniques. In this regard, in this work, we studied the possibility of using the dating method by radiation centers in quartz the concentration of which was measured with electron paramagnetic resonance (EPR) spectroscopy. When dating lava fl ows with an age of less than 50 thousand years, there is a good convergence of the results obtained by EPR dating, radiocarbon and comparative geomorphological methods. Due to the low thermal stability of radiation centers in quartz, in some cases, the measured values of age refl ect a later thermal impact of overlying lava fl ows or intrusive bodies. The improved technique of EPR dating with intermediate annealing led to the results that have better convergence with the data obtained by 40Ar/39Ar and K-Ar methods for rocks older than 50 thousand years.

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Eusden, Jr., J. Dykstra, et Daniel R. Lux. « Slow late Paleozoic exhumation in the Presidential Range of New Hampshire as determined by the 40Ar/39Ar relief method ». Geology 22, n^o 10 (1994) : 909. http://dx.doi.org/10.1130/0091-7613(1994)022<0909:slpeit>2.3.co;2.

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Forster, M. A., et G. S. Lister. « The interpretation of 40Ar/39Ar apparent age spectra produced by mixing : application of the method of asymptotes and limits ». Journal of Structural Geology 26, n^o 2 (février 2004) : 287–305. http://dx.doi.org/10.1016/j.jsg.2003.10.004.

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Bukovská, Zita, Petr Jeřábek, Ondrej Lexa, Jiří Konopásek, Marian Janák et Jan Košler. « Kinematically unrelated C—S fabrics : an Lexample of extensional shear band cleavage from the Veporic Unit (Western Carpathians) ». Geologica Carpathica 64, n^o 2 (1 avril 2013) : 103–16. http://dx.doi.org/10.2478/geoca-2013-0007.

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Abstract Discontinuous and kinematically unrelated C-S fabrics have been recognized along the contact between the Gemeric and Veporic Units in the Western Carpathians. The formation of S and C fabrics within orthogneiss, quartzite and chloritoid-kyanite schist of the Veporic Unit is associated with Cretaceous syn-burial orogen-parallel flow and subsequent exhumational unroofing. The formation of the two fabrics characterized by distinct quartz deformation microstructure and metamorphic assemblage is separated by an inter-tectonic growth of transversal chloritoid-, kyanite-, ± monazite-bearing assemblage. The monazite U-Th-Pb concordia age of 97 ± 4 Ma was obtained by the laser ablation ICP-MS dating method. The age of this inter-tectonic metamorphic stage together with existing 40Ar/39Ar ages on exhumation of the Veporic Unit indicate that despite the similar appearance to shear bands or C-S mylonites there is a time span of at least 10 Myr between the formation of homogeneous S fabrics and superposed discrete C fabrics in the studied rocks

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Vrolijk, Peter, David Pevear, Michael Covey et Allan LaRiviere. « Fault gouge dating : history and evolution ». Clay Minerals 53, n^o 3 (septembre 2018) : 305–24. http://dx.doi.org/10.1180/clm.2018.22.

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ABSTRACTRadiometric dating of fault gouges has become a useful tool for regional tectonics studies and for exploring and understanding fault and earthquake processes. Methods to define the absolute age of faults achieved a solid scientific foundation almost 25 years ago when the development and application of illite age analysis for investigating sedimentary burial and thermal histories found a new potential application – defining the age of fold-and-thrust development. Since then, the methods have benefitted from further development and incorporation of the 40Ar/39Ar micro-encapsulation method and quantitative clay mineral evaluation to distinguish polytypes (Wildfire). These refinements to the methods have improved their application in fold-and-thrust terrains and have opened up applications in normal and strike-slip fault environments. Another important development is the use of absolute dating methods in retrograde clay gouges in which clays in a fault develop from igneous or metamorphic wall rocks that contain no clays. In addition, the method has also been shown to be useful at dating folds in fold-and-thrust belts. We think the method is now an established part of the geological toolkit, look forward to future fault structural and tectonic studies that incorporate fault ages and hope that researchers continue to probe and discover ways that the method can assist fault process studies, including earthquake fault studies.

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Deino, Alan L., Giovanni Orsi, Sandro de Vita et Monica Piochi. « The age of the Neapolitan Yellow Tuff caldera-forming eruption (Campi Flegrei caldera – Italy) assessed by 40Ar/39Ar dating method ». Journal of Volcanology and Geothermal Research 133, n^o 1-4 (mai 2004) : 157–70. http://dx.doi.org/10.1016/s0377-0273(03)00396-2.

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Anechitei-Deacu, Valentina, Alida Timar-Gabor, Kathryn Fitzsimmons, Daniel Veres et Ulrich Hambach. « Multi-method luminescence investigations on quartz grains of different sizes extracted from a loess section in Southeast Romania interbedding the Campanian Ignimbrite ash layer ». Geochronometria 41, n^o 1 (1 mars 2014) : 1–14. http://dx.doi.org/10.2478/s13386-013-0143-4.

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Abstract In this study we present luminescence investigations of four samples of loess bracketing the Campanian Ignimbrite/Y5 tephra at the Rasova-Valea cu Pietre site, on the eastern bank of the Danube River, southeastern Romania. Investigations involved SAR-OSL dating on aliquots of fine (4–11 μm) and medium-grained (63–90 μm) quartz, as well as single grain analyses on 125–180 μm quartz. Luminescence dating results coupled with glass-shard chemical fingerprinting assign the depositional age and origin of the ash layer to that of the Campanian Ignimbrite/Y5 tephra, dated elsewhere using 40Ar/39Ar to 39.28 ± 0.11 ka. Fine-grained (4–11 μm) quartz SAR-OSL analyses yielded ages of 44.4 ± 4.5 ka below the ash, and 41.4 ± 4.2 ka above the ash layer. Single grain analysis on coarse-grained quartz, however, demonstrates that coarse material from these samples exhibits low sensitivity and responds poorly to internal checks of the SAR protocol in comparison with the finer sediment. This observation highlights the need for more extensive investigations into the luminescence properties of quartz as well as into the origin of quartz contributions from different primary sources in the Lower Danube loess steppe.

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Li, Yang, et Pieter Vermeesch. « Short communication : Inverse isochron regression for Re–Os, K–Ca and other chronometers ». Geochronology 3, n^o 2 (2 août 2021) : 415–20. http://dx.doi.org/10.5194/gchron-3-415-2021.

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Abstract. Conventional Re–Os isochrons are based on mass spectrometric estimates of 187Re/188Os and 187Os/188Os, which often exhibit strong error correlations that may obscure potentially important geological complexity. Using an approach that is widely accepted in 40Ar/39Ar and U–Pb geochronology, we here show that these error correlations are greatly reduced by applying a simple change of variables, using 187Os as a common denominator. Plotting 188Os/187Os vs. 187Re/187Os produces an “inverse isochron”, defining a binary mixing line between an inherited Os component whose 188Os/187Os ratio is given by the vertical intercept, and the radiogenic 187Re/187Os ratio, which corresponds to the horizontal intercept. Inverse isochrons facilitate the identification of outliers and other sources of data dispersion. They can also be applied to other geochronometers such as the K–Ca method and (with less dramatic results) the Rb–Sr, Sm–Nd and Lu–Hf methods. Conventional and inverse isochron ages are similar for precise datasets but may significantly diverge for imprecise ones. A semi-synthetic data simulation indicates that, in the latter case, the inverse isochron age is more accurate. The generalised inverse isochron method has been added to the IsoplotR toolbox for geochronology, which automatically converts conventional isochron ratios into inverse ratios, and vice versa.

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Yang, Tianjian, Xiaoming Sun, Guiyong Shi et Ying Liu. « LA-ICP-MS U–Pb Dating of Cenozoic Rutile Inclusions in the Yuanjiang Marble-Hosted Ruby Deposit, Ailao Shan Complex, Southwest China ». Minerals 11, n^o 4 (19 avril 2021) : 433. http://dx.doi.org/10.3390/min11040433.

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Among the marble-hosted ruby deposits in the Himalayan tectonic belt, which yields the highest-quality rubies in the world, the Yuanjiang deposit is the only economically viable one located in China. More attempts are necessary to put constraints on the ore-forming age of these marble-hosted ruby deposits. Here, we dated rutile inclusions in the Yuanjiang rubies using the LA-ICP-MS U–Pb method, which yielded a lower intercept 206Pb/238U age of 20.2 ± 1.2 Ma on the Tera-Wasserburg plot, close to the 22.5–22.2 Ma 40Ar/39Ar ages of phlogopite from the ruby host matrix assemblage. Our U–Pb rutile age put a constraint on the cooling history of the Yuanjiang rubies deposit. The new rutile age is consistent with our previous model that shows the ca. 28–22 Ma left lateral shearing plays an important role in transporting the ruby deposit toward the surface. This study provides the first example of in-situ U–Pb dating of rutile in the Himalayan tectonic belt, demonstrating the great potential of U–Pb rutile geochronology for Cenozoic mineral deposits.

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Krautz, Jana, Mandy Hofmann, Andreas Gärtner, Ulf Linnemann et Arno Kleber. « Capability of U–Pb dating of zircons from Quaternary tephra : Jemez Mountains, NM, and La Sal Mountains, UT, USA ». E&G Quaternary Science Journal 67, n^o 1 (31 janvier 2018) : 7–16. http://dx.doi.org/10.5194/egqsj-67-7-2018.

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Abstract. Two Quaternary tephras derived from the Jemez Mountains, New Mexico – the Guaje and Tsankawi tephras – are difficult to distinguish due to their similar glass-shard chemical composition. Differences in bulk chemical composition are small as well. Here we examine the feasibility to assign an age to a distal tephra layer in the La Sal Mountains, Utah, by U–Pb dating of zircons and to correlate it with one of the two Jemez eruptions. We also dated original Jemez tephras for comparison. Even though the tephras are very young, we obtained reasonable age determinations using the youngest cluster of zircon grains overlapping in age at 2σ. Thereafter, the Guaje tephra is 1.513 ± 0.021 Myr old. The La Sal Mountains tephra is correlated with the Tsankawi tephra. Three samples yielded a common age range of 1.31–1.40 Myr. All ages are in slight disagreement with published age determinations obtained by 40Ar ∕ 39Ar dating. These findings indicate that distal Jemez tephras can be distinguished by U–Pb dating. Furthermore, we encourage giving this method a try for age assignments even of Quaternary volcanic material.

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Guillou, Hervé, Christophe Hémond, Brad S. Singer et Jérôme Dyment. « Dating young MORB of the Central Indian Ridge (19°S) : Unspiked K-Ar technique limitations versus 40Ar/39Ar incremental heating method ». Quaternary Geochronology 37 (février 2017) : 42–54. http://dx.doi.org/10.1016/j.quageo.2016.10.002.

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Oliver, Douglas H., J. Dykstra Eusden, Jr. et Daniel R. Lux. « Slow late Paleozoic exhumation in the Presidential Range of New Hampshire as determined by the 40Ar/39Ar relief method : Comment and Reply ». Geology 23, n^o 8 (1995) : 763. http://dx.doi.org/10.1130/0091-7613(1995)023<0763:slpeit>2.3.co;2.

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Lykhin, D. A., V. V. Yarmolyuk, A. A. Vorontsov et A. V. Travin. « Structure and age of the fluorite-berillium deposit of the field Rainbow, Western Sayan Mountains : to the problem of assessment of metallogenic prospects of the territory ». Доклады Академии наук 488, n^o 3 (26 septembre 2019) : 282–87. http://dx.doi.org/10.31857/s0869-56524883282-287.

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The age and geochemical parameters of the muscovite-fluorite-euclase-beryl Raduga deposit, which is located within the Kizir-Kazyr zone of rare-metal magmatism, are determined. In contrast to other deposits and ore occurrences of the zone, represented by alkaline granites characterized by rare metal mineralization, the Raduga deposit is associated with metasomatites in carbonate rocks. The age of the deposit, estimated at 40Ar/39Ar by the muscovite method of beryllium fluorite-muscovite greisens, is 469.3± 4.5Ma. It corresponds to the age of the ore-bearing alkaline granites of the zone. The dikes which occur within the deposit are identical by the composition to the dikes of rare-metal alkaline granitic massifs, one of which is located in a few kilometers from the deposit. The nature of the ore Be-Li mineralization of the deposit is in good agreement with the geochemical specialization of the Early Paleozoic Kizir-Kazyr metallogenic zone. The revealed features of the relationship between Raduga deposit and rare-metal deposits in alkaline granites suggests a variety of mechanisms involved in the formation of rare-metal deposits of the Kizir-Kazyr zone. Thus, it allows to expand approaches for prediction and exploration of rare-metal deposits in the region.

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Costa, S., et H. Maluski. « Use of the 40Ar- 39Ar stepwise heating method for dating mylonite zones : An example from the St. Barthe´le´my massif (Northern Pyrenees, France) ». Chemical Geology : Isotope Geoscience section 72, n^o 2 (mars 1988) : 127–44. http://dx.doi.org/10.1016/0168-9622(88)90061-9.

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Zhai, Degao, Anthony E. Williams-Jones, Jiajun Liu, David Selby, Chao Li, Xiao-Wen Huang, Liang Qi et Donghang Guo. « Evaluating the Use of the Molybdenite Re-Os Chronometer in Dating Gold Mineralization : Evidence from the Haigou Deposit, Northeastern China ». Economic Geology 114, n^o 5 (1 août 2019) : 897–915. http://dx.doi.org/10.5382/econgeo.2019.4667.

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Abstract The Haigou lode gold deposit (>40 tons [t] at 3.4 g/t), which is located near the eastern boundary of the Central Asian orogenic belt and the North China craton, is one of the largest gold deposits in northeastern China. Native gold is intergrown with molybdenite and pyrite in auriferous quartz veins hosted by a monzogranite-monzonite stock and locally by Proterozoic gneiss, thereby offering an excellent opportunity to directly date the mineralizing event. Uranium-Pb age determinations for zircon yielded ages for the monzogranite and monzonite of 327.1 ± 1.1 and 329.5 ± 1.0 Ma, respectively. Numerous mafic to felsic dikes, which are crosscut by ore veins (pre-ore), parallel to these veins (possibly synore), or crosscut by them (post-ore), were carefully examined and dated. Their zircon 206Pb/238U ages are 318.3 ± 1.0, 310.9 ± 1.1, and 134.9 ± 0.4 Ma, respectively, thereby placing the timing of gold mineralization within the relatively large interval of 318.3 ± 1.0 to 134.9 ± 0.4 Ma. The age of mineralization was determined directly using the Re-Os method applied to molybdenite. A total of 19 molybdenite samples separated from auriferous quartz veins yielded widely differing Re-Os model ages of 467 to 155 Ma, and replicate analyses of individual samples also yielded widely differing ages. Significantly, the wide range is attributable entirely to the results obtained for some coarse-grained molybdenite samples and is interpreted to be due to Re and Os isotope decoupling, the considerable spatial Re heterogeneity, the analytical procedure (e.g., use of small sample aliquots), and the post-ore deformation. Nine of the samples, which are all fine grained, yielded a robust weighted mean model age of 310 ± 3 Ma and an isochron age of 309 ± 8 Ma. Thus, the molybdenite Re-Os ages are identical, within uncertainty, to those of the dikes that are parallel to the ore veins, indicating that these dikes were emplaced contemporaneously with the ore and that they and the Haigou gold mineralization are of late Paleozoic age (ca. 310 Ma). Finally, a sericite sample obtained from an auriferous vein returned a 40Ar/39Ar plateau age of 165.3 ± 1.2 Ma, which is much younger than the age of the mineralization constrained by Re-Os age determinations of molybdenite. This indicates that the 40Ar/39Ar isotope system was reset by post-ore thermal events. Our new geochronological data provide evidence for late Paleozoic gold mineralization in Haigou, which makes it the oldest known lode gold deposit in the easternmost Central Asian orogenic belt, a finding that has important implications for precious metal mineral exploration in the eastern part of the Solonker-Xar Moron-Changchun-Yanji suture zone between the Central Asian orogenic belt and the North China craton. This study also indicates that accurate and reproducible molybdenite Re-Os ages representing the true timing of ore deposition need an integrated combination of careful petrography, proper sampling procedures, sufficiently large analyzed aliquots, multiple analyses of individual samples, and multiple dating methods.

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Trayler, Robin B., Mark D. Schmitz, José I. Cuitiño, Matthew J. Kohn, M. Susana Bargo, Richard F. Kay, Caroline A. E. Strömberg et Sergio F. Vizcaíno. « An improved approach to age-modeling in deep time : Implications for the Santa Cruz Formation, Argentina ». GSA Bulletin 132, n^o 1-2 (23 mai 2019) : 233–44. http://dx.doi.org/10.1130/b35203.1.

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AbstractAccurate age-depth models for proxy records are crucial for inferring changes to the environment through space and time, yet traditional methods of constructing these models assume unrealistically small age uncertainties and do not account for many geologic complexities. Here we modify an existing Bayesian age-depth model to foster its application for deep time U-Pb and 40Ar/39Ar geochronology. More flexible input likelihood functions and use of an adaptive proposal algorithm in the Markov Chain Monte Carlo engine better account for the age variability often observed in magmatic crystal populations, whose dispersion can reflect inheritance, crystal residence times and daughter isotope loss. We illustrate this approach by calculating an age-depth model with a contiguous and realistic uncertainty envelope for the Miocene Santa Cruz Formation (early Miocene; Burdigalian), Argentina. The model is calibrated using new, high-precision isotope dilution U-Pb zircon ages for stratigraphically located interbedded tuffs, whose weighted mean ages range from ca. 16.78 ± 0.03 Ma to 17.62 ± 0.03 Ma. We document how the Bayesian age-depth model objectively reallocates probability across the posterior ages of dated horizons, and thus produces better estimates of relative ages among strata and variations in sedimentation rate. We also present a simple method to propagate age-depth model uncertainties onto stratigraphic proxy data using a Monte Carlo technique. This approach allows us to estimate robust uncertainties on isotope composition through time, important for comparisons of terrestrial systems to other proxy records.

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McWilliams, C. K., M. J. Kunk, R. P. Wintsch et D. L. Bish. « Determining ages of multiple muscovite-bearing foliations in phyllonites using the 40Ar/39Ar step heating method : Applications to the Alleghanian orogeny in central New England ». American Journal of Science 313, n^o 10 (1 décembre 2013) : 996–1016. http://dx.doi.org/10.2475/10.2013.02.

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Garnier, V., H. Maluski, G. Giuliani, D. Ohnenstetter et D. Schwarz. « ArAr and UPb ages of marble-hosted ruby deposits from central and southeast Asia ». Canadian Journal of Earth Sciences 43, n^o 4 (1 avril 2006) : 509–32. http://dx.doi.org/10.1139/e06-005.

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To date the formation of ruby deposits and link it to the regional metamorphism associated with Tertiary Himalayan orogenesis, 40Ar39Ar stepwise heating experiments were performed on single grains of phlogopite syngenetic with ruby, and zircon inclusions in ruby and spinel were dated with the UPb method by ion-probe. The ArAr ages of phlogopites associated with ruby are Oligocene (24.7 ± 0.3 Ma) at Jegdalek in Afghanistan; Miocene at Mogok in Myanmar (18.7 ± 0.2 to 17.1 ± 0.2 Ma), at Hunza in Pakistan (10.8 ± 0.3 to 5.4 ± 0.3 Ma), and Chumar in Nepal (5.6 ± 0.4 Ma); and Pliocene (4.6 ± 0.1 Ma) at Ruyil in Nepal. In Vietnam, a zircon included in a ruby from the Quy Chau deposit yielded a 238U206Pb age of 53.8 ± 4.6 Ma, whereas in the Red River shear zone, ruby formed at around 4036 Ma during ductile deformation under peak metamorphic conditions. The ages obtained in this study are in agreement with those previously published for the ruby-bearing metamorphic belts and document extensional tectonics that were active from Afghanistan to Vietnam between the Oligocene and the Pliocene. Ruby-bearing marbles define a high-quality gem belt linked to the high-temperature metamorphism of the Himalayan fold belt that developed during the Tertiary collision of the Indian plate with Asia.

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