Littérature scientifique sur le sujet « 40Ar/39Ar method »

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.

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.

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.

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

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.

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.

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.

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.

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.

L’Himalaya è classicamente considerata una catena orogenica strutturalmente cilindrica per l’impressionante continuità laterale, da ovest ad est, delle principali unità lito-tettoniche e zone di taglio, caratteristica peculiare di questa catena collisionale. L’esumazione del cuore metamorfico della catena, il Greater Himalayan Sequence (GHS), è favorita dall’attività di due zone di taglio regionali a cinematica opposta: la Main Central Thrust zone (MCTz) a cinematica compressiva e il South Tibetan Detachment System (STDS) a cinematica normale, rispettivamente alla base e al tetto del GHS stesso. In questa tesi ho studiato l’evoluzione strutturale e geocronologica del STDS e della MCTz, con particolare focus su quest’ultima, in due transetti nell’Himalaya indiana occidentale: le valli dell’Alaknanda – Dhauli Ganga e la valle del Bhagirathi – Gangotri nella regione del Garhwal. A questo scopo, ho adottato un approccio multidisciplinare che combina studi microstrutturali, chimici e geocronologici e stime di vorticità cinematica. Poiché le miche sono ubiquitarie nelle zone di taglio, il metodo geocronologico 40Ar/39Ar su biotitie e muscovite è stato ampiamente utilizzato in passato e viene utilizzato tutt’oggi per vincolare l’età della deformazione per taglio. Il metodo 40Ar/39Ar step-heating, il più adatto per questo tipo di studi, è un metodo chiave per risolvere complessità petrologiche e chimiche grazie al riconoscimento di età differenti dovute a un differente rilascio dell’Ar caratterizzato da diversi rapporti Cl/K e Ca/K. Ho applicato questo metodo, combinato con una nuova procedura, l’Ar Differential Release Plot (DRP), che permette di identificare chiaramente l’influenza della coesistenza di fillosilicati nel trend di rilascio dell’Ar durante gli step di riscaldamento, permettendo di selezionare gli step ottimali che corrispondono al degassamento delle miche in senso stretto. Questa nuova procedura permette di determinare l’età in modo molto più accurato in rocce polideformate come quelle studiate. I risultati ottenuti con il metodo 40Ar/39Ar supportano la diacronicità della MCTz e del STDS nelle aree studiate, suggerendo che i modelli di esumazione del GHS dovrebbero tener conto della mancanza di contemporaneità tra queste due strutture. Inoltre, è stato sviluppato un nuovo approccio tridimensionale basato sulla X-ray micro Computed Tomography (microCT) e applicato per lo studio della vorticità cinematica usando il metodo dei porfiroclasti stabili. Un esame dei dati di letteratura ha dimostrato che il nostro approccio nello studio di vorticità usando il metodo dei porfiroclasti stabili non era mai stato applicato prima. Questo metodo è stato applicato a rocce provenienti dalla MCTz nella valle del Bhagirathi e i risultati suggeriscono che l’esumazione del GHS sia stata guidata da una variazione di deformazione all’interno della MCTz da un deformazione dominata da taglio semplice a cui è seguita una deformazione dominata da taglio puro, coerentemente con i dati di letteratura lungo la catena. Riassumendo, combinando studi meso e microstrutturali con analisi chimiche e geocronologia 40Ar/39Ar, i risultati della mia tesi indicano che la MCTz e il STDS non sono coevi nei transetti studiati e che la MCTz mostra una variazione di regime deformativo durante la sua evoluzione.
The Himalaya is commonly regarded as a cylindrical belt from west to east due to the impressive lateral continuity of the main litho-tectonic units and faults/shear zones, which is a peculiar feature of this mountain range. The exhumation of the metamorphic core of the belt, the Greater Himalayan Sequence (GHS), was favored by two regional scale opposite-kinematics ductile to brittle shear zones: the contractional Main Central Thrust zone (MCTz) at the bottom and the normal-sense South Tibetan Detachment System (STDS) at the top of the GHS itself. In this thesis, I investigated the structural and geochronological evolution of the STDS and the MCTz, with a particular focus on the latter, in two transects in the Indian Western Himalaya: the Alaknanda – Dhauli Ganga Valleys and the Bhagirathi – Gangotri Valley in the Garhwal region. To this aim, I used a multidisciplinary approach, which combines microstructural, chemical and geochronological studies, as well as a kinematic estimate. As micas are ubiquitous in strongly deformed shear zones, the 40Ar/39Ar geochronological method on biotite and muscovite has commonly been employed to constrain the ages of mylonitization. The 40Ar/39Ar step-heating approach, the most useful for the present study, is a key procedure to unravel petrological and chemical complexities because of the recognition of different ages due to different steps of Ar release characterized by different Cl/K and Ca/K ratios. I applied this method, combined with a new methodological approach first developed in detail during this Ph.D. thesis work, to rocks coming from the bounding shear zones of the GHS. This new procedure, named Ar Differential Release Plot (DRP), that allows to clearly identify the influence of the co-existence of phyllosilicates on the trend of Ar release during the heating steps, allowing to select the optimal steps corresponding to the degassing of micas sensu stricto, which leads to more reliable age determinations in such metamorphic polydeformed rocks. The results of 40Ar/39Ar method support the diachroneity of the MCTz and the STDS in the studied areas, suggesting that the models of exhumation of the GHS should account for their lack of contemporaneity. Moreover, a new three-dimensional approach based on the X-ray micro Computed Tomography (microCT) has been developed and applied for the study of the kinematic vorticity using the stable porphyroclasts method. A close examination of literature data shows that our study approach in the study of kinematic vorticity using the microCT has never been applied before. This method has been applied to MCTz rocks from the Bhagirathi valley and the results suggest that the exhumation of the GHS has been affected by a shift of deformation within the MCTz from simple to pure shear dominated flow, coherent with literature data all along the belt. In summary, combining meso‐ and micro‐structural studies as well as chemical analyses and 40Ar/39Ar geochronology, my thesis results points out how MCTz and STDS are not coeval in the studied structural transects and that the MCTz shows a shift in deformation regime during its evolution.

ABSTRACT The last major mass extinctions in Earth history (e.g., end-Guadalupian, end-Permian, end-Triassic, and end-Cretaceous) are all correlated closely in time with the main-phase eruptions of major flood basalt provinces (Emeishan, Siberian, Central Atlantic Magmatic Province, and Deccan Traps, respectively). The causal relationship between flood volcanism and mass extinction is not clear, but likely involves the climate effects of outgassed volatile species such as CO2, SO2, Cl, F, etc., from some combination of magma and country rocks. In a surprising “coincidence,” the end-Cretaceous (K-Pg boundary) micro-faunal extinction also corresponds precisely in time to what may have been the largest meteor impact of the past billion years of Earth history, the Chicxulub crater at 66.05 Ma. The Deccan Traps eruptions were under way well before K-Pg/Chicxulub time and are most likely the result of the mantle plume “head” that initiated the presently active Reunion hotspot track—thus the Deccan Traps were clearly not generated, fundamentally, by the impact. However, recent high-precision 40Ar/39Ar geochronology indicates that conspicuous changes in basalt geochemistry, lava flow morphology, emplacement mode, and a possible 50% increase in eruption rate at the Lonavala/Wai subgroup transition in the Deccan Traps lava group corresponded, within radioisotopic age precision, to the K-Pg boundary and the Chicxulub impact. This has led to the testable hypothesis that the Mw ~11 seismic disturbance of the Chicxulub impact may have affected the Deccan eruptions. Here we review a broad landscape of evidence regarding Deccan volcanism and its relation to the K-Pg boundary and attempt to define what we see as the most important questions than can and should be answered by further research to better understand both the onshore and largely unknown offshore components of Deccan-related volcanism, and what their climate and environmental impacts at K-Pg time may have been.