Subgrain 40Ar/39Ar dating of museum-quality micas reveals intragrain heterogeneity

Naumenko-Dèzes, Maria O.; Villa, Igor M.; Rolland, Yann; Gallet, Sylvain; Lanari, Pierre

doi:10.1016/j.chemgeo.2021.120215

Cited by 3 publications

(2 citation statements)

References 29 publications

(41 reference statements)

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“…Relatively few previous studies of pegmatites present spatially resolved isotopic dates that are supplemented by detailed petrological characterisation of the analysed materials, and they often report significant scatter in the obtained apparent ages [24][25][26][27][28][29][30][31][32][33]. Some of these results were or can be taken as evidence for protracted, multi-stage and potentially polygenic origin of individual pegmatites, which challenges the very foundation of thinking of them as fast-forming products of the late-stage evolution of felsic magmas.…”

Section: Introductionmentioning

confidence: 99%

Do Pegmatites Crystallise Fast? A Perspective from Petrologically-Constrained Isotopic Dating

Popov

2023

Geosciences

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Most recent studies consider the formation of individual pegmatite bodies to be a fast process with estimated crystal growth rates reaching a walloping 10 m/day. This opinion is presumably underpinned by the traditional way of thinking of them as the end products of magmatic fractionation. Indeed, modelling has shown that if a pegmatite-forming substance with a temperature near granitic solidus intrudes into a much colder host rock, as recorded in some outcrops, it must cool rapidly. From here, a conclusion is made that the crystallisation must likewise be rapid. However, this view is challenged by several studies that published isotopic dates supported by petrological characterisation of the analysed materials, which suggested or can be used to suggest that some minerals in pegmatites grew over millions of years. Surprisingly, such in-depth work on the geochronology of individual pegmatite bodies is relatively uncommon, so it is early to make generalisations. Here, I highlight some of the existing evidence with the aim to stimulate further research into the timescales of pegmatite crystallisation, including the use of petrologically constrained isotopic dating.

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Section: Introductionmentioning

confidence: 99%

Do Pegmatites Crystallise Fast? A Perspective from Petrologically-Constrained Isotopic Dating

Popov

2023

Geosciences

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“…Lanari et al, 2014]. In addition, the interpretation of 40 Ar/ 39 Ar ages of K-bearing minerals (most often phyllosilicates, more rarely feldspars or amphiboles) is complicated by the fact that deformation introduces microstructural defects in these minerals, favoring their partial or complete fluid-induced recrystallization that have a great effect on the distribution and the retention of argon in their crystalline lattice at the nanometer scale [Naumenko-Dèzes et al, 2021, Villa, 2022. It should also be stressed that for these deformed rocks the interpretation of the 40 Ar/ 39 Ar data in terms of cooling age or (re)crystallization age remains a subject of debate as well as the mechanisms of age reset during deformation [Villa et al, 2014, Villa, 2016.…”

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confidence: 99%

40 Ar/ 39 Ar geochronology of crustal deformation

Monié,

Münch,

Milesi

et al. 2024

Comptes Rendus. Géoscience

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Crustal deformation is characterized by brittle and ductile faults that accommodate at different scales the strain imposed by plate tectonics. The aim of this contribution is to show with the help of different examples how the in situ 40 Ar/ 39 Ar dating of synkinematic neocrystallized minerals in ductile shear zones and the step-heating 40 Ar/ 39 Ar dating of synkinematic authigenic clays in fault gouges can bring information on the timing of fault activity. However, due to their complex evolution, interpretation of the argon signature in fault zones requires consideration of several effects among which re-or neocrystallization, inheritance and fluid interaction processes are dominant.

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<sup>40</sup>Ar/<sup>39</sup>Ar dating of a hydrothermal pegmatitic buddingtonite–muscovite assemblage from Volyn, Ukraine

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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Subgrain 40Ar/39Ar dating of museum-quality micas reveals intragrain heterogeneity

Cited by 3 publications

References 29 publications

Do Pegmatites Crystallise Fast? A Perspective from Petrologically-Constrained Isotopic Dating

Do Pegmatites Crystallise Fast? A Perspective from Petrologically-Constrained Isotopic Dating

40 Ar/ 39 Ar geochronology of crustal deformation

<sup>40</sup>Ar/<sup>39</sup>Ar dating of a hydrothermal pegmatitic buddingtonite–muscovite assemblage from Volyn, Ukraine

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Subgrain 40Ar/39Ar dating of museum-quality micas reveals intragrain heterogeneity

Cited by 3 publications

References 29 publications

Do Pegmatites Crystallise Fast? A Perspective from Petrologically-Constrained Isotopic Dating

Do Pegmatites Crystallise Fast? A Perspective from Petrologically-Constrained Isotopic Dating

40 Ar/ 39 Ar geochronology of crustal deformation

&lt;sup&gt;40&lt;/sup&gt;Ar/&lt;sup&gt;39&lt;/sup&gt;Ar dating of a hydrothermal pegmatitic buddingtonite–muscovite assemblage from Volyn, Ukraine

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<sup>40</sup>Ar/<sup>39</sup>Ar dating of a hydrothermal pegmatitic buddingtonite–muscovite assemblage from Volyn, Ukraine