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Identifying the mechanism(s) of 40Ar redistribution and loss in feldspar during protracted residence in high-temperature fluid-free geologic environments

Applicant Popov Daniil
Number 191478
Funding scheme Early Postdoc.Mobility
Research institution Department of Earth Sciences The Open University
Institution of higher education Institution abroad - IACH
Main discipline Geochronology
Start/End 01.07.2020 - 31.12.2021
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Keywords (4)

thermochronology; Ar-Ar; feldspar; geochronology

Lay Summary (French)

La date isotopique d'un minéral est une horloge de décroissance radioactive qui nous montre combien de temps s'est écoulé après un certain événement dans l'histoire de la Terre. Pour une horloge donnée, cet événement pourrait être la cristallisation du magma, le refroidissement sous une certaine température ou l'interaction avec les fluides circulant dans la croûte terrestre. Ce projet vise à améliorer notre compréhension pour lesquels ces événements sont dates par l'horloge qui utilise la désintégration du potassium (K) en argon (Ar) dans des feldspaths.
Lay summary
Les dates K-Ar (et leur variété Ar-Ar) des feldspaths ont généralement été interprétées comme enregistrant quand les roches se refroidissaient en dessous de 350-150°C. Cependant, un nombre croissant de données suggère que, dans de nombreux cas, une telle interprétation est incorrecte, et ces dates nous montrent quand les feldspaths interagissent avec les fluides circulant dans la croûte terrestre. Cette étude vise à identifier dans quels cas l'horloge K-Ar dans le feldspath enregistre le moment du refroidissement des roches. L'objectif spécifique de ce projet est d'identifier quel mécanisme a démarré l'horloge K-Ar dans les feldspaths des roches métamorphiques à haute température en Norvège. Les études pétrologiques suggèrent que ces roches n’ont presque pas interagi avec les fluides circulant dans la croûte terrestre. Par conséquent, les dates K-Ar (Ar-Ar) des feldspaths de ces roches peuvent enregistrer le moment du refroidissement. Ce projet vérifiera si c'est le cas en (i) collectant des échantillons de roches non affectées par l'interaction avec les fluides, (ii) en caractérisant les feldspaths pour confirmer qu'ils n'ont pas interagi avec les fluides et (iii) en obtenant des dates Ar-Ar pour ces feldspaths.
Direct link to Lay Summary Last update: 09.12.2019

Responsible applicant and co-applicants


Inclusions of Amorphous and Crystalline SiO2 in Minerals from Itrongay (Madagascar) and Other Evidence for the Natural Occurrence of Hydrosilicate Fluids
Popov Daniil V., Spikings Richard A., Razakamanana Théodore (2022), Inclusions of Amorphous and Crystalline SiO2 in Minerals from Itrongay (Madagascar) and Other Evidence for the Natural Occurrence of Hydrosilicate Fluids, in Geosciences, 12(1), 28-28.
Numerical Modelling of Radiogenic Ingrowth and Diffusion of Pb in Apatite Inclusions with Variable Shape and U-Th Zonation
Popov Daniil V., Spikings Richard A. (2021), Numerical Modelling of Radiogenic Ingrowth and Diffusion of Pb in Apatite Inclusions with Variable Shape and U-Th Zonation, in Minerals, 11(4), 364.
Thermochronology of Alkali Feldspar and Muscovite at T > 150 °C Using the 40Ar/39Ar Method: A Review
Spikings Richard A., Popov Daniil V. (2021), Thermochronology of Alkali Feldspar and Muscovite at T > 150 °C Using the 40Ar/39Ar Method: A Review, in Minerals, 11(9), 1025.

Scientific events

Active participation

Title Type of contribution Title of article or contribution Date Place Persons involved
AGU Fall Meeting 2021 Talk given at a conference A Holistic Approach to Understanding the Migration Mechanisms of Pb in Apatite and Ar in Alkali Feldspar from Proterozoic Granitic Batholiths from the Mt. Isa Inlier (Australia): Inferences from Multi-Method Petrological Characterisation and Isotope Analy 14.12.2021 online, United States of America Popov Daniil;
Goldschmidt 2021 conference Talk given at a conference Multi-method approach to understanding the migration mechanisms of Pb in apatite and Ar in alkali feldspar from Proterozoic granitic batholiths from the Mt. Isa Inlier (Australia) 06.07.2021 online, France Popov Daniil;


Title Date Place
session "Rates and Mechanisms of Chemical and Isotopic Transport in Geomaterials with Implications for Geochronology" at Goldschmidt 2021 conference 06.07.2021 online, France

Associated projects

Number Title Start Funding scheme
160052 Advancing alkali feldspar and muscovite 40Ar/39Ar thermochronology 01.12.2015 Project funding (Div. I-III)
202872 Determination of argon diffusivity in alkali feldspar samples that fracture in response to laboratory heating 01.01.2022 Postdoc.Mobility


Our understanding of Earth history is underpinned by knowledge of when geological events happened and how long they lasted. One of the approaches to determine the timing and the duration of geological events is using a radioactive decay clock: the ratio of daughter to parent isotopes in a mineral or a rock indicates how long the clock has been ticking and provides an isotopic date. Different radioactive decay clocks, namely different radioactive parent-daughter isotope pairs in different minerals, can be started or reset by different processes, such as crystallisation, fluid-mediated alteration or cooling below the temperatures at which daughter isotopes can escape minerals by diffusion. Accurate interpretation of an isotopic date therefore requires an understanding of which process is responsible in a given case. This project seeks to improve the present-day understanding of which processes can start and reset the clock utilising the decay of 40K to 40Ar in feldspars (covering both plagioclase and alkali feldspar). Feldspars are the most common mineral group in the Earth’s crust, and occur in a wide range of rock types. Therefore, their 40Ar/39Ar dates (a modern variant of K-Ar dates) can provide a valuable insight into Earth history. For decades these dates were interpreted to record the time of cooling of feldspars’ host rocks through ~350-150°C (e.g. Lovera et al., 1989; Lovera et al., 2002) and used to constrain the rates and timescales of numerous geologic phenomena, including the exhumation of deep crust and slow cooling of large magma bodies. Such interpretations rely on the assumption that volume diffusion is the sole mechanism of 40Ar migration over geologic timescales. However, an increasing amount of data suggests that 40Ar may be redistributed and removed from feldspars more rapidly and effectively by fluid-mediated alteration and recrystallisation (e.g. Parsons et al., 1999; Chafe et al., 2014). This includes the data obtained during my PhD research. Diffusive loss of 40Ar may therefore be the exception rather than the norm, throwing doubt on hundreds of tectonic interpretations that were obtained assuming that 40Ar/39Ar dates of feldspars constrain thermal histories of rocks.Importantly, however, all previous data suggesting that 40Ar migration within feldspars is controlled by fluid-related processes were obtained from rocks that exhibit abundant evidence for interaction with fluids and contain abundant water-bearing minerals, which could release fluids in metamorphic reactions. Although such ‘wet’ rocks dominate those exposed at the Earth’s surface, some rocks are ‘dry’ and show no evidence of interaction with fluids. Maybe feldspars from these ‘dry’ rocks loose 40Ar by diffusion? There is no data available to answer this question, which hinders the development of a fully-constrained framework for interpreting 40Ar/39Ar dates of this critically important mineral. This study therefore aims to acquire such data and improve our understanding of the drivers of 40Ar redistribution and loss in feldspars.In detail, I aim to identify the mechanism(s) of 40Ar migration in feldspar from granulite-facies metamorphic and magmatic rocks from the Bergen Arcs and the Western Gneiss Region in Norway. These >0.9 Ga rocks have preserved their original mineral compositions and textures through a later eclogite-facies metamorphic event at ~0.4 Ga. This preservation is thought to be related to the lack of fluids that are considered to catalyse mineral reactions (e.g. Kühn et al., 2000; Krabbendam et al., 2000; Wain et al., 2001). These rocks are therefore an ideal natural laboratory for seeking evidence for diffusive redistribution and loss of 40Ar in feldspar. The specific objectives of the proposed study include (i) sampling a variety of granulite-faces felsic and mafic gneisses and magmatic rocks, (ii) selecting samples that exhibit the least evidence for interaction with fluids, (iii) separating feldspars from the chosen samples and characterising their texture using optical and electron microscopy and cathodoluminescence imaging, (iv) collecting bulk-grain and in situ 40Ar/39Ar data, and (v) comparing the obtained 40Ar/39Ar data with feldspar textures.In summary, this study will provide an important benchmark for interpreting 40Ar/39Ar dates of feldspars from a variety of different rock types. This will allow for obtaining more accurate constraints on processes which shaped the Earth.