Project

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Advancing alkali feldspar and muscovite 40Ar/39Ar thermochronology

Applicant Spikings Richard A.
Number 160052
Funding scheme Project funding
Research institution Département des sciences de la Terre Université de Genève
Institution of higher education University of Geneva - GE
Main discipline Geochronology
Start/End 01.12.2015 - 30.11.2019
Approved amount 311'294.00
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All Disciplines (4)

Discipline
Geochronology
Geology
Other disciplines of Earth Sciences
Mineralogy

Keywords (4)

Diffusion; 40Ar/39Ar thermochronology; Geochronology; U-Pb thermochronology

Lay Summary (French)

Lead
Notre objectif est d’améliorer la précision des histoires thermales continues entre les températures de ~150 - 550°C.
Lay summary

Notre objectif est d’améliorer la précision des histoires thermales continues entre les températures de ~150 – 550°C. L’histoire thermale des solutions >150°C est très utile en Sciences de la Terre car elle permet d’enregistrer les processus fondamentaux depuis l’éjection à l’impacte d’astéroïde, jusqu’aux processus des croûtes terrestres supérieure et intermédiaire. Nous testerons la précision de l’histoire thermale de solutions par analyses isotopiques d’Ar sur des feldspaths alcalins et muscovites. La plupart des systèmes thermochronologiques connus sont sensibles aux températures < ~180°C, et seulement quelques techniques pourraient l’être entre 150 – 550°C. Des études récentes ont montré que la thermochronologie U-Pb des apatites peut fournir des données précises et quantitatives entre ~350 – 550°C. Cependant, obtenir des histoires thermales entre 150 – 350°C en utilisant des données isotopiques reste très controversé. Des solutions incluent le modèle de diffusion multi-domaine de Lovera et al. (1989), celui de trajet-multiple de Lee (1995), et quelques expériences semblent valider la présence de profils de volume de diffusion, et la récupération de l’information de l’histoire thermale de feldspaths alcalins de qualité gemme (e.g. Flude et al., 2014). Au contraire, divers auteurs (e.g. Villa, 2006; Chafe et al., 2014) rapportent que la distribution d’isotopes Ar dans les feldspaths est fonction de l’interaction de fluides, empêchant la récupération de donnée quantitative t-T. La vérification du rôle joué par le volume de diffusion durant la perte de l’isotope fils à l’échelle géologique est primordiale pour valider la précision des études thermochronologiques. Nous étudierons la distribution des isotopes Ar dans les feldspaths alcalins et les muscovites en comparant les histoires thermales continues obtenues par des systèmes isotopiques indépendants dans différentes phases minérales d’une même roche.

Direct link to Lay Summary Last update: 06.08.2015

Responsible applicant and co-applicants

Employees

Publications

Publication
Diffusion and fluid interaction in Itrongay pegmatite (Madagascar): Evidence from in situ 40Ar/39Ar dating of gem-quality alkali feldspar and U Pb dating of protogenetic apatite inclusions
Popov Daniil V., Spikings Richard A., Scaillet Stéphane, O'Sullivan Gary, Chew David, Badenszki Eszter, Daly J. Stephen, Razakamanana Théodore, Davies Joshua H.F.L. (2020), Diffusion and fluid interaction in Itrongay pegmatite (Madagascar): Evidence from in situ 40Ar/39Ar dating of gem-quality alkali feldspar and U Pb dating of protogenetic apatite inclusions, in Chemical Geology, 556, 119841-119841.
Pathways for 39Ar loss during step-heating of alkali feldspar megacrysts from the Shap granite (UK): Combined evidence from diffusion experiments and characterisation of heating-induced texture modifications
Popov Daniil V., Spikings Richard A., Kouzmanov Kalin (2020), Pathways for 39Ar loss during step-heating of alkali feldspar megacrysts from the Shap granite (UK): Combined evidence from diffusion experiments and characterisation of heating-induced texture modifications, in Chemical Geology, 547, 119677-119677.
Diffusion vs. fluid alteration in alkali feldspar 40Ar/39Ar thermochronology: Does cross-correlation of log(r/r0) and age spectra validate thermal histories?
Popov Daniil V., Spikings Richard A. (2020), Diffusion vs. fluid alteration in alkali feldspar 40Ar/39Ar thermochronology: Does cross-correlation of log(r/r0) and age spectra validate thermal histories?, in Chemical Geology, 539, 119506-119506.
Removing a mask of alteration: Geochemistry and age of the Karadag volcanic sequence in SE Crimea
Popov Daniil V., Brovchenko Valeria D., Nekrylov Nikolai A., Plechov Pavel Yu, Spikings Richard A., Tyutyunnik Oksana A., Krigman Lyubov V., Anosova Maria O., Kostitsyn Yuri A., Soloviev Alexey V. (2019), Removing a mask of alteration: Geochemistry and age of the Karadag volcanic sequence in SE Crimea, in Lithos, 324-325, 371-384.

Collaboration

Group / person Country
Types of collaboration
Stephen Daly, Geology, School of Earth Sciences, University College, Dublin Ireland (Europe)
- Research Infrastructure
David Chew, Geology, Trinity College, Dublin Ireland (Europe)
- in-depth/constructive exchanges on approaches, methods or results
- Research Infrastructure
Othmar Muentener, Institut des Sciences de la Terre, UNIL Switzerland (Europe)
- Research Infrastructure

Associated projects

Number Title Start Funding scheme
146332 Advancing U-Pb high temperature thermochronology by combining single grain and intra-grain dating 01.04.2013 Project funding
177026 Re-equiping the noble gas laboratory to perform state of the art science, University of Geneva 01.03.2018 R'EQUIP
202872 Determination of argon diffusivity in alkali feldspar samples that fracture in response to laboratory heating 01.01.2022 Postdoc.Mobility
191478 Identifying the mechanism(s) of 40Ar redistribution and loss in feldspar during protracted residence in high-temperature fluid-free geologic environments 01.07.2020 Early Postdoc.Mobility

Abstract

This proposal requests funds for three years for a new PhD student, who will work on a project designed to advance the science of quantitative thermochronology and its application to Earth Sciences. The principal motivation for this proposal is to significantly improve the accuracy of continuous time (t) - Temperature (T) solutions between the temperatures of ~150 - 550°C, which are currently far more controversial than at lower temperatures. Thermal history solutions at >150°C are extremely useful in Earth Sciences because they can constrain fundamental processes ranging from the timing of ejection and impact of asteroids, to terrestrial processes within the Earth’s middle and lower crust. We aim to raise funds to test the accuracy of continuous t-T solutions obtained from argon isotopic data acquired from alkali feldspars and muscovite micas. Can we account for the Ar isotope data by solely invoking volume diffusion, or is Ar redistribution dominated in most cases by the presence of retrogressed phases and fluid interaction?Our approach is to i) generate t-T models from (bulk and in-situ) Ar isotope data, assuming volume diffusion is the principal mechanism for Ar loss in nature and in the laboratory, ii) compare the Ar degassing behaviour with microtextural and compositional variations within single crystals, and iii) compare these t-T solutions with independent numerical t-T solutions obtained from apatite U-Pb dates from the same rock, facilitating an assessment of the accuracy of each solution, and thus testing the premise of volume diffusion.A majority of well-constrained thermochronological systems are sensitive to temperatures that are lower than ~180°C (e.g. fission track, (U-Th)/He methods), and only a few techniques have been developed that may be able to quantify t-T between 150 - 550°C. Studies during the previous decade (e.g. Cochrane et al., 2014) have shown that apatite U-Pb thermochronology can provide accurate, quantitative t-T information between ~350 - 550°C. However, solving t-T at temperatures between 150 - 350°C using isotopic data remains highly controversial. Solutions include the multi-domain diffusion model of Lovera et al. (1989), and the multi-path model of Lee (1995), and some experiments appear to validate the presence of volume diffusion profiles, and thus the recovery of t-T paths from gem-quality alkali feldspars (e.g. Flude et al., 2014). Similarly, Harrison et al. (2009) suggest that muscovite micas may also preserve Ar diffusion profiles, which can be measured to generate quantitative t-T information between ~340 - 450°C (cooling at 10°C/My, 100micron diffusion length), although this approach has only been used once (Harrison and Lovera, 2014). On the contrary, several authors (e.g. Villa, 2006; Chafe et al., 2014) report that the distribution of Ar isotopes in feldspars is a function of fluid interaction and sub-solidus transformations, precluding the recovery of quantitative t-T information. Experimental verification of the role played by volume diffusion during daughter isotope loss over geological time-scales is therefore paramount to validate the accuracy of thermochronological studies. In this proposal, we aim to examine the causes of the distribution of Ar isotopes in alkali feldspars and muscovite by comparing continuous t-T paths obtained from independent isotopic systems in different mineral phases in the same rock. Overlap between the t-T solutions obtained from the 40Ar/39Ar and U-Pb isotopic systems would strongly support a hypothesis that diffusion modelling of Ar isotopic data can recover accurate t-T information. Non-systematic discrepancies between t-T paths would suggest that Ar concentration profiles are dominated by sub-solidus transformations, and these will be investigated with microtextural and compositional analyses.
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