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Diffusion kinetics of U, Th, Pb, Hf and REEs in baddeleyite, and development of local electrode atom probe tomography as a means to analyze experimentally induced diffusion profiles in crystalline materials

English title Diffusion kinetics of U, Th, Pb, Hf and REEs in baddeleyite, and development of local electrode atom probe tomography as a means to analyze experimentally induced diffusion profiles in crystalline materials
Applicant Bloch Elias
Number 173988
Funding scheme Ambizione
Research institution Institut des sciences de la Terre Université de Lausanne
Institution of higher education University of Lausanne - LA
Main discipline Geochemistry
Start/End 01.10.2017 - 30.09.2021
Approved amount 871'598.00
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All Disciplines (4)

Discipline
Geochemistry
Geochronology
Other disciplines of Earth Sciences
Mineralogy

Keywords (4)

Baddeleyite; Diffusion; Geochronology; Atom Probe

Lay Summary (French)

Lead
Bien qu'il y ait une eu augmentation significative de données de diffusion expérimentale au cours de ces dernières décennies, il existe encore des lacunes importantes dans cette base de données. De plus, les techniques analytiques utilisées actuellement pour mesurer les profils de diffusion, nécessite généralement que les expériences de diffusion soient menées à des températures dépassant largement les températures d'intérêt géologique. Compte tenu de ces problèmes, je propose de déterminer expérimentalement les diffusivités de U, Th, Pb, Hf et des éléments de terres rares dans la baddeleyite, afin de développer un protocole d'utilisation de la sonde atomique tomographique (LEAP). Cette méthode permet d’analyser des profils de diffusion induit lors d’expériences de basse température et d’éléments chimique a diffusion lente.
Lay summary

La baddeleyite est un minéral accessoire des roches magmatiques et métamorphiques. Avec l’avancée des techniques analytiques, il est devenu possible de dater ce minéral à l’aide du système U-Pb. Cependant, l’interprétation de ces âges dépend de la connaissance des coefficients de diffusion, hors aucun coefficient de diffusion pour la baddeleyite n’a été directement mesuré.

Le premier volet de ce projet a pour but de quantifier les paramètres cinétiques de diffusion de l’U, Th, Pb, Hf et REEs dans la baddeleyite. Les paramètres cinétiques de diffusion de tout système géochronologique sont d’une importance fondamentale quand on veut interpréter un âge radiométrique. En effet, la diffusion de ces éléments peut rendre l’interprétation de ces âges extrêmement complexe.

Le deuxième volet de ce projet est de développer une routine analytique utilisant la tomographie LEAP (local electrode atom probe) afin d’améliorer la résolution spatiale lors de la mesure de profils de diffusion dans les matériaux cristallins. La barrière majeure pour appliquer ces études expérimentales de diffusion aux matériaux géologiques – caractérisés par des vitesses de diffusion extrêmement lentes – est due au fait que les techniques disponibles actuellement peuvent seulement résoudre des coefficients de diffusion plus élevé que ~10-24 m2/s. Comme la vitesse de diffusion dans les matériaux est corrélée à la température, les expériences de diffusion sont communément menées à des températures relativement hautes (~1200-1500 °C) et les résultats sont extrapolés à plus basse température ce qui a pour effet d’augmenter sensiblement les incertitudes. La capacité de mesurer des profils courts grâce à la tomographie LEAP représenterait une avance significative en permettant aux chercheurs de procéder à des expérimentations de diffusion à des températures pertinentes géologiquement parlant et à s’intéresser à des éléments caractérisés par des coefficients de diffusion extrêmement lents. .

Direct link to Lay Summary Last update: 04.09.2017

Lay Summary (English)

Lead
Although there has been a significant rise in the collection of experimental diffusion data over the past several decades, there are still important gaps in the existing dataset. In addition, the current state of analytical techniques used to measure experimentally induced diffusion profiles typically requires that diffusion experiments be conducted at temperatures far exceeding the temperatures of interest to geoscientists. With these issues in mind, here I have proposed to experimentally determine the diffusivities of U, Th, Pb, Hf and REEs in baddeleyite, and to develop a protocol for using local electrode atom probe tomography (LEAP) to analyze the products of low-temperature experiments and slow-diffusing species.
Lay summary

The first component of this project involves quantifying the diffusion kinetic parameters of U, Th, Pb, Hf, and rare earth elements (REEs) in baddeleyite. The diffusion kinetic parameters of the parent and daughter nuclides of any given geochronological system are of fundamental importance when interpreting radiometric ages. For example, a given mineral age can correspond to the formation age of the mineral itself, the time at which the mineral cooled below a critical temperature at which the isotopes constituting the decay system were no longer sufficiently mobile via diffusion, or can take on complex meanings depending on the diffusivities of the parent and daughter nuclides. Although baddeleyite has recently emerged as a popular mineral to retrieve U-Pb ages, to date not one diffusion coefficient from baddeleyite has been reported in the literature.

            The second component of this project is to develop an analytical routine for utilizing local electrode atom probe (LEAP) tomography to analyze short diffusion profiles in crystalline materials. A major limitation of applying experimental diffusion studies to geologic materials – which typically exhibit slow diffusion – is that currently available analytical techniques can only resolve diffusion coefficients as low as ~10-24 m2/s. As a result, experiments are commonly run at relatively high temperatures (~1200-1500 °C) and the retrieved Arrhenius trends are extrapolated down to geologically realistic temperatures, which introduces significant error into calculations utilizing these data. The ability to measure short, low-concentration diffusion profiles via LEAP would represent a significant advance in experimental diffusion kinetic studies in that it would enable researchers to run diffusion experiments at geologically relevant temperatures.

Direct link to Lay Summary Last update: 04.09.2017

Responsible applicant and co-applicants

Employees

Publications

Publication
Diffusion of calcium in forsterite and ultra-high resolution of experimental diffusion profiles in minerals using local electrode atom probe tomography
Bloch E.M., Jollands M.C., Gerstl S.S.A., Bouvier A-S, Plane F., Baumgartner L.P. (2019), Diffusion of calcium in forsterite and ultra-high resolution of experimental diffusion profiles in minerals using local electrode atom probe tomography, in Geochimica et Cosmochimica Acta, 265, 85-95.

Collaboration

Group / person Country
Types of collaboration
ScopeM/ETH Zurich Switzerland (Europe)
- in-depth/constructive exchanges on approaches, methods or results
- Publication
- Research Infrastructure

Abstract

I propose a systematic approach to experimentally quantifying the diffusion kinetics of U, Th, Pb, Hf and REEs in baddeleyite, and developing local electrode atom probe tomography (LEAP) as a means to analyze extremely short (~5-100 nm) diffusion profiles. The potential significance of these projects with respect to U-Pb baddeleyite geochronology and experimental diffusion kinetics is summarized through an evaluation of the current state of these fields and discussion of preliminary data. Throughout this project Dr. Bloch will be working in the experimental petrology and SIMS laboratories at the University of Lausanne, which are run by Prof. Lukas Baumgartner, as well as in the LEAP laboratory at ETH Zürich in collaboration with Dr. Stephan Gerstl.Baddeleyite experiments will focus on determining the diffusion kinetics of U, Th, Pb, Hf and rare earth elements (REEs) as functions of temperature and oxygen fugacity (fO2) in baddeleyite in order to improve existing geochronological and geochemical techniques. The U, Th and Pb data will be applied to analytical and numerical models in order to provide an improved framework for interpreting baddeleyite U-Th-Pb ages. Diffusion data for Hf will be used to evaluate under what circumstances Hf isotopes in baddeleyite preserve the initial 176Hf/177Hf ratio of the host rocks. Data for REE diffusion are needed in order to evaluate whether REE trends in baddeleyite, which are used to infer various characteristics of the host magma during baddeleyite crystallization, correspond to the U-Th-Pb age retrieved from the same sample and whether they reflect equilibrium or fractional crystallization/melting. The U-Pb system in baddeleyite is ideal for dating large igneous provinces (LIPs), which generate magmas that are dominantly mafic in composition and seldom contain zircon, the far more routinely used mineral for U-Pb geochronology. It has been argued that the thermal activity associated with the plumes responsible for LIPs may be the driving force in the break-up of supercontinents (Hill, 1991), and the eruption of LIPs has significant effects on global climate change (McElwain et al., 1999). Furthermore, LIPs are often correlated with major extinction events (Courtillot, 1994). As such, U-Pb baddeleyite ages of LIPs are increasingly being used for paleo-tectonic reconstructions, paleoclimatology and paleontology. Baddeleyite is also an extremely useful geochronological/geochemical tool for studying meteorites, which are predominantly mafic in composition and seldom contain zircon. Determining the diffusive systematics of this geochronometer would thus have direct applications to multiple fields in Earth and planetary sciences, especially considering the fact that to date not one diffusion coefficient from baddeleyite has been reported in the literature.A major limitation of applying experimental diffusion studies to geologic materials - which typically exhibit slow diffusion - is that currently available analytical techniques can only resolve diffusion coefficients as low as ~10-24 m2/s. As a result, experiments are commonly run at relatively high temperatures (~1200-1500 °C) and the retrieved Arrhenius trends are extrapolated down to geologically realistic temperatures, which introduces significant error into calculations utilizing these data. Furthermore, analytical techniques such as Rutherford Backscattering Spectrometry (RBS) that can resolve short (tens of nm) diffusion profiles have relatively poor sensitivity and are therefore problematic for the trace element diffusion studies that are critical for most geochronological interpretations. Developing LEAP as a means to analyze experimentally induced diffusion profiles would allow researchers to conduct diffusion studies at geologically relevant temperatures, and thus allow the acquisition of diffusion data that is far more directly applicable than traditional analytical tools allow.
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