Skip to Main content Skip to Navigation
New interface
Theses

Comportement géochimique du chronomètre U-Th-Pb dans la monazite : approche par analyses in-situ au LA-ICP-MS

Abstract : Monazite is regarded as a robust geochronometer in magmatic and metamorphic crustal rocks because it contains high concentrations of Th, and to a lesser extent U. Insensitive to lead diffusion, it possibly records high temperature geological events without resetting. On the other hand, monazite strongly controls the lanthanide and actinide budget in the host-rock and is involved in numerous chemical reactions, which makes it a good petrogenetic tracer as well. Therefore, the coupling of age and chemical data in monazite allows an accurate interpretation of the geological event which is recorded by monazite crystallization. However, monazite is very sensitive to interactions with fluids, which possibly result in its partial or complete dissolution, followed by recrystallization of new grains. The present study has shown that the behavior of monazite during fluid-rock interaction is controlled by several parameters: fluid and rock composition, ligands, pH and temperature. At low temperatures (350°C - 450°C) in a microgranite, monazite is easily disturbed by dissolutionprecipitation in presence of F and CO2 -rich fluid, while an alkali-bearing fluid has no effects on it. By contrast, at higher temperatures (> 600 ° C) in metapelitic rocks, the same alkalibearing fluid induces dissolution-precipitation of monazite. The impact of this process on the geochronometer of depends on Pb, Th and U mobility in the fluid: as a result, the newly formed grained record either the same age as the initial crystal, the age of fluid/rock interaction, or an apparent age without any geological significance. The U-Pb and Th-Pb isotopic systems in monazite can also be affected by the incorporation of common Pb during crystallization (up to several hundred ppm), then increasing artificially the recorded ages. This work has demonstrated that the correct interpretation of the age data in monazite relies on a careful characterization of each geological example and cannot rely simply on commonly accepted paradigms. In the future, optimizing the use of monazite as geochronometer implies to (1) improve the analytical methods (increasing spatial resolution to solve geological problems at the nanometric scale, use of homogeneous monazites as standards) and (2) use a range of geochemical tools (major- and trace elements, U-Th-Pb dating, oxygen isotopes). This would help to better interpret the ages recorded by monazite.
Document type :
Theses
Complete list of metadata

Cited literature [61 references]  Display  Hide  Download

https://theses.hal.science/tel-00966588
Contributor : ABES STAR :  Contact
Submitted on : Tuesday, June 3, 2014 - 3:57:12 PM
Last modification on : Tuesday, October 19, 2021 - 7:01:39 PM
Long-term archiving on: : Tuesday, April 11, 2017 - 3:24:20 AM

File

DIDIER_2013CLF22418.pdf
Version validated by the jury (STAR)

Identifiers

  • HAL Id : tel-00966588, version 2

Citation

Amélie Didier. Comportement géochimique du chronomètre U-Th-Pb dans la monazite : approche par analyses in-situ au LA-ICP-MS. Sciences de la Terre. Université Blaise Pascal - Clermont-Ferrand II, 2013. Français. ⟨NNT : 2013CLF22418⟩. ⟨tel-00966588v2⟩

Share

Metrics

Record views

580

Files downloads

1327