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MONA : La monazite hydrothermale

6 décembre 2012 ( dernière mise à jour : 2 juin 2017 )

MONA – La monazite hydrothermale : l’accessoire indispensable


Durée : 2012 - 2017

Financement : ANR Jeunes Chercheurs, Jeunes Chercheuses - JCJC

Coordinateur : Emilie Janots (ISTerre)

Equipes ISTerre impliquées :
- Minéralogie

Partenaires :
GéoSciences Environnements Toulouse, Laboratoire Magmas et Volcans (LMV, Clermont-Ferrand)

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Résumé du projet :

Dating hydrothermal mineralization is difficult because most chronometers are commonly reset in open system. Although monazite is rare in hydrothermal deposits, it is very attractive to date mineralization because it has a very robust isotopic system (U-Th-Pb) that retains the age of its precipitation. In presence of successive fluid circulations, monazite (re)crystallizes sporadically. Growth episodes are identifiable from zoning in trace elements composition (REE+Y, Th, U and Pb). Using in-situ high resolution techniques (spot size 5µm), it is possible to date these distinct domains and to obtain the age of the successive mineralization stages. In monazite taken from Alpine clefts (Central Alps), preliminary SIMS U-Th-Pb dating gave very promising results. Results show that Miocene monazite can be dated with a resolution of 0.2 Ma. Within a single grain, it is possible to distinguish episodic growth stages separated by an interval <2Ma. Furthermore, monazite is zoned with extreme trace elements fractionation (Th/U up to 600), which remains to be understood.

The aim of this project is to investigate the potential of monazite as chronometer and geochemical tracer of fluid mineralization via detailed characterization of natural samples and hydrothermal experiments. The objectives are two-fold : (1) Obtaining monazite growth ages and growth duration for different areas of the Alps and to link them with the local deformation and exhumation history, (2) trying to understand chemical and isotopic behaviour of the REE+Y, Th and U during monazite/fluid interaction.

Project achievement will require a pluridisciplinar approach combining mineralogy, experimental petrology, geochronology and structural geology. For the experimental part (ISTerre, Grenoble), we expect to perform 32 experiments in cold-seal vessels in order to evaluate the role of monazite composition, T, fO2 and fluid speciation on the trace elements behaviour. For the natural case study, we dispose of large hydrothermal monazite crystals taken from different parts of the Alps. Identification of experimental and natural phases requires use of advanced and expensive analytical techniques, including XRD, EMP, SEM, TEM, FIB, LA-ICP-MS, SIMS and fluid inclusions analyses (GET, Toulouse). High-resolution Th-Pb isotopic age dating of cleft monazites will be obtained using LA-ICP-MS (Laboratoire Magmas-Volcans, Clermont-Ferrand). For the realization of the project, we demand for the funding of a PhD student.

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