Micro-drilling ID-TIMS U-Pb dating of single monazites: A new method to unravel complex poly-metamorphic evolutions. Application to the UHT granulites of Andriamena (North-Central Madagascar)

• Published in: Contributions to mineralogy and petrology 2004-03, Vol.147 (1), p.110-122
• Main Authors: Paquette, Jean-Louis, Goncalves, Philippe, Devouard, Bertrand, Nicollet, Christian
• Format: Article
• Language: English
• Subjects: Crystals; Drilling; Grain growth; High temperature; Mass spectrometry; Metamorphism; Mineralogy; Minerals; Petrology; Zirconium
• ISSN: 0010-7999; 1432-0967
• DOI: 10.1007/s00410-003-0549-8

In terranes subject to complex metamorphic evolution, zircon and monazite U-Pb system can record successive growth and/or resetting episodes. Conventional isotope dilution-thermal ionisation mass spectrometry (ID-TIMS) dating of fractions or single grains from conventionally separated zircon and monazite populations may produce inaccurate ages. A new technique combining textural analysis and ID-TIMS dating on single monazite crystals is proposed. This method is applied directly on thin sections with three successive steps comprising (1) a characterisation of textural relationships using electron microprobe analyses and images, (2) extraction by micro-drilling of selected monazites and (3) U-Pb dating of each extracted grain by ID-TIMS. The potential for this in situ dating technique are tested on Ultra High Temperature (UHT) granulites from North-Central Madagascar. Four distinct events on a single thin section were distinguished. The UHT metamorphism was dated at 2.5 Ga, while two retrogression events were dated at 790 and 500 Ma. Finally, a single crystal shielded in quartz seems to preserve a pre-metamorphic age around 2.7 Ga. [PUBLICATION ABSTRACT]