Monazite “in situ” 207Pb/206Pb geochronology using a small geometry high-resolution ion probe. Application to Archaean and Proterozoic rocks
Résumé
This paper reports the application of secondary ion mass spectrometry (SIMS) using a small geometry Cameca IMS4f ion probe to provide reliable in situ 207Pb/206Pb ages on monazite populations of Archaean and Proterozoic age. The reliability of the SIMS technique has been assessed on two samples previously dated by the conventional ID-TIMS method at 2661±1 Ma for monazites extracted from a pelitic schist from the Jimperding Metamorphic Belt (Yilgarn Craton, Western Australia) and 1083±3 Ma for monazites from a high-grade paragneiss from the Northampton Metamorphic Complex (Pinjarra Orogen, Western Australia). SIMS results provide 207Pb/206Pb weighted mean ages of 2659±3 Ma (n=28) and 1086±6 Ma (n=21) in close agreement with ID-TIMS reference values for the main monazite growth event. Monazites from the Northampton Complex document a complex history. The spatial resolution of about 30 μm and the precision achieved successfully identify within-grain heterogeneities and indicate that monazite growth and recrystallisation occurred during several events.