Monazite "in situ" 207Pb/206Pb geochronology using a small geometry high-resolution ion probe.: Application to archaean and proterozoic rocks

(T)his paper reports the application of secondary ion mass spectrometry (SIMS) using a small geometry Cameca IMS4f ion probe to provide reliable in situ Pb-207/Pb-206 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 Pb-207/Pb-206 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 mum and the precision achieved successfully identify within-grain heterogeneities and indicate that monazite growth and recrystallisation occurred during several events. This includes detection of one inherited grain dated at ca. 1360 Ma, which is identical to the age of the youngest group of detrital zircons in the paragneiss. Younger ages at ca. 1120 Ma are tentatively interpreted as dating a growth event during the prograde stages of metamorphism. These results demonstrate that the closure temperature for lead diffusion in monazite can be as high as 800 degreesC. At last, ages down to ca. 990 Ma are coeval with late pegmatitic activity and may reflect either lead losses or partial recrystallisation during the waning stages of metamorphism. A third unknown sample was analysed to test the capability of the in situ method to date younger monazite populations. The sample, a pelitic metatexite from Northwestern Hoggar (Algeria), contains rounded metamorphic monazites that provide a Pb-207/Pb-206 weighted mean age of 603 +/- 11 Ma (n = 20). This age is interpreted as recording emplacement of a gabbronoritic body during amphibolite facies regional metamorphism and is representative of the late pulse of the Pan-African tectonometamorphic evolution in the western part of the Tuareg shield. In situ SIMS analyses using a widely available, small geometry ion probe, can thus be successfully used to accurately determine ages for complex Precambrian monazite populations. (C) 2002 Elsevier Science B.V. All rights reserved.