Géochronologie U-Pb par ablation laser et ICP-MS (LA-ICP-MS ...

Chemical Geology 184 (2002) 151–165
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Monazite ‘‘in situ’’ 207 Pb/ 206 Pb geochronology using a small
geometry high-resolution ion probe. Application to
Archaean and Proterozoic rocks
Delphine Bosch a, *, Dalila Hammor b , Olivier Bruguier c , Renaud Caby a ,
Jean-Marc Luck d
a Laboratoire de Tectonophysique, Université Montpellier II, CNRS-UMR 5568, cc 066, Place Eugène Bataillon,
34095 Montpellier Cedex 05, France
b Département de Géologie, Université d’Annaba, B.P. 12, El Hadjar Annaba, Algeria
c Service ICP-MS, ISTEEM, Université Montpellier II, cc 049, Place Eugène Bataillon, 34 095 Montpellier Cedex 5, France
d Laboratoire de Géophysique, Tectonique et Sédimentologie, Université Montpellier II, CNRS-UMR 5573,
cc 060, Place Eugène Bataillon, 34 095 Montpellier Cedex 5, France
Received 19 March 2001; accepted 2 August 2001
Abstract
This paper reports the application of secondary ion mass spectrometry (SIMS) using a small geometry Cameca IMS4f ion
probe to provide reliable in situ 207 Pb/ 206 Pb 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 F 1
Ma for monazites extracted from a pelitic schist from the Jimperding Metamorphic Belt (Yilgarn Craton, Western Australia)
and 1083 F 3 Ma for monazites from a high-grade paragneiss from the Northampton Metamorphic Complex (Pinjarra
Orogen, Western Australia). SIMS results provide 207 Pb/ 206 Pb weighted mean ages of 2659 F 3Ma(n = 28) and 1086 F 6Ma
(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 mm 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 °C. 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 207 Pb/ 206 Pb
weighted mean age of 603 F 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
* Corresponding author.
E-mail addresses: bosch@dstu.univ-montp2.fr (D. Bosch), bruguier@dstu.univ-montp2.fr (O. Bruguier), caby@dstu.univ-montp2.fr
(R. Caby), jmluck@dstu.univ-montp2.fr (J.-M. Luck).
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