Constraints on the U-Pb systematics of metamorphic rutile from in situ LA-ICP-MS analysis

摘要

In situ laser ablation ICP-MS U-Pb dating of metamorphic rutile from granulite facies metapelitic rocks of the Archaean Pikwitonei granulite domain (Manitoba, Canada) provides constraints on Pb diffusion and characterizes the closure behavior of rutile. The analysis of transects of 35-mu m spots across 15 rutile grains having a size of 120 to 280 mu m yielded concordant ages with core ages of ca. 2450 Ma and core-to-rim younging towards 2280 Ma. Age profiles indicate that volume diffusion of Pb occurs in rutile implying that the ages represent cooling ages. To investigate the closure behavior of Pb in rutile closure temperature profiles (T-c(x)) were constructed based on different models combined with experimentally-determined diffusion parameters. The classical T-c(x) model of Dodson (1986; Mat. Sci. Forum 7, 145-154) indicates a rapid decrease of T-c in the rims of grains, providing unrealistic estimates for the cooling rate when combined with U-Pb ages. A new T-c(x) model was constructed based on the analyzed age profiles that are described by an error function. This model shows a more steady decrease in T-c throughout the grain from ca. 640 degrees C in the core (depending on grain size) to a rim intercept (T-c.rim) of 490 degrees C (+/- 7, 2 sigma), which is interpreted to be the extrapolated theoretical absolute temperature of insignificant Pb diffusion in rutile. The new model provides a better description of the relation between age and T-c for the analyzed grains. However, both T-c(x) models demonstrate that even in small grains the variations of T-c can be significant making it impossible to derive one representative T-c for Pb in rutile. The error function-based T-c(x) model allows the determination of cooling rates, which show a decrease over time from ca. 2.2 to 0.4 degrees C/Ma agreeing well with previous estimates for the Pikwitonei granulite domain. This consistency supports the validity of our model and indicates that cooling rates can be estimated from single grains by LA-ICP-MS U-Pb dating of rutile providing constraints on the cooling history of a metamorphic terrane. The slow cooling rates imply that exhumation was slow (<0.1 mm/yr) and was controlled by surface erosion.