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

Abstract

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-μm spots across 15 rutile grains having a size of 120 to 280μm yielded concordant ages with core ages of ca. 2450Ma and core-to-rim younging towards 2280Ma. 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 (Tc(x)) were constructed based on different models combined with experimentally-determined diffusion parameters. The classical Tc(x) model of Dodson (1986; Mat. Sci. Forum 7, 145–154) indicates a rapid decrease of Tc in the rims of grains, providing unrealistic estimates for the cooling rate when combined with U–Pb ages. A new Tc(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 Tc throughout the grain from ca. 640°C in the core (depending on grain size) to a rim intercept (Tc,rim) of 490°C (±7, 2σ), 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 Tc for the analyzed grains. However, both Tc(x) models demonstrate that even in small grains the variations of Tc can be significant making it impossible to derive one representative Tc for Pb in rutile. The error function-based Tc(x) model allows the determination of cooling rates, which show a decrease over time from ca. 2.2 to 0.4°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 (<0.1mm/yr) and was controlled by surface erosion.