High-precision U [sbnd]Pb ages of metamorphic rutile: application to the cooling history of high-grade terranes

Abstract

Metamorphic rutiles occurring in granulite and upper amphibolite facies metapelitic rocks of the Archean Pikwitonei granulite domain (Manitoba) and the Proterozoic Adirondack terrane (New York) give concordant and near concordant U Pb ages. The Pb concentrations in rutile range from 2.85 to 168 ppm, U concentrations range from 10.9 to 390 ppm and the measured 206Pb/ 204Pb ratios range from 182 to 22,100 corresponding to 238U/ 204Pb ratios of 398–75,100. The proportions of radiogenic 208Pb are very low, ranging from 0.0 to 6.9% of total radiogenic Pb. The habits of the rutile crystals range from stubby to acicular, the physical properties vary from opaque/black to transparent/reddish-brown. Separate batches of black and reddish-brown rutile grains from the same samples have similar U and Pb concentrations, Pb-isotope ratios, and yield the same U/Pb ages within analytical uncertainty. No correlation of U concentration and 206Pb/ 204Pb ratios with morphology or color of the rutiles was observed among the samples analyzed. Most rutiles yield concordant U Pb ages which are reproducible within analytical uncertainty, i.e. generally ±2 Ma. The U Pb ages for prograde rutile are younger than the time of peak metamorphism given by U Pb ages for garnet and zircon, and also younger than U Pb ages for sphene and monazite, and 40Ar/ 39Ar and K Ar ages for hornblende but older than 40Ar/ 39Ar and K Ar ages for biotite from the same area. This suggests that the rutile ages reflect cooling below closure temperatures. Within a single hand-specimen, and thus for an identical thermal history, larger rutile grains give older ages than do smaller grains. This suggests that volume diffusion is the most probable mechanism responsible for the ages being younger than the time of peak metamorphism. It also suggests that the dimensions for such diffusion are directly related to the dimensions of the rutile crystal and not to the dimensions of sub-grain domains, as is the case for Ar diffusion in hornblende and feldspar. The concordant U Pb ages as well as the similar ages over a large area suggest that rutile has a well-defined temperature of closure. At a cooling rate of about 0.5–1°C/Ma the closure temperature for U Pb diffusion in rutile is about 420°C for grains with a radius of 0.009–0.021 cm, and about 380°C for grains with a radius of 0.007–0.009 cm. U Pb rutile ages obtained on different grain sizes indicate a cooling rate of about 0.5°C/Ma for the Pikwitonei granulite domain at 2460-2300 Ma. U Pb ages for rutile thus provide high-precision ages for evaluating the cooling history of high-grade terranes and can be critical to the quantitative development of thermal models for crustal evolution.