Lu–Hf and Sm–Nd garnet geochronology: Chronometric closure and implications for dating petrological processes

To investigate the systematics of the 176Lu–176Hf and 147Sm–143Nd garnet chronometers, we performed REE and isotope analyses on garnet crystals of different size (0.55–3.1 mm radius) from a single granulite specimen (Archean Pikwitonei Granulite Domain, Manitoba, Canada). The Lu–Hf dates are similar among grains having radii larger than 1.3 mm (weighted mean: 2714 ± 6 Ma ; 2 s.d.) and up to 20 Myr younger for finer fractions. In contrast, Sm–Nd dates are equal to or younger than the 2641–2637 Ma age of peak metamorphism. The roles of concurrently digested inclusions and core–rim Lu/Sm fractionation are quantitatively evaluated and excluded as causes for the observed age heterogeneity. Instead, the isotope dates are interpreted to reflect partial loss of radiogenic 176Hf and 143Nd, and are used to constrain the systematics of the garnet chronometers at high temperature. The data constrain the grain sizes and thermal histories for which garnet dates can be validly linked to prograde or peak P–T conditions. In addition, garnet dates can now be used to place quantitative constraints on initial cooling from high- and ultrahigh temperature—a temperature segment that is inaccessible to other thermochronometers. The results indicate that garnet could play a major role in governing 176Hf/177Hf disequilibria and decoupling of 176Hf/177Hf from 143Nd/144Nd at a mineral scale in much of the mid- to lower crust and lithospheric mantle.