176Lu–176Hf geochronology of garnet I: experimental determination of the diffusion kinetics of Lu3+ and Hf4+ in garnet, closure temperatures and geochronological implications

The 176 Lu– 176 Hf and 147 Sm– 143 Nd decay systems are routinely used to determine garnet (Grt)–whole-rock (WR) ages; however, the 176 Lu– 176 Hf age of garnet is typically older than the 147 Sm– 143 Nd age determined from the same aliquots. Here we present experimental data for Lu 3+ and Hf 4+ diffusion in garnet as functions of temperature, pressure and oxygen fugacity and show that the diffusivity of Hf 4+ in almandine/spessartine garnet is significantly slower than that of Lu 3+ . The diffusive closure temperature ( T C ) of Hf 4+ is significantly higher than that of Nd 3+ , and although this property is partly responsible for the observed 176 Lu– 176 Hf and 147 Sm– 143 Nd Grt–WR age discrepancies, the difference between the T C -s of Lu 3+ and Hf 4+ could lead to apparent Grt–WR 176 Lu– 176 Hf ages that are skewed from the age of Hf 4+ closure in garnet. In addition, the slow diffusivity of Hf 4+ indicates that the bulk of metamorphic garnets retain a substantial fraction of prograde radiogenic 176 Hf throughout peak metamorphic conditions, a phenomenon that further complicates the interpretation of 176 Lu– 176 Hf garnet ages and invalidates the use of analytical T C expressions. We argue that the diffusion of trivalent rare earth elements in garnet becomes much faster when their concentration level falls below a few hundred ppm, as in the experiments of Tirone et al. (Geochim Cosmochim Acta 69: 2385–2398, 2005 ), and further argue that this low-concentration mechanism is appropriate for modeling the susceptibility of 147 Sm– 143 Nd garnet ages to diffusive resetting.