Lu-[sup.176]Hf geochronology of garnet I: experimental determination of the diffusion kinetics of [Lu.sup.3+] and [Hf.sup.4+] in garnet, closure temperatures and geochronological implications

The [sup.176]Lu-[sup.176]Hf and [sup.147]Sm-[sup.143]Nd decay systems are routinely used to determine garnet (Grt)-wholerock (WR) ages; however, the [sup.176]Lu-[sup.176]Hf age of garnet is typically older than the [sup.147]Sm-[sup.143]Nd age determined from the same aliquots. Here we present experimental data for [Lu.sup.3+] and [Hf.sup.4+] diffusion in garnet as functions of temperature, pressure and oxygen fugacity and show that the diffusivity of [Hf.sup.4+] in almandine/spessartine garnet is significantly slower than that of [Lu.sup.3+]. The diffusive closure temperature ([T.sub.C]) of [Hf.sup.4+] is significantly higher than that of [Nd.sup.3+], and although this property is partly responsible for the observed [sup.176]Lu-[sup.176]Hf and [sup.147]Sm-[sup.143]Nd Grt-WR age discrepancies, the difference between the [T.sub.C]-s of [Lu.sup.3+] and [Hf.sup.4+] could lead to apparent Grt-WR [sup.176]Lu-[sup.176]Hf ages that are skewed from the age of [Hf.sup.4+] closure in garnet. In addition, the slow diffusivity of [Hf.sup.4+] indicates that the bulk of metamorphic garnets retain a substantial fraction of prograde radiogenic [sup.176]Hf throughout peak metamorphic conditions, a phenomenon that further complicates the interpretation of [sup.176]Lu-[sup.176]Hf garnet ages and invalidates the use of analytical [T.sub.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 [sup.147]Sm-[sup.143]Nd garnet ages to diffusive resetting.