HR: 14:00h
AN: V12F-02 INVITED     [PDF]
TI: REE and Trace-Element Diffusivities in Garnet Determined From Resorption-Induced Stranded Diffusion Profiles
AU: * Carlson, W D
EM: wcarlson@mail.utexas.edu
AF: Department of Geological Sciences, University of Texas at Austin, Austin, TX 78712 United States
AB: Resorption of garnet at very high temperatures may produce stranded diffusion profiles for trace elements; these can be exploited by measurement and modeling to yield quantitative estimates of diffusion coefficients for species that often defy definitive experimental determination of diffusion rates. Knowledge of intracrystalline diffusion rates in garnet for major and trace components provides vital constraints on the rates and mechanisms of diverse processes in the crust and mantle, providing information on time-temperature relationships across the broad P-T range of garnet stability. Trace-component diffusivities in particular are needed for proper application of garnet-based geochronometers, and the relatively slow diffusion of trace components also makes their diffusion rates especially crucial for the determination of uplift and cooling histories in the deep crust and mantle. Unfortunately, experimental determination of diffusion rates for trace elements in garnet (e.g., Y, REE) has proven extremely difficult, because sluggish diffusion yields transport only over micron or submicron distances for laboratory time scales. As a result, few experimental data exist. A promising complementary approach is to measure and model the stranded diffusion profiles produced by garnet resorption in nature, as has been done recently for the major divalent cations (Carlson, 2002, Am Min 87:185). A special requirement is to find rocks in which original garnet crystallization took place at temperatures high enough to homogenize the internal distribution of trace components, and in which later resorption occurred at temperatures high enough and times long enough to produce measurable diffusion profiles. The contact aureole of the Makhavinekh Lake Pluton (Nain Plutonic Suite, Labrador) provides rocks suitable for such a study. LA-ICP-MS analyses of Y and REE profiles in garnets, combined with thermal models for the contact aureole (McFarlane et al., 2003, JMG 21:405) allow extraction of diffusivities using the coupled resorption-diffusion model of Carlson (2002). Preliminary results expand the available database for Y and REE diffusion in garnet, yielding values generally consistent with the sparse experimental data.
DE: 3660 Metamorphic petrology
DE: 3670 Minor and trace element composition
DE: 5120 Plasticity, diffusion, and creep
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting