HR: 1340h
AN: V13A-0522    [Abstracts]
TI: Intracrystalline Diffusivities of REE, Y, V, and Cr in Garnet at 700 - 900 °C
AU: * Carlson, W D
EM: wcarlson@mail.utexas.edu
AF: Dept Geological Sciences, University of Texas at Austin, 1 University Station C1100, Austin, TX 78712 United States
AB: Partial resorption of garnet crystals in pelitic granulites from the inner aureole of the Makhavinekh Lake Pluton (Labrador) produced stranded diffusion profiles for numerous trace elements, including Y, V, Cr, and REEs. Diffusivities of these elements at temperatures from ~700-900 °C (at 5.3 kbar) have been extracted from the profiles by numerical simulation of their development. Garnet crystallized in restitic gneisses during a partial melting event that caused nearly complete internal homogenization of trace-element concentrations. Subsequent resorption, induced by intrusion of troctolite/anorthosite, produced coronas of orthopyroxene + cordierite with accessory monazite. Profiles for Y, V, Cr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tb, Yb, and Lu were measured by LA-ICPMS; Y, V, and Cr were also measured by EPMA to validate laser-ablation protocols. Laser data were acquired as continuous line traverses, measured in triplicate along adjacent paths and averaged to reduce noise. The 20-micrometer laser spot was large in relation to the length scale of the profiles, requiring deconvolution of overlap to recover accurate concentrations. Profile shapes correspond to the sense and degree of partitioning of elements between relict garnet and the coronal assemblage: Nd and Sm concentrations (at levels just above detection limits) decrease toward the rims of relict garnet crystals; Gd profiles are flat; and all other measured elements show increases in concentration toward garnet rims. Diffusivities were retrieved from numerical simulations in which the computed effects of diffusion during resorption are fitted to measured profiles. The diffusivities derived from the simulations are relatively insensitive to uncertainties in all inputs except the thermal history. Conductive thermal models, constrained by Al-in-opx thermometry, yield reliable heating/cooling histories as a function of distance from the intrusive contact, allowing tracer diffusion coefficients (in m²/sec) to be determined to within ca. ñ 0.3 log₁₀ units. The results confirm prior findings that REE diffusion in garnet is insensitive to the size of the diffusing ion. Although comparisons require long extrapolations in pressure and composition, these results appear consistent with the bulk of existing high-temperature experimental data. They extend the range of determinations to temperatures, pressures, and garnet compositions relevant to crustal conditions.
DE: 1065 Major and trace element geochemistry
DE: 3620 Mineral and crystal chemistry (1042)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005