MODELING DIFFUSION LIMITED UPTAKE OF TRACE ELEMENTS BY ECLOGITE GARNETS

Laser ablation ICP-MS analysis of garnets from several eclogites of the Zermatt-Saas Fe zone (ZSZ) of the Western Swiss Alps reveal extremely sharp central peaks of Lu. The central peak is radial symmetric and concentrations decrease exponentially. The size of the peak is approximately 1/5 of the radius. Towards the rim of the garnets an additional, much less pronounced peak was found. The rapid decrease in Lu cannot be modeled using a Raleigh fractionation model because the concentration of the central peak decreases over too short a distance and rim values do not approach zero. We have adapted a trace element incorporation model to explain the data, where trace element uptake is limited by diffusion in the matrix sounding the porphyroblast, while equilibrium is maintained at the surface of the growing crystal. The matrix is approximated by a continuum. Garnet growth is assumed to be independent of the trace element concentration, and is solely governed by major element availability, pressure, and temperature. Lu-concentrations are well fit using this model. A garnet growth interval of ca. 10 Ma is assumed for the calculations following Lapen et al. (2003) for nearby eclogites from the ZSZ of the Val Tournanche area. Assuming a constant radial growth rate for the porphyroblast, an effective diffusion constant in the matrix of D= 1.5e-8cm²/year best fits the measured profile. Changes in diffusion coefficient due to temperature increase or changing growth velocities produce additional small peaks similar to those found in the outer parts of the garnets. Trace element enrichment or depletions are therefore not necessarily due to breakdown of trace element rich minerals. Raleigh fractionation can be viewed as the limiting case for elemental zonations in garnet in these systems, in which diffusion is so fast that depletion profiles surrounding neighboring garnets overlap significantly. Calculations show that for a typical garnet porphyroblast spacing of 1cm large effective diffusion coefficients of 1.e-4 cm²/year is needed, assuming a 10 Ma growth periods.

Lapen, T.J., Johnson, C.M., Baumgartner, L.P., Mahlen, N.J., Beard, B.L., Amato, J.A EPSL 6811 1-16