Constraints on the duration of high-pressure metamorphism in the Tauern Window from diffusion modelling of discontinuous growth zones in eclogite garnet

摘要: An eclogite sample from the Grossgockner region of Hohe Tauern, Austria contains garnet with a pronounced compositional discontinuity between Mn-rich core and an Fe-rich rim. This jump in composition was caused by garnet-consuming reaction followed growth rim + omphacite marks prograde transition epidote–amphibolite to facies metamorphism. Garnet ended at peak metamorphic conditions 570 °C, 17 kbar, but intracrystalline diffusion continued until about 450 °C, 4 kbar on retrograde path. overgrowth texture represents natural couple time span 1 Myr calculated profile developing out original sharp step. For typical crustal densities, this corresponds minimum average velocity range 4.6–7.4 cm yr−1 (for vertical movement), which is one fastest exhumation rates reported. The all divalent cations four profiles modelled, both analytically numerically. Both approaches gave comparable results, times computed for each element were always discrepant up factor 2. Variations coefficients within 2 analytical calculations remedied consistent upper limits. Numerical modelling does not require simplifications introduced approach. On other hand, error propagation computationally unfeasible method.