UNEXPECTEDLY NARROW TRACE-ELEMENT DIFFUSION PROFILES IN NATURAL GARNETS: EFFECTS OF LIMITED NUCLEATION KINETICS?

Stranded diffusion profiles (SDPs) of trace elements in partially resorbed garnet porphyroblasts in outer portions of the Makhavinekh Lake Pluton (MLP) aureole are narrower than predicted by extrapolation of diffusion data (derived in part from inner-aureole garnet profiles), suggesting possible kinetic limitations to nucleation of coronal product phases during resorption.

Trace-element zoning in garnet is an increasingly important source of information on the prograde and retrograde history of metamorphic rocks, especially considering the homogenization effects on major-element zoning at high temperatures and the continued analytical innovations toward extracting trace concentrations at high spatial resolution. To broaden the use of trace-element analysis as a tool for petrology and geochronology, diffusion rates are needed for all relevant metamorphic conditions. Recently determined intracrystalline diffusion rates for trace elements in garnet accurately reproduce SDPs in MLP inner-aureole garnets at characteristic temperatures (T_c) as low as 809 °C. To expand this effort, lower-temperature rocks in outer portions of the aureole and one additional locality (T_c = ~625-790 °C) are under study.

In the MLP aureole, garnet growth and prolonged residence at granulite-facies conditions produced nearly flat trace-element zoning in garnet. Subsequent contact metamorphism consumed garnet rims and grew cordierite + orthopyroxene coronal structures, and resulted in SDPs at the edges of relict garnet. Concentrations of Y and Cr measured by EPMA along several rim traverses in multiple garnets exhibit typical diffusion profile shapes with penetration distances of 10-15 μm at characteristic temperatures of 748 and 789 °C. However, extrapolation from higher temperatures predicts that these distances should be larger by a factor of three to four.

If predicted penetration distances are accurately determined, then the diffusivities in these rocks are not unusually slow, but instead may reflect kinetic impediments to the resorption reaction. In these lower temperature rocks, kinetic impediments to nucleation during the early stages of contact metamorphism would delay reaction and promote late garnet consumption and shorter durations of diffusion that would cause the appearance of slow diffusion rates.