COMPOSITIONAL ZONING IN GARNET FROM POINT ANALYSES TO TRACE ELEMENT MAPS: A TRIBUTE TO ROBERT TRACY
Major element mapping via EPMA of amphibolite and some granulite facies garnet show bell-shaped Mn zoning profiles and increasing Fe and Mg/Fe from core to rim, indicating prograde growth. The EPMA mapping also shows more complex zoning and subtle growth features including sector zoning, Ca spikes, and oscillatory zoning. Trace element mapping via LA-ICPMS reveals examples of simple bell-shaped zoning profiles in HREE. Garnet from other rocks have oscillatory HREE zoning, high HREE cores, and high REE annuli. Interpretation of the complex zoning profiles is in progress and initial work indicates that some zoning patterns may be caused by reaction history involving trace element rich accessory minerals, transport-induced depletion halos, and external fluxes, e.g. melt. In summary, major element zoning remains the most reliable indicator of prograde zoning and sharp concentration gradients of REE provide evidence of limited intracrystalline diffusion during and after growth. Challenges remain for integrating trace element data into the growth history of garnet, but the observations by Robert Tracy provide an excellent foundation.