COMPOSITIONAL ZONING IN GARNET FROM POINT ANALYSES TO TRACE ELEMENT MAPS: A TRIBUTE TO ROBERT TRACY

The first Electron Probe microanalysis (EPMA) of garnet was restricted to points and major constituents (e.g., Fe, Ca, Mg, Mn, Si, Al). These were used to infer zoning patterns based on lines of analyses and interpret element incorporation into growing garnet crystals (e.g., Hollister, 1966). Automation of EPMA led to contoured grids of analyses to produce compositional zoning maps (e.g., Tracy, 1982). Subsequently, new developments in energy dispersive detectors and automation improvements led to major element compositional mapping. More recently, laser ablation inductively-coupled plasma mass spectrometry (LA-ICPMS) revolutionized trace element analysis allowing routine analysis of ≤25 um spots. These data raise numerous questions and allow new interpretations, but the observations and interpretations on garnet zoning made by Robert Tracy still hold true today and are important for our understanding of growth and post growth processes. Our current work focuses on understanding the effects of complex trace element zoning on garnet geochronology. Here we discuss the combined utility of major and trace element zoning in garnet using examples from amphibolite and granulite facies rocks.

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.