Trace Element Chemistry in Zircon: Testing the Inverted Magma Chamber Model of the Picture Gorge Ignimbrite

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Mark Schmitz


The Picture Gorge Ignimbrite (PGI) is among the most voluminous and laterally extensive silicic pyroclastic deposits in central Oregon and is a prominent marker bed within the fossil-bearing stratigraphy of the John Day Formation. Previous research on the PGI promoted a model of magma chamber inversion upon eruption to explain compositional variation in stratigraphy (Fisher, 1966). We sought to test this model through investigation of the age and TE composition of zircon to determine timing and temperature of crystallization and degree of crystallization with other minerals. Our predictions included: 1) Increasing crystallization temperature, 2) Decreasing concentrations of incompatible trace elements in zircon, 3) A decrease in the magnitude of Europium anomaly in zircon with stratigraphic height. Scanning electron microscope cathodoluminescence images revealed a dominant group of grains with generally darker sector-zoned cores truncated and over grown by bright rims. The population included rarer bright whole crystals. These were interpreted to illustrate a change in the intensive properties of the magma chamber during crystallization resulting in crystal resorption and regrowth. Cores and rims have overlapping temperature ranges, which supports the model of reheating of the system, resulting in undersaturation of zircon in the mama. Cores are enriched in Yttrium and heavy rare earth elements compared to rims, suggesting a change in the composition of the magma during the reheating. We found that Fisher’s model predicted changes in parameters within zircon crystals though not necessarily in the stratigraphy of the ignimbrite. Evidence requires a modification to Fisher’s model, namely an episode of mafic recharge in the PGI source magma. Zircons from later in the eruptive sequence, presumably from deeper within the magma chamber appear to have thicker rim over growth. This implies a greater degree of reaction of crystals in proximity to mafic recharge at the base of the magma chamber.

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