TRACE ELEMENT CHEMISTRY IN ZIRCON: TESTING THE INVERTED MAGMA CHAMBER MODEL OF THE PICTURE GORGE IGNIMBRITE

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 John Day Formation. Previous research on the PGI promoted a model of magma chamber inversion upon eruption to explain a decrease in degree of melt evolution with increasing stratigraphic height (Fisher, 1966). We sought to test this model through investigation of the age and trace element composition of zircon to determine timing, temperature, and degree of crystal fractionation and melt differentiation. Our predictions included: increasing Ti-in-zircon crystallization temperature; decreasing concentrations of incompatible trace elements in zircon; and a decrease in the magnitude of the negative europium anomaly in zircon with stratigraphic height.

Scanning electron microscope cathodoluminescence images revealed a dominant group of zircon crystals with generally darker sector-zoned cores truncated and overgrown by bright rims. Tandem LA-ICPMS and CA-TIMS U-Pb geochronology on these crystals demonstrate that the cores are not xenocrystic. These features are instead 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 a model of reheating of the system, with transient Zr undersaturation and resorption of zircon in the magma, followed by renewed down-T crystallization. Cores are enriched in yttrium and heavy rare earth elements compared to rims, suggesting an accompanying change in the composition of the magma during the reheating.

We found that Fisher’s model predicted changes in parameters among different periods of zircon crystal growth, however the zircon evidence requires a modification, namely an episode of mafic recharge into the PGI source magma. Zircons from later in the eruptive sequence, presumably from deeper within the magma chamber, have thicker rim overgrowths implying a greater degree of reaction of crystals in proximity to recharge at the base of the chamber. The mean CA-TIMS age of zircon crystallization is 29.120 ± 0.013 Ma, and all of the aforementioned magma dynamics occurred within the ca 30 ka uncertainty of single crystal CA-TIMS analyses.