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Quantifying the oxygen fugacity (fo2) of high temperature lithospheric fluids, including hydrothermal systems, presents a challenge because these fluids are difficult to capture and measure in the same manner as quenched glasses of silicate melts. The chemical properties of fluids can however be inferred through mineral proxies that interacted with the fluids through precipitation or recrystallization. Here, we present hydrothermal experiments to quantify the partition coefficients of rare earth elements (REEs) – including redox-sensitive Ce and Eu – between zircon and fluid. Experiments were conducted in a piston cylinder device at temperatures that range from 1200 to 800 ◦C under fo2-buffered conditions in a SiO2-ZrO2-NaCl-REE-oxide system, and similar experiments were performed in the absence of NaCl (31 total experiments). The fo2 was buffered to values that range from approximately 3 log units below to 7 log units above the fayalite magnetite quartz equilibrium. Zircon REE concentrations were quantified using laser ablation inductively coupled plasma mass spectrometry whereas the quenched fluids were extracted and measured by solution-based inductively coupled plasma mass spectrometry. Zircon Ce anomalies, quantified relative to La and Pr, exhibit sensitivity to oxygen fugacity and temperature and our preferred calibration is: log [􀀀 Ce Ce* ) D 􀀀 1 ] = (0.237 ± 0.040)× log(fo2) + 9437±640 T(K) 􀀀 5.02 ± 0.38 where the Ce anomalies are calculated from the partition coefficients for La, Ce, and Pr. Zircon Eu anomalies are also a function of oxygen fugacity though they exhibit no systematic dependence on T. Our preferred calibration is described by: 􀀀 Eu Eu* ) D = 1 1+100.30±0.04􀀀 [0.27±0.03]×ΔFMQ We performed additional calculations, in which lattice strain parabolas were fit to all non-redox sensitive rare earth elements that were added to the starting composition (i.e., La, Pr, Sm, Gd, Dy, Ho, Tm, Lu) as an alternate means to calculate anomalies. This method yields broadly similar results, though we prefer the La-Pr calibrations due to the non-systematic REE patterns frequently encountered with hydrothermal zircons; e.g., LREE zircon enrichment relative to other REEs. These experiments are applied to quantify the fo2 of fluids during mineralization of critical element-bearing systems, and separately to calculate the oxygen fugacity values of fluids formed during plate boundary processes.