EVALUATING THE APPLICATION OF CE-IN-ZIRCON AS A MAGMA FO2 ESTIMATOR

Zircon-melt partitioning behavior of Ce is sensitive to the redox conditions of magmas as zircon crystallizes, because Ce⁴⁺ is far more compatible than Ce³⁺ in zircon. Previous works suggest that the Ce concentration in zircon is an effective tool to estimate magma oxygen fugacity (Trail et al., 2012; Smythe and Brenan, 2016). The goal of this study is to understand the application and limitation of different Ce-in-zircon oxygen barometers and to quantitatively identify Ce⁴⁺ and Ce³⁺ in zircon and the contributions of inclusions to Ce measurements. We evaluate multiple experimentally derived models by applying them to natural sample data from a variety of systems: Mt. St. Helens, WA, USA (continental arc), Peach Spring Tuff, NV, USA (continental extension zone) and Iceland (mid-ocean ridge/hot spot). We estimate magma fO₂ using different methods and compare the results with other measures of magmatic conditions for these systems. However, light rare earth element (LREE) concentrations in zircon analyses are easily inflated by inclusions. In order to more accurately estimate magma fO₂ using the models, we try to quantify the contributions of inclusions to Ce concentrations measured in zircon. The Ce value (which we denote as Ce**) obtained by projecting a log-linear relationship from chondrite-normalized Sm value through Gd may better represent the concentration of Ce³⁺ in zircon, than the commonly used Ce*, calculated from La and Sm, as this reports Ce³⁺ in zircon plus the contribution from inclusions. Calculating Ce** allows us to use Ce as a more reliable tool to understand magma redox conditions.