Oxygen Fugacity Across Tectonic Settings

Experiment and observation have established the centrality of oxygen fugacity ( f O 2 ) to determining the course of igneous differentiation, and so the development and application of oxybarometers have proliferated for more than half a century. The compositions of mineral, melt, and vapor phases determine the f O 2 that rocks record, and the activity models that underpin calculation of f O 2 from phase compositions have evolved over time. Likewise, analytical method development has made new sample categories available to oxybarometric interrogation. Here we compile published analytical data from lithologies that constrain f O 2 (n=860 volcanic rocks - lavas and tephras - and n=326 mantle lithologies - the majority peridotites) from ridges, back-arc basins, forearcs, arcs, and plumes. Because calculated f O 2 varies with choice of activity model, we recalculate f O 2 for each dataset from compositional data, applying the same set of activity models and methodologies for each data type. Additionally, we compile trace element concentrations (e.g., vanadium) which serve as an additional f O 2 -proxy. The compiled data show that, on average, volcanic rocks and mantle rocks from the same tectonic setting yield similar f O 2 s, but mantle lithologies span a much larger range in f O 2 than volcanics. Multiple Fe-based oxybarometric methods and vanadium partitioning vary with statistical significance as a function of tectonic setting, with f O 2 ridges < back arcs < arcs. Plume lithologies are more nuanced to interpret, but indicate f O 2 s ≥ ridges. We discuss the processes that may shift f O 2 after melts and mantle lithologies physically separate from one another. We show that the effects of crystal fractionation and degassing on the f O 2 of volcanics are smaller than the differences in f O 2 between tectonic settings and that effects of subsolidus metamorphism on the f O 2 values recorded by mantle lithologies remain poorly understood. Finally, we lay out challenges and opportunities for future inquiry.