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3 Oxygen Fugacity Across Tectonic Settings
Elizabeth Cottrell1, Suzanne K. Birner2, Maryjo Brounce3, Fred A. Davis4, Laura E. Waters5, and Katherine A. Kelley6
1 Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, DC, USA
2 Division of Natural Sciences, Nursing, and Mathematics, Berea College, Berea, KY, USA
3 Department of Earth and Planetary Sciences, University of California Riverside, Riverside, CA, USA
4 Department of Earth and Environmental Sciences, University of Minnesota Duluth, Duluth, MN, USA
5 Department of Earth and Environmental Science, NewMexico Institute of Mining and Technology, Socorro, NM, USA
6 Graduate School of Oceanography, University of Rhode Island, Narragansett, RI, USA
ABSTRACT
Experiment and observation have established the centrality of oxygen fugacity (fO2) 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 fO2 that rocks record, and the activity models that underpin calculation of fO2 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 fO2 (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 fO2 varies with choice of activity model, we recalculate fO2 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 fO2‐proxy. The compiled data show that, on average, volcanic rocks and mantle rocks from the same tectonic setting yield similar fO2s, but mantle lithologies span a much larger range in fO2 than volcanics. Multiple Fe‐based oxybarometric methods and vanadium partitioning vary with statistical significance as a function of tectonic setting, with fO2 ridges < back arcs < arcs. Plume lithologies are more nuanced to interpret, but indicate fO2s ≥ ridges. We discuss the processes that may shift fO2 after melts and mantle lithologies physically separate from one another. We show that the effects of crystal fractionation and degassing on the fO2 of volcanics are smaller than the differences in fO2 between tectonic settings and that effects of subsolidus metamorphism on the fO2 values recorded by mantle lithologies remain poorly understood. Finally, we lay out challenges and opportunities for future inquiry.
3.1. INTRODUCTION
The role that oxygen fugacity (fO2) plays in producing the unique topography of Earth’s surface, defined by high‐standing continents and low‐lying ocean basins, was recognized in the