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 with 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 re-calculate 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.