Beyond Oxygen Fugacity: A Compositional Metric to Probe Earth's Redox Structure

Abstract We quantify the redox capacity ( d O 2 ) by redox titration—defined as the quantity of oxygen required to fully oxidize a sample—for rocks spanning Earth's upper mantle to surface sediments. The values span three orders of magnitude, with the greatest variability in metamorphic and surface rocks that host both highly reduced (rich in carbon and sulfur) and oxidized materials. In contrast, exhumed lower‐crustal and mantle rocks display more uniform values of d O 2 . This distribution reflects not a progressive net planetary oxidation, but active redox partitioning , shaped by far‐from‐equilibrium biological chemistry, surface processes, and tectonic cycling. Unlike thermodynamic potentials such as μ O 2 , the compositional variable d O 2 directly constrains the redox structure of Earth's shallow lithospheric reservoirs. This framework provides an experimentally grounded and physically intuitive metric for quantifying redox transfers and assessing their geophysical consequences.