Magmatic oxygen fugacity (or fO 2 ) is an important control on elemental behavior, such as solubility (e.g., soluble Fe 2+ vs. insoluble Fe 3+ ), mobility (e.g., mobile UO 4 + vs. immobile U 4+ ), chemical affinity (e.g., siderophile S at low fO 2 vs. lithophile S at high fO 2 ), and compatibility (e.g., compatible Eu 2+ vs. incompatible Eu 3+ in plagioclase). Understanding the fO 2 of the upper mantle in the early Earth is particularly valuable for quantitatively describing processes associated with planetary differentiation (e.g.,