Modelling redox state via V-Sc-Ti-Yb partitioning in mantle derived melts

The ratios of multivalent V to homovalent elements (e.g., V/Sc, V/Ti, V/Yb) are pivotal proxies for assessing mantle oxygen fugacity (fO2), yet their geochemical behaviours during partial melting remain debatable. Here, we conducted petrological modelling to explore the responses of these ratios to fO2 variations in sub-arc and mid-ocean ridge basalt (MORB) mantle sources, considering diverse mineral assemblages and chemical compositions. Our results suggest that V/Sc is relatively unaffected by changes in mantle source characteristics and melting extent, positioning it as a more dependable indicator of mantle fO2 and as a tool for tracing the evolution of mantle fO2 in terrestrial planets. In contrast, V/Ti and V/Yb are both more sensitive to the degree of melting and V/Ti is notably affected by mantle chemical composition. These insights refine our understanding of redox conditions across various tectonic environments on Earth. For instance, applying our model to data sets of primitive basalts reveals that the contemporary sub-arc mantle is generally more oxidised than the MORB mantle, consistent with impacts of slab-derived fluids and sediments in arc regions.