<p>The redox state of the sub-arc mantle impacts magma degassing at arc volcanoes and arc-related ore deposits. However, the evolution of the oxygen fugacity (<i>f</i>O₂) of the sub-arc mantle during the Neoproterozoic Oxygenation Event (NOE) remains poorly understood. Here, we applied machine learning to identify arc basalts, then calculated V/Sc ratios of primitive arc basalts since ~0.8 billion years ago (Ga). Our results show that average V/Sc remained nearly constant at ~7.16, suggesting an oxidized sub-arc mantle since ~0.8 Ga. This implies that increased surficial oxygen levels during the NOE may not directly oxidize the sub-arc mantle. Instead, its oxidation likely stems from the influx of oxidized fluids and melts via subducted serpentinized oceanic crust. The persistently oxidized sub-arc mantle may have controlled the long-term oxidation of arc magmas and thus influenced the oxidation of Earth’s surficial environments since ~0.8 Ga, though it was not the NOE’s primary driver.</p>