Sub-Arc Mantle Redox Evolution since the Archean

The redox state of the sub-arc mantle is critical in contraining the behavior of redox-sensitive elements, particularly volatile species, within the magmatic system. However, the long-term evolution of redox conditions in the sub-arc mantle and its impact on atmospheric oxygen levels remain unclear. In this study, we address two key challenges in understanding the sub-arc mantle redox throughout geological history: 1) the need for more effective methods to identify arc samples in different tectonic settings, and 2) the lack of constraints on the evolution of redox-sensitive trace elements from the mantle source. To overcome these challenges, we used an XGBoost machine learning model trained on multiple elements and elemental ratios (e.g., Nb, Ta, Ti, Pb, Th/Nb, Nb/La, and U/Nb) to accurately classify arc basalts and basalts (N ≈ 14,000) derived from other tectonic settings. Subsequently, we modeled the evolution of trace elements (V, Ti, and Sc) partitioning in the hydrous and dry depleted mantle beneath the arc using non-modal near-fractional melting. Finally, we applied the trained machine learning model and redox-sensitive elemental ratios to a refined global dataset of basalts (N ≈ 19,000) to calculate the redox evolution since 3.4 Ga. Our findings reveal that there have been minimal fluctuations in the melting T-P conditions since 3.4 Ga, indicating a consistent oxidized state in the sub-arc mantle. The average ΔFMQ of 0.96 ± 0.52 (1 SD) observed in our study is similar to that of the modern arc mantle since the early Archean. Interestingly, it is unlikely that the long-term oxidation of the sub-arc mantle has significantly impacted the progressive increase in atmospheric oxygen levels over time.