A machine learning-based vanadium-in-magnetite-clinopyroxene oxybarometer for constraining oxygen fugacity in mafic–ultramafic intrusion

Zhong-Jie Bai, Jian-Feng Gao, Wen-Jun Hu, Jun-hua Yao, Wei-Guang Zhu, Guang-Shao Wang

Reliable reconstruction of primary magmatic oxygen fugacity (fO2) in plutonic systems remains challenging because existing oxybarometers either require equilibrium melt compositions or preferentially record late-stage oxide equilibration, limiting their ability to constrain primary magmatic redox conditions. Here we develop a machine-learning framework that transfers experimentally calibrated V partitioning between clinopyroxene (Cpx)–melt and magnetite (Mt)–melt to a melt-independent Mt–Cpx oxybarometer. The framework also yields complementary V-in-Cpx and V-in-Mt oxybarometers for systems in which equilibrium melts are available or can be independently reconstructed. The calibrated models accurately reproduce experimental V partitioning and fO2 over a wide range of temperatures, fO2, and compositions. Model interpretation indicates that V partitioning provides the primary control on predicted fO2, whereas temperature and mineral–melt chemistry mainly account for equilibrium-related corrections. Comparisons with independently constrained experimental fO2 and established mineral-based oxybarometers demonstrate robust predictive performance and broad applicability to natural magmatic systems. Application of the melt-independent V-in-Mt–Cpx oxybarometer to the Panzhihua intrusion and the Upper Zone of the Bushveld Complex reveals contrasting redox evolution during magma differentiation. Panzhihua preserves persistently oxidized conditions (ΔFMQ= +1.29 to +1.99), consistent with redox buffering by repeated recharge of relatively oxidized magma, whereas the Bushveld Upper Zone records progressive reduction (ΔFMQ= +0.06 to +1.31) associated with differentiation in a predominantly closed magmatic system. These results demonstrate that mineral-pair V partitioning provides a practical means of reconstructing magmatic redox evolution in plutonic rocks and offers new insights into the links among magma dynamics, oxygen fugacity, and Fe–Ti–V oxide