A magnetite–clinopyroxene vanadium oxybarometer for reconstructing magmatic redox conditions in plutonic rocks

Guang-Shao Wang, Zhong-Jie Bai, Wen-Jun Hu, Jun-hua Yao, Wei-Guang Zhu, Haiyang Liu

Reliable reconstruction of primary magmatic oxygen fugacity (fO2) in plutonic systems remains challenging because most existing oxybarometers either require equilibrium melts or are restricted to specific mineral assemblages. Here we develop the first melt-independent vanadium-based magnetite–clinopyroxene (Mt–Cpx) oxybarometer by transferring experimentally calibrated V partitioning from clinopyroxene–melt and magnetite–melt systems into an experimentally constrained mineral-pair framework using machine learning. 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. Independent experimental tests and comparisons with established mineral-based oxybarometers support the reliability of the models within the investigated calibration and compositional ranges. 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 or carbonate wall rocks assimilation, 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 establish V partitioning between magnetite and clinopyroxene as a practical melt-independent proxy for reconstructing magmatic fO2 during magnetite saturation. The method is particularly valuable for oxide-bearing mafic–ultramafic intrusions and hydrous, oxidized arc magmas in which magnetite crystallization is favored. By directly linking magnetite crystallization with magmatic redox evolution, the method offers new opportunities to investigate the coupled evolution of oxygen fugacity, magma differentiation, and open-system processes across diverse magmatic environments.