A novel experimental V-Sc olivine-melt oxybarometer for arc magmas

This manuscript is a preprint. A substantially revised and peer-reviewed version has been published in Geochimica et Cosmochimica Acta (DOI: https://doi.org/10.1016/j.gca.2025.12.025), which should be cited instead. Redox conditions significantly affect phase equilibria, the availability and mobility of heterovalent elements, including volatiles (i.e., S) and metals (e.g., Fe, Cu) in silicate melts. Gaining a deeper understanding of the initial redox state of magmas may help better understand magmatic ore fertility, volcanic degassing, and the redox evolution of Earth's crust and atmosphere. This study reports an optimized V and novel V-Sc olivine-melt oxybarometers, developed using existing V-partitioning data and new results from a series of fractional crystallization experiments. Experiments were conducted in a rapid-quench molybdenum-hafnium carbide pressure vessel apparatus equipped with a custom-designed hydrogen membrane for flexible, precise, and accurate oxygen fugacity (fO2) control. They were performed at constant pressure (P = 200 MPa) and variable temperatures (T = 1019-870 °C), under water-saturated conditions at fO2 ranging from -1 to +3.5 log units relative to the FMQ buffer. The impact of the system’s composition (X) was evaluated by comparing two distinct liquid lines of descent (medium-K-calk-alkaline and shoshonitic) through simultaneous experiments using two capsules in parallel. The results show that the partition coefficient of V between olivine and melt (D-V [Ol/melt]) is not systematically affected by varying P-T-X and highly correlates with changing fO2, thus suggesting that the minor variations observed on a global fO2 scale rather reflect analytical and experimental uncertainties. The updated empirical calibrations allow the determination of logfO2 as ∆FMQ from measured 1) D-V [Ol/melt] and 2) KD-V/Sc [Ol/melt], expressed by the following equations: 1) ∆FMQ = -1.72333 ± 0.04723 - (log10[(1.2273-log(D-V [Ol/melt]))/(log(D-V [Ol/melt])+3.11912)]/-0.09895 ± 0.00175) for NBO/T (non-bridging oxygens per tetrahedrally coordinated cation) ≤ 0.6: 2) ∆FMQ = (log(KD-V/Sc [Ol/melt])+0.98079 ± 0.02244)/-0.20567 ± 0.01231 In the logfO2 range of FMQ -1 to FMQ +3.5, relevant for arc magmatism, and at hydrous conditions, both oxybarometers show 2σ calibration uncertainty below 0.5 log units. The updated and new V-based oxybarometers enhance robustness across a wide P-T-X-fO2 range, enabling accurate quantification of the redox state of magmatic systems. They can, in turn, be applied to volcanic rocks ranging from basaltic to andesitic compositions by using suitable olivine-hosted silicate melt inclusions to reconstruct the redox history of deep-seated magma reservoirs in subduction zones. In addition, we demonstrate that the olivine–melt FeT-Mg exchange coefficient (where FeT is total iron expressed as FeO) correlates with changing fO₂ and can serve as an Fe-Mg oxybarometer within the calibrated range of this study. Owing to the rapid diffusion of Fe in olivine, this approach can record rapid redox fluctuations and provides complementary constraints to the more robust V-based oxybarometers.