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Experimental development of new oxybarometers based on V-partitioning between mafic minerals and hydrous silicate melts

Enzo-Enrico Cacciatore 1, Alexandra Tsay1. Ivano Gennaro1 and Zoltán Zajacz1

Abstract

Redox conditions in magmatic environments control phase equilibria and the behavior of volatiles like sulfur. Understanding the interplay between oxygen fugacity (fO2) and magmatic differentiation provides insights into the roles of magmatism and volcanic degassing in Earth's redox evolution. However, there are only a few studies on oxybarometric methods calibrated at elevated pressure and water content, conditions typical of magma differentiation at convergent plate boundaries. We conducted experiments (Pressure (P) = 200 MPa, Temperature (T): 1020−920 °C, logfO2: -1 to +3.5 log∆FMQ) to develop oxybarometers based on vanadium (V) partitioning between mafic minerals and hydrous silicate melts. The results show that V partitioning is largely independent of  P, T, and silicate melt composition (X). For olivine-silicate melt pairs, the V partition coefficient (DV[Ol/melt]) is modeled using a sigmoidal regression equation: logfO2(∆FMQ) = LOGX0-log10[([(A2-y)/(y-A1)-1)]2)0.5]/p, with A1 = -3.10 ± 0.13, A2 = 1.30 ± 0.12, LOGX0 = -1.98 ± 0.25, p = -0.10 ± 0.01, and y = logDV[Ol/melt]. This equation reproduces fO2 within a 2σ median error of one log unit over a wide range of T-P-X. This study expands previous data for olivine to lower temperatures and higher water activities, improving robustness and reducing calibration error. It also highlights the influence of fO2 on fractional crystallization in intermediate magmas at subduction zones. The olivine-based oxybarometer is applicable to mafic volcanic rocks with suitable silicate melt inclusion-host mineral pairs for reconstructing magma reservoir redox histories. For more evolved magmatic systems, orthopyroxenes and amphiboles offer additional potential to similarly constrain fO2.