Experimental calibration of V–Sc oxybarometers based on clinopyroxene- and orthopyroxene-melt equilibria

Abstract

We present new experimental calibration of V–Sc exchange between clinopyroxene, orthopyroxene, and silicate melt at physical–chemical conditions characteristic of arc magmatism. The calibration combines previously published experimental datasets with new high-pressure rapid-quench experiments conducted in a molybdenum-hafnium carbide pressure vessel assembly equipped with a custom-designed semi-permeable hydrogen membrane to control oxygen fugacity (fO₂). Experiments were performed at 200 MPa under water-saturated conditions, covering temperatures of 1030–870 °C and fO₂ from − 1 to + 7 log units relative to fayalite-magnetite-quartz buffer (FMQ). Melt composition effects were evaluated by comparing liquid lines of descent of medium-K calc-alkaline basaltic and shoshonitic magmas. The resulting orthopyroxene-melt oxybarometer accurately reproduces fO₂, yielding a median absolute error of 0.36 log units over the range FMQ − 2 to + 5.5. Precise calibration of the clinopyroxene-melt oxybarometer was more challenging due to the clinopyroxene’s more complex composition and potential kinetic effects. Nevertheless, the proposed equation reproduces fO₂ with an overall median absolute error of 1.01 log units over the calibrated range FMQ − 2.5 to + 5. Provided that the equilibrium melt composition can be constrained by the analysis of silicate melt inclusions or groundmass glass, the proposed pyroxene-melt oxybarometers can be used to track the variation of magmatic fO₂ in diverse settings and over a broad temperature range.