Experimental calibration of a new oxybarometer for silicic magmas based on vanadium partitioning between magnetite and silicate melt

Partition coefficients of vanadium between magnetite and rhyolitic silicate melt, D-V(mgt/melt), were experimentally determined as a function of oxygen fugacity (0.7-4.0 log units above the fayalite-magnetite-quartz buffer), temperature (800-1000 degrees C), melt alumina saturation index (ASI = 0.74-1.14), magnetite composition (0.2-14 wt% TiO2) and pressure (1-5 kbar; at H2O saturation). Experiments were performed by equilibrating small (<= 20 mu m), V-free magnetite grains in V-doped silicate melts (similar to 100 ppm V) and then analyzing both phases by LA-ICP-MS. Attainment of equilibrium was demonstrated by several reversal experiments. The results suggest that D-V(mgt/melt) depends strongly on fO(2), increasing by 1.5-1.7 log units from the MnO-Mn3O4 buffer to the Ni-NiO buffer, and to lesser (but still considerable) extents on melt alumina saturation index (ASI; increasing by 0.3-0.7 log units over 0.4 ASI units) and temperature (increasing by 0.3-0.7 log units over a 200 degrees C interval at a fixed fO(2) buffer). Magnetite composition and melt water content seem to have negligible effects. The data were fitted by the following linear regression equation: