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

Abstract

Partition coefficients of vanadium between magnetite and rhyolitic silicate melt, DVmgt/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°C), melt alumina saturation index (ASI=0.74–1.14), magnetite composition (0.2–14wt% TiO2) and pressure (1–5kbar; at H2O saturation). Experiments were performed by equilibrating small (≤20µm), V-free magnetite grains in V-doped silicate melts (∼100ppmV) and then analyzing both phases by LA-ICP-MS. Attainment of equilibrium was demonstrated by several reversal experiments. The results suggest that DVmgt/melt depends strongly on fO2, 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°C interval at a fixed fO2 buffer). Magnetite composition and melt water content seem to have negligible effects. The data were fitted by the following linear regression equation:logDVmgt/melt=0.3726∗10,000T+2.0465∗ASI-0.4773∗ΔFMQ-2.1214,in which temperature is given in K, ASI refers to molar Al2O3/(CaO+Na2O+K2O) and ΔFMQ refers to the deviation of fO2 (in log units) from the fayalite-magnetite-quartz buffer. This equation reproduces all of our data within 0.3 log units, and 89% of them within 0.15 log units. The main advantages of this new oxybarometer over classical magnetite–ilmenite oxybarometry are (1) that it can be applied to rocks that do not contain ilmenite, and (2) that it is easier to apply to slowly-cooled rocks such as granites.