Partitioning of Fe3+/Fetotal between amphibole and basanitic melt as a function of oxygen fugacity

We present the first microbeam measurements of Fe3+/Fetotal and H contents in amphiboles and glasses synthesized from a basanite at high temperature and pressure where oxygen fugacity (fO2) was buffered and water activity was monitored. The amphiboles were synthesized from basanite (San Carlos, AZ, USA) at 1100–1175°C and 1.5–2.0 GPa, at four fO2 values from the iron–wüstite (IW) buffer to the magnetite–hematite (MH) buffer. The Fe3+ contents of amphiboles reflect the fO2 or fH2 of the melt from which they have crystallized. Synthesized amphiboles below the MH buffer are pargasites and above that buffer, they are magnesiohastingsites. The change in amphibole type is accompanied by the crystallization of a Ti-bearing oxide or spinel, thus the magnesiohastingsites have relatively low Ti contents. At near constant aH2O, as fO2 increases, Fe3+ content increases in both amphiboles and glasses. The Fe3+/Fetotal partitioning between amphibole and melt is close to unity for the IW and HM samples. Samples prepared at intermediate fO2 have a Fe3+/Fetotal partition coefficient that is slightly greater than unity, probably because the calibration curves for Fe2+ and Fe3+ depend on coordination. If Fe3+/Fetotal partitioning is unity throughout the fO2 range, and the system is closed to oxygen and hydrogen exchange, then a melt that only crystallizes amphibole will have constant Fe3+/Fetotal.