Partitioning of Fe3+/Fe-total between amphibole and basanitic melt as a function of oxygen fugacity

We present the first microbeam measurements of Fe3+/Fe-total and H contents in amphiboles and glasses synthesized from a basanite at high temperature and pressure where oxygen fugacity (fO(2)) was buffered and water activity was monitored. The amphiboles were synthesized from basanite (San Carlos, A Z, USA) at 1100-1175 degrees C and 1.5-2.0 GPa, at four fO(2) values from t he iron-wustite (IW) buffer to the magnetite-hematite (MH) buffer. The Fe3 contents of amphiboles reflect the fO(2) or fH(2) of the melt from which t hey have crystallized. Synthesized amphiboles below the MH buffer are parga sites and above that buffer, they are magnesiohastingsites. The change in a mphibole type is accompanied by the crystallization of a Ti-bearing oxide o r spinel, thus the magnesiohastingsites have relatively low Ti contents. At near constant aH(2)O, as fO(2) increases, Fe3+ content increases in both a mphiboles and glasses. The Fe3+/Fe-total partitioning between amphibole and melt is close to unity for the IW and HM samples. Samples prepared at inte rmediate fO(2) have a Fe3+/Fe-total partition coefficient that is slightly greater than unity, probably because the calibration curves for Fe2+ and Fe 3+ depend on coordination. If Fe3+/Fe-total partitioning is unity throughou t the fO(2) range, and the system is closed to oxygen and hydrogen exchange , then a melt that only crystallizes amphibole will have constant Fe3+/Fe-t otal. (C) 2000 Elsevier Science B.V. All rights reserved.