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 (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-wustite (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 Fe^3-/Fe(total) partitioning between amphibole and melt is close to unity for the IW and HM samples. Samples prepared at intermediate fO₂ have a Fe³⁺/Fe(total) partition coefficient that is slightly greater than unity, probably because the calibration curves for Fe²⁺ and Fe³⁺ depend on coordination. If Fe³⁺/Fe(total) 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⁺/Fe(total). (C) 2000 Elsevier Science B.V. All rights reserved.