An experimental determination of the effect of pressure on the Fe^3/∑Fe ratio of an anhydrous silicate melt to 3.0 GPa

The effect of pressure on the Fe³⁺/∑Fe ratio of an anhydrous andesitic melt was determined from 0.4 to 3.0 GPa at 1400 °C with oxygen fugacity controlled internally by the Ru + RuO₂ buffer. Values of Fe³⁺/∑Fe were determined by Mössbauer spectroscopy on quenched glasses with a precision of ±0.01, one standard deviation. This precision was verified independently by XANES spectroscopy of the same samples. The XANES spectra show a systematic increase in energy and decrease in intensity of the 1s → 3d transition with increasing pressure. The results to 2.0 GPa are in good agreement with predictions from density and compressibility measurements fitted to a Murnaghan equation of state, but the datum at 3.0 GPa has higher Fe³⁺/∑Fe than predicted from the trend established by the lower-pressure data. This might be due to a coordination change in Fe³⁺ at high pressure; although there is no evidence for this in the Mössbauer spectra, such a change could account for the change in intensity of the 1s → 3d transition in the XANES spectra with pressure.