Abstract
The effect of pressure on the Fe 3+ /∑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 + RuO 2 buffer. Values of Fe 3+ /∑Fe were determined by Mossbauer 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 Fe 3+ /∑Fe than predicted from the trend established by the lower-pressure data. This might be due to a coordination change in Fe 3+ at high pressure; although there is no evidence for this in the Mossbauer spectra, such a change could account for the change in intensity of the 1s → 3d transition in the XANES spectra with pressure.
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