Empirical relationships between ferric-ferrous ratio, oxygen fugacity, temperature and bulk composition of natural silicate melts have been calibrated for temperatures above 1200°C by different authors, most recently by Kress & Carmichael (1991; equation 1), and Nikolaev et al. (1996; equation 2). In order to test whether these equations can be applied at lower temperatures that are relevant to processes in magma chambers, we performed Mössbauer spectroscopy on synthetic ferro-basaltic glasses synthesised at controlled fO₂ (FMQ-2 to FMQ+1) in the temperature range 1132 - 1221°C.

Samples equilibrated below the liquidus contain plag (T£1178°C) or plag+ol (T£1160°C). In these samples the ferric-ferrous ratio of interstitial glass was determined using the Mössbauer milliprobe which allows spectra to be collected on spots with a diameter in the order of 100 mm (McCammon, 1994). The analytical data were compared to the ferric-ferrous ratios calculated using equations 1 and 2. For products of runs equilibrated at near-liquidus conditions (1174-1221°C) both equations consistently give lower ferric-ferrous ratios than the Mössbauer data. At lower temperatures, between 1162 and 1132°C, there is a very good agreement between the Mössbauer results and those calculated with equation 1, whereas equation 2 yields higher ferric-ferrous ratios at the lowest temperatures (1132°C).

In conclusion, the equation of Kress & Carmichael (1991) can be used to calculate ferric-ferrous ratios and the oxygen fugacity in quenched residual melts from a ferro-basaltic composition at sub-liquidus temperatures down to 1130°C, i.e. clearly outside the calibration range of the equation. The agreement between measured and calculated values is worse at near-liquidus temperatures, which are close to the calibration conditions.