The temperature dependence of the partitioning of Al2O3 between forsterite-rich olivine and Cr-rich spinels has been experimentally calibrated at 100 kPa between 1250 and 1450 °C under reducing conditions. For spinel with values of YCr between 0.07 and 0.69, and containing <0.1 Fe3+ atoms and <0.025 Ti atoms per 4 O atoms the experimental data can be fit to an equation:
where YCr = Cr/(Cr + Al) in spinel in atomic proportions and KD = Al2O3ol/Al2O3sp in wt%. This equation reproduces the temperature of the experiment for the calibration data set within ±22 °C. Empirical tests suggest that this geothermometer has little pressure dependence within the range of pressures encountered in the crust and upper mantle.