A method to estimate the oxygen fugacity (fO2) during the crystallization of kimberlites is developed using the Fe content of CaTiO3 perovskite (Pv), a common groundmass phase in these rocks. With increasing fO2, more Fe exists in the kimberlitic liquid as Fe3+, and thus partitions into Pv. Experiments to study the partitioning of Fe between Pv and kimberlite liquid were conducted at 100 kPa on simple and complex anhydrous kimberlite bulk compositions from 1130 to 1300°C over a range of fO2 from NNO − 5 to NNO + 4 (where NNO is the nickel–nickel oxide buffer), and at Nb and rare earth element (REE) contents in the starting materials of 0–5 wt % and 1500 ppm, respectively. The partitioning of Fe between Pv and kimberlite liquid is influenced mostly by fO2, although the presence of Nb increases the partition of Fe3+ into perovskite at a given T and fO2. Multiple linear regression (MLR) of all the experimental data produces a relationship that describes the variation of Fe and Nb in Pv with fO2 relative to the NNO buffer:
(uncertainties at 2σ, and Nb and Fe as cations per three oxygens). Over the range of conditions of our experiments, this relationship shows no temperature (T) dependence, is not affected by the bulk Fe content of the kimberlite starting material and reproduces experimental data to within 1 log fO2 unit.