Ferric Iron in CaTiO3 Perovskite as an Oxygen Barometer for Kimberlitic Magmas I: Experimental Calibration

A method to estimate the oxygen fugacity (fO₂) during the crystallizationof kimberlites is developed using the Fe content of CaTiO₃ perovskite(Pv), a common groundmass phase in these rocks. With increasingfO₂, more Fe exists in the kimberlitic liquid as Fe³⁺, and thuspartitions into Pv. Experiments to study the partitioning ofFe between Pv and kimberlite liquid were conducted at 100 kPaon simple and complex anhydrous kimberlite bulk compositionsfrom 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 startingmaterials of 0–5 wt % and 1500 ppm, respectively. Thepartitioning of Fe between Pv and kimberlite liquid is influencedmostly by fO₂, although the presence of Nb increases the partitionof Fe³⁺ into perovskite at a given T and fO₂. Multiple linearregression (MLR) of all the experimental data produces a relationshipthat describes the variation of Fe and Nb in Pv with fO₂ relativeto the NNO buffer:

(uncertaintiesat 2, and Nb and Fe as cations per three oxygens). Over therange of conditions of our experiments, this relationship showsno temperature (T) dependence, is not affected by the bulk Fecontent of the kimberlite starting material and reproduces experimentaldata to within 1 log fO₂ unit. KEY WORDS: kimberlites; oxygen fugacity; perovskite; ferric iron; magma