The Oxygen Fugacity Relationships At The Contact of The E-F Horizons of The Eastern Bushveld

Abstract

A review of the theory and evolution of fO2 cells is given. Particular attention is paid to the double cell design of Sato (1971). The incorporation of a servo mechanism after Sato (1971) is discussed. Calibration procedures for the entire system are presented with some emphasis on the use of nickel-neckel oxide and synthetic magnetite. The petrology of the Bushveld is reviewed in light of a possible fO2 hypothesis for the Critical Zone. Results of an examination of the E-F contact, Critical Zone, Eastern Bushveld, show a last equilibration at 1075 C and at a log10 fO2 value of -12.20. Two explanations are suggested for this intersection point: 1. It is the solidus, fO2 and temperature of the Bushveld magma at this horizon. 2. It is the post-cumulus, i.e., sub-solidus, fO2 and temperature of equilibration for the Bushveld rocks at the F-horizon. The first explanation is favored by the author and is being pursued currently. The second explanation requires ferrous/ferric equilibration but not chemical equilibration. This phenomenon is discussed from the consideration of diffusion rates for various ions.