The geochemistry of titanomagnetite in magnetite layers and their host rocks of the eastern Bushveld Complex

More than 430 analyses of magnetite separates revealed pronounced differences in composition between magnetite from magnetite layers and disseminated magnetite from the host rocks. Magnetite from the massive layers is among others enriched in elements such as Ti, Mg, Al, and Si, which suggests that this magnetite crystallized under conditions of disequilibrium with the magma, i.e., by spontaneous nucleation and rapid crystallization. Lower vanadium values in the magnetites from the massive layers suggest crystallization at higher f (sub O 2 ) than from the rocks containing the disseminated magnetite.Variation diagrams reveal a cyclicity in compositional trends upward in the sequence which is related to nucleation of the magma in the roof zone of the intrusion, the periodic collapse of this crystal-laden magma, and its incorporation in the zone of crystallization at the base of the magma chamber. Deviations of this normal trend are considered to reflect a stratification of the roof zone and a gradual incorporation of the stratified roof zone liquid into the convecting magma. A pronounced upward increase in the Cr content of magnetite is tentatively ascribed to a combination of olivine crystallization at the expense of pyroxene in the upper subzones and a possible increase in the partition coefficient of chromium between spinel and liquid with a decrease in the crystallization temperature.