Abstract
New Mössbauer measurements of Fe3+/ΣFe ratios from chromites in the Upper Critical Zone of the Rustenberg Layered Suite of the Bushveld Complex, South Africa vary between 0.139 and 0.232 (mean=0.185±0.025 (1σ)). A more oxidised group have Fe3+/ΣFe ratios in the range 0.766–0.738. Textural studies indicate that these more oxidised samples experienced post-magmatic oxidation during brecciation.The chromitites have a narrow range of cr# (0.618–0.671) and are Fe-rich. There is some variability in cr# with stratigraphic height implying variation in the composition of the parental magmas.Variation within the less oxidised group of chromitites is attributed to variations in the oxidation state of the parent magma, which in turn is thought to reflect different degrees of mixing between magmatic sources with differing oxidation states. Comparison with geologically recent chromitites of similar composition suggests that upper mantle redox conditions have remained unchanged since 2.06Ga.
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