Variation in oxygen fugacity with depth in the upper mantle beneath the Kaapvaal craton, Southern Africa

Abstract

Oxygen fugacity ( fO 2) is an important parameter in many geochemical processes in the Earth’s mantle. To assess how fO 2 varies with depth, Fe 3+ contents in garnet and spinel from peridotite xenoliths were determined by Mössbauer spectroscopy. A total of 49 xenoliths were investigated from localities on the southern flank of the Kaapvaal craton in South Africa (Kimberley, Jagersfontein, Frank Smith Mine, Monastery) and Lesotho (Letseng-la-Terae, Liqhobong, Matsoku). These samples provide a depth coverage from ∼80 to 220 km. For the Lesotho and South African xenoliths there is a systematic, but not monotonic, decrease in fO 2 with depth. Between about 80 and 150 km depth there is a decrease of ∼3 log units. At shallow depths, where spinel peridotites are stable, Δlog fO 2 values of ∼FMQ−1 (i.e. one log unit below the fayalite–magnetite–quartz reference oxygen buffer) are obtained, similar to other worldwide occurrences. At greater depths the decrease in fO 2 is less, amounting to ∼0.8 log units over the depth interval of 150–220 km. At ∼220 km depth the Δlog fO 2 lies just below FMQ−4 and is expected to decrease further with depth, reaching conditions of metal saturation near the 410 km discontinuity. Under such fO 2 conditions a coexisting fluid phase would be dominantly composed of H 2O and CH 4. Thus in the deeper portions of the upper mantle the necessary conditions for ‘redox melting’ are met, namely a region where CH 4-rich fluids can exist and migrate upward into more oxidised peridotite. The lowering of fO 2 with depth follows in part from a negative Δ V for the reaction describing the incorporation of Fe 3+ in garnet. The change in the rate of fO 2 decrease is attributable to a change in the bulk composition, the sheared peridotites at depth being relatively fertile compared to the overlying depleted peridotites. Variable degrees of oxidation appear to have attended metasomatism, as recorded in samples from Kimberley. Our results emphasise that the upper mantle cannot be treated as monolithic in terms of redox state. The early development of the Earth’s atmosphere was directly influenced by degassing of the mantle via volcanic activity. Since the majority of magmas, such as MORB, are produced under conditions within the spinel peridotite field, this implies that mantle degassing will be dominated by CO 2 and H 2O, although S should be present mostly as sulphide. This is consistent with recent data on Cr and V abundances in lavas. However, hydrothermal or volcanic emissions with fO 2 values in the range of FMQ should not be seen as ‘oxidising’ since they had a capacity to react with O 2 and inhibit the build-up of free O 2 in the early atmosphere.