Other
Scientific paper
Sep 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003e%26psl.214..295w&link_type=abstract
Earth and Planetary Science Letters, Volume 214, Issue 1-2, p. 295-310.
Other
30
Upper Mantle, Redox State, Oxygen Fugacity, Kaapvaal Craton, Peridotite Xenoliths, Archean Atmosphere
Scientific paper
Oxygen fugacity (fO2) is an important parameter in many geochemical processes in the Earth's mantle. To assess how fO2 varies with depth, Fe3+ 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 fO2 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, ΔlogfO2 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 fO2 is less, amounting to ~0.8 log units over the depth interval of 150-220 km. At ~220 km depth the ΔlogfO2 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 fO2 conditions a coexisting fluid phase would be dominantly composed of H2O and CH4. Thus in the deeper portions of the upper mantle the necessary conditions for `redox melting' are met, namely a region where CH4-rich fluids can exist and migrate upward into more oxidised peridotite. The lowering of fO2 with depth follows in part from a negative ΔV for the reaction describing the incorporation of Fe3+ in garnet. The change in the rate of fO2 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 CO2 and H2O, 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 fO2 values in the range of FMQ should not be seen as `oxidising' since they had a capacity to react with O2 and inhibit the build-up of free O2 in the early atmosphere.
Koch Manfred
Woodland Alan B.
No associations
LandOfFree
Variation in oxygen fugacity with depth in the upper mantle beneath the Kaapvaal craton, Southern Africa does not yet have a rating. At this time, there are no reviews or comments for this scientific paper.
If you have personal experience with Variation in oxygen fugacity with depth in the upper mantle beneath the Kaapvaal craton, Southern Africa, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Variation in oxygen fugacity with depth in the upper mantle beneath the Kaapvaal craton, Southern Africa will most certainly appreciate the feedback.
Profile ID: LFWR-SCP-O-1152483