Redox conditions in the cratonic lithosphere and implications for metasomatism

Abstract

The redox state of the cratonic lithosphere and its impact on metasomatism have been examined using garnet peridotite xenoliths from two kimberlite pipes (Wesselton and Kimberley) in the Kaapvaal Craton, South Africa. Conventional petrologic techniques, along with electron probe microanalysis (EPMA) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), were used to characterise the samples and identify metasomatic events. Evidence of at least one such event was found in the Wesselton samples. Iron K-edge X-ray absorption near-edge structure (XANES) spectroscopy was used to determine the Fe3+/∑Fe of garnet from both suites, and then calculate the oxygen fugacity (fO2). The Wesselton samples were found to show a decrease in fO2 (relative to the fayalite-magnetite-quartz buffer) with increasing pressure, which was overprinted by an oxidation event in zoned garnet crystals, which were observed in one sample. The fO2 of the core of the zoned crystals was consistent with equilibrium with a carbonated silicate melt, and is possible that the oxidised nature of this melt is related to formation of the metasomatised rim observed, which is more oxidised than the garnet core. XANES spectroscopy was used to produce the first fully quantitative map of the distribution of Fe3+/∑Fe in garnet for one of these crystals, which confirmed the variation in fO2 between the core and rim, strengthening the relationship between oxidation and metasomatism. The interface between the two compositional zones showed a very short profile and this was examined using both electron probe microanalysis and NanoSIMS. The compositional profile at this interface was modelled as a diffusive process and it was found that the zonation formed in 2 - 10 years. The diffusion coefficients (relative to Ca) of Na, Cr, Ti and Y in mantle garnet were also determined for the first time.