Fe3+ in garnet and the V/Cr redox proxy: Two methods, one result. Oxygen fugacity of Paleoarchean ky/cor eclogites from the Kaapvaal craton – subducted equivalents of former troctolites

The Earth’s mantle was at metal saturation shortly after accretion (ΔFMQ ~ -4.5; Frost and McCammon, 2004). This supposedly changed very rapidly to ΔFMQ ~ 0 (= today’s oxidation state of the MORB mantle) due to the disproportionation of Fe2+ in bridgmanite to Fe3+ + Fe0. The latter was transferred to the core leaving an oxidized residue behind. Magmas from the mantle should therefore indicate the same oxygen fugacity through time. This is tested here with 3.2 Ga old kyanite and corundum eclogite xenoliths from the Bellsbank diamond mine on the Kaapvaal craton, i.e. metamorphosed troctolites (= olivine-plagioclase cumulates; Shu et al. 2016).

One method to estimate the oxygen fugacity of troctolites is based on the V/Cr ratio of the accumulating olivine (Mallmann and O’Neill, 2009). Modern day troctolites from the Pacific and the Atlantic form a correlation of V and Cr with a slope corresponding to ΔFMQ ~ 0. The ky/cor eclogites yield a steeper slope corresponding to ΔFMQ ~ -2 meaning that the Fe3+ content of the garnets should be close to zero. Our second method, the determination of Fe3+/ΣFe in garnet with the flank method by EPMA (Hoefer and Brey 2007), gave Fe3+/ΣFe < 0.03 that yielded a mean ΔFMQ value of -1.6 as calculated with the oxybarometer of Vasilyev (2016). This indicates a gradual increase of oxygen fugacity in the mantle after core formation (see also Aulbach and Stagno 2016) and confirms that the oxidation state of Fe can be preserved in eclogites since the Archean.