A new tool to trace the redox-state of the upper mantle in the Archaean

The redox state of the upper mantle in the Archaean through to the Proterozoic is a key parameter as it would have buffered atmospheric composition and interacted with the ocean-atmosphere system. There have been multiple approaches using geochemical proxies, such as V-Sc and redox sensitive stable isotopes (e.g. Fe) applied to mantle-derived rocks to investigate this problem. As whole rock samples are prone to overprinting (alteration, metamorphism) and as mafic rocks in particular are difficult to date, a technique using a robust U-bearing accessory mineral might allow better and more trustworthy temporal constraints to be measured. Recent work developing an oxybarometer based on S in apatite using µ-XANES has shown great promise as apatite can seamlessly incorporate reduced and oxidised S species and directly reflect the fugacity of host magmas. Nonetheless, apatite crystals in a matrix rock are prone to alteration and recrystallisation, but apatite inclusions trapped in zircon during magmatic crystallisation are robust, with the advantage that the enclosing zircon can be dated and the mantle source traced via Lu-Hf and O isotopes. To demonstrate that this approach works, we have studied 2.35 Ga TTGs and 2.13 Ga sanukitoids from the Mineiro Belt, Brazil. These rocks temporally straddle the Great Oxidation Event. Apatite inclusions in zircons from this TTG-sanukitoid transitional magmatic record reveal a change from reduced to more oxidised conditions from pre- to post-GOE. We then discuss how this approach has and can be further applied to Archaean rocks as a tracing tool for earlier oxygenation events.