The redox conditions of anhydrous and hydrous xenoliths of suprasubduction and intraplate lithospheric mantle

The oxidation state of the upper mantle, its relationship with C-H-O fluids speciation and tectonic settings has been debated for decades and the various modelling have considered the prevalent role of the hydrous minerals over nominally anhydrous minerals (and the opposite) as well as the dissolution of silicate minerals (as providers of Fe3+ to the system) as directly related to water activity and oxygen fugacity. Each of these modelling has different implications for mantle rheology, seismic structure, and the evolution of the lithosphere (i.e.: Karato and Jung, 1998, Hirshmann, 2006). Upper mantle is the only part of the Earth's mantle where the oxygen fugacity can be directly measured, its values/variation being dependent on various processes such as partial melting and metasomatism often operating in time and space without solution of continuity. Recent general reviews of oxygen thermobarometry measurements (Forst & McCammon, 2008; Foley, 2011) indicate that the oxygen fugacity at the top of the upper mantle falls within ±2 log units of the fayalite-magnetite-quartz (FMQ) oxygen buffer. There is also a general consensus in considering H2O as the strongest oxidizing agent in mantle metasomatic fluids, its activity leading to the formation of amphibole and raising the mantle redox state. This contribution presents fO2 and water activity results from three spinel-bearing mantle xenolith localities and distinct geodynamic settings: Ichinomegata (Japan) amphibole-bearing peridotites entrained in calc-alkaline basalts and Cerro Fraile (South Patagonia, Argentina), mostly anhydrous lherzolites and pirossenites brought up to the surface by alkaline basalts representing fragments of sub-arc mantle and Baker Rocks, Victoria Land (Antarctica), amphibole-bearing lherzolites representing portion of intraplate subcontinental lithospheric mantle. The three mantle sectors records fO2 values in the range of -1.9 to +0.8 log units of the FQM buffer. and low to very low aH2O even in presence of amphibole (< 0.0008). The interesting result emerging by this study is that amphibole-bearing peridotites record more reduced conditions with respect to anhydrous peridotites of the same mantel column, as also observed in amphibole-bearing mantle peridotite from Nüshan, eastern China (Li & Zhang, 20021) and in hydrous Cerro Fraile mantle xenoliths (Wang et al., 2009). These results dismantle the straightforward relationship between the presence of amphibole and relatively high oxidation conditions. For these mantle fragments it seems that the metasomatic processes able to stabilize amphibole in the spinel-peridotite is not necessary triggered by high water activity, which in turn is not leading to enhance oxygen fugacity. It is conceivable that amphibole being the main acceptor of H2O among the peridotite minerals, acts as a H-O buffer, preventing further fluids circulation. Karato and Jung (1998). Earth Planet. Sci. Letter 157: 193-207 Hirshmann (2006). Annu. Rev.Earth Planet. Sci. 34:629-53 Forst & McCammon (2008). Annu. Rev. Earth Planet. Sci. 36:389-420 Foley (2011). J. Petrol. 52:1363-1391 Li & Zhang (2002) Science in China 45: 349-357 Wang et al. (2009). Contrib. Mineral. Petrol. 153:607-624.