Forward modelling of redox equilibria in the Earth mantle: simple versus complex systems

Oxygen fugacity (fO2) is conventionally used in petrology as a variable describing the oxygen chemical potential (μO2). In multiphase systems fO2 is classically determined by the distribution of Fe3+ in mineral phases. For peridotite mineral assemblages fO2 can be evaluated from several equilibria involving Fe3+-garnet components, where Fe3+ occurs in octahedral coordination. For the olivine + orthopyroxene + Fe3+-garnet assemblage two of these reactions are represented by: (1) 2 Ca3Fe3+2Si3O12 + 2 Mg3Al2Si3O12 + 2 Fe2Si2O6 = 2 Ca3Al2Si3O12 + 4Fe2SiO4 + 3 Mg2Si2O6 + O2 And (2) 2 Fe2+3Fe3+2Si3O12 (skiagite) = 4 Fe2SiO4 + FeSi2O6 + O2 Equilibrium (1) by Luth et al. (1990) has been recently experimentally tested by Stagno et al. (2013), while equilibrium (2) was calibrated by Gudmundsson & Wood (1995). Both equilibria have advantages and limits in their applicapibility. This contribution aims to compare the use of the two oxybarometers in simple and complex natural systems, selecting garnet peridotite samples of well studied suprasubduction mantle from the Sulu belt (China) and Ulten Zone (Italy) as case studies. Solid solution models will be applyed to both equilibria in a forward modelling of fO2, using the available thermodynamic database of Holland & Powell (2011) for andradite and grossular garnet end- members and that of Malaspina et al. (2009) which combined experimental and thermochemical data for skiagite. The results suggest that the Sulu and Ulten peridotites apparently record higher fO2s (FMQ to FMQ+2) than garnet peridotite xenoliths from the sub-cratonic mantle equilibrated at similar pressure conditions. These results will be also discussed in terms of extensive thermodynamic properties (moles of O2), demostrating that fO2 is not a simple, monotonically increasing function of the quantity of O2, and that high μO2 can be attained by lowering the bulk oxygen proportion in the system. The μO2, and therefore its conventional representation in fO2 space, exhibits a complex variation as a function of the variable phase assemblages developed in metasomatised peridotites. Gudmundsson G. & Wood B.J. 1995. Experimental tests of garnet peridotite oxygen barometry. Contrib. Mineral. Petrol., 119, 56-67. Holland T. & Powell R. 2011. An improved and extended internally consistent thermodynamic dataset for phases of petrological interest, involving a new equation of state for solids. J. Metamorph. Geol. 29, 333-383. Luth R.W., Virgo D., Boyd F.R. & Wood B.J. 1990. Ferric iron in mantle-derived garnets; implications for thermobarometry and for the oxidation state of the mantle. Contrib. Mineral. Petrol., 104, 56-72. Malaspina N., Poli S. & Fumagalli P. 2009. The oxidation state of metasomatized mantle wedge: insights from C-O-H- bearing garnet peridotite. J. Petrol., 50, 1533-1552. Stagno V., Ojwang D.O., McCammon C.A. & Frost D. 2013. The oxidation state of the mantle and the extraction of carbon from Earth’s interior. Nature, 493, 84-88.