The Behavior of Fe3+/∑Fe During Partial Melting of Spinel Lherzolite

Abstract:

The use of wet chemistry and X-ray absorption near edge structure (XANES) spectroscopy to determine the oxidation state of Fe in submarine glasses and olivine-hosted melt inclusions has provided important new insights into the global systematics of Fe³⁺/∑Fe in mid-ocean ridge basalts (MORB) [1, 2]. Because MORB are aggregates of near-fractional partial melts formed by decompression melting of variably depleted peridotite, it is difficult to judge the extent to which they directly reflect the oxidation state of the oceanic upper mantle. To provide a theoretical framework within which to interpret Fe³⁺/∑Fe in MORB, I have developed a model that describes the behavior of Fe³⁺/∑Fe during spinel lherzolite partial melting in a system closed to oxygen.

Modeling is carried out by calculating the Fe³⁺/∑Fe of olivine using the point defect model of [3], and determining Fe³⁺/∑Fe of the bulk peridotite from mineral-mineral partitioning. The inter-mineral Fe³⁺/Fe²⁺ exchange coefficients are derived from Mössbauer data on natural spinel peridotites, and are parameterized in terms of oxygen fugacity, temperature, and the Fe content of the olivine. The Fe³⁺/∑Fe of the melt is determined by combining mass-balance with an equation relating the Fe³⁺/∑Fe of the melt to the fugacity of oxygen [4]. Spinel lherzolite partial melting is modeled after [5].

Modeling results indicate that oxygen fugacity does not follow the fayalite-magnetite-quartz (FMQ) buffer during partial melting. For isobaric partial melting, the system becomes reduced relative to FMQ with increasing extent of melting. This results from an increase in the FMQ buffer with increasing temperature, whereas oxygen fugacity in the peridotite remains nearly constant. Conversely, during polybaric partial melting the oxidation state of the residual peridotite increases relative to FMQ. The effective partition coefficient for Fe³⁺is larger than previously thought, so that a redox couple with S is not required to explain its compatibility during partial melting.

[1] Bézos and Humler (2005) Geochim Cosmochim Acta 69:711-725; [2] Cottrell and Kelley (2011) Earth Planet Sci Lett 305:270-282; [3] Dohmen and Chakraborty (2007) Phys Chem Min 34:409-430; [4] Kress and Carmichael (1991) Contrib Min Pet 108:82-92; [5] Kinzler (1997) J Geophys Res 102:853-874