D log fO2 (FMQ) - i.e. the oxygen fugacity defined by a mineral assemblage relative to the fayalite-magnetite-quartz buffer - is widely used as a monitor of the oxidation state of the Earth's mantle. Different D log fO2 (FMQ) are thought to reflect different oxidation states, with the consequence that models of a stratified or of a laterally inhomogeneous mantle are usually favored. Changes in D log fO2 (FMQ), in particular during the ascent of mantle magmas, are thought to be related to redox processes.
The present note aims at showing that D log fO2 (FMQ) is not a reliable indicator of the oxidation state and at exemplifying some consequences for redox models of the mantle and its melt products.
D log fO2 (FMQ) can be only estimated from mineral assemblages that are related to a solid oxygen buffer equilibrium. For upper mantle peridotites, the most widely used assemblage is olivine-orthopyroxene-spinel which relies on the fayalite-ferrosilite-magnetite (FFsM) equili-brium:
(6 Fe2+SiO3)opx+ (2 Fe2+Fe3+2O4)spl = (6 Fe2+2SiO4)ol + O2 , with the following expression for the oxygen fugacity:
fO2 = KFFsM . (aFeSiO3)6. (aFe3O4)2 . (aFe2SiO4)-6 (1).
D log fO2 (FMQ) = log fO2 - log fO2 (FMQ) (2).
There are several calibrations of fO2 for the FFsM equilibrium in the literature.
D log fO2 (FMQ) may be dependent on temperature, because the assemblages considered do not necessarily have the same fO2 -T relation than the FMQ buffer. Moreover, temperature-dependent Fe-Mg exchanges also indirectly influence D log fO2 (FMQ) because they affect the activity term in the log fO2 calculation (eq. 1). The effect of the modal composition is related to that of temperature: as soon as the isopleths of the components involved in the buffer equilibrium are not parallel in a log fO2 vs. T diagram, the temperature-dependence of D log fO2 (FMQ) becomes a function of the modal ratio of the corresponding phases. If changes in the bulk chemical composition translate into changes of the mineral compositions, the activity term (eq. 1) and, hence, D log fO2 (FMQ) are also affected. The influence of pressure on D log fO2 (FMQ) is two-fold. The equilibrium constant of any oxygen buffer reaction, and hence its fO2 , is dependent on pressure via the reaction volume of the solids. Indirectly, mineralogical changes with depth in the mantle also affect fO2.
Changes in D log fO2 (FMQ) due to any of the factors enumerated above do not reflect any redox change. In particular, increasing D log fO2 (FMQ) with decreasing pressure in an ascending magma cannot be related to an oxidation process, as proposed in several papers (e.g. Carmichael & Ghiorso, 1986; Ballhaus, 1993). Similarly, a decrease of D log fO2 (FMQ) with increasing depth in the mantle is not synonymous of reduction (cf. Ballhaus, 1995).
Since changes in D log fO2 (FMQ) need not reflect any oxidation or reduction, this parameter cannot adequately represent the oxidation state. There is no clear definition of the oxidation state of a paragenesis, a rock or any geological entity, because geological materials have complex chemical compositions. However, since iron is the only major transition element in the mantle, it is reasonable to express oxidation ratios in the mantle by the ratio "OF"=SOFe/SFe, whereby OFe is the oxygen linked to iron in all valencies (incl. Fe°). The OF-ratio is dependent on the modal composition of the rock and there is no simple relation between it and D log fO2 (FMQ). It must be stressed that if the bulk chemical composition (incl. the bulk Fe3+/SFe) is assumed to be constant throughout the mantle (O'Neill et al., 1993; Ballhaus, 1995), the OF-ratio is also constant and oxidised or reduced domains are excluded.
Different D log fO2 (FMQ) translate into different compositions of the C-H-O fluids that may coexist with solid mantle parageneses. In particular, decreasing values of D log fO2 (FMQ) of the solid assemblage may result in a reduction of the coexisting mantle fluids. The oxygen thus released must, in turn, oxidise the solid assemblage. The related increase in D log fO2 (FMQ) will depend on the buffering capacity of the solid assemblage and may be only slight. In conclusion, mantle domains with low D log fO2 (FMQ) may have more oxidised solid parageneses than those with higher D log fO2 (FMQ). It is, however, important to note that at sufficient pressure, relatively oxidised domains - i.e. with relatively high OF ratios - may contain an iron-bearing metallic phase coexisting with Fe3+- bearing silicates and oxides.
Ballhaus, C., Contrib. Mineral. Petrol. 114, 331-348 (1993).
Ballhaus, C., Earth planet. Sci. Lett. 132, 75-86 (1995).
Carmichael, I. S. E. & Ghiorso, M.S., Earth planet. Sci. Lett. 78, 200-210 (1986).
O'Neill, H. St. C. et al. In Evolution of the Earth and Planets, IUGG Vol. 14, 73-88 (1993).