Oxygen fugacity profile of the oceanic upper mantle and the depth of redox melting beneath ridges

Abstract:

Oxygen fugacity (f_O2) of a mantle mineral assemblage, controlled primarily by Fe redox chemistry, sets the depth of the diamond to carbonated melt reaction (DCO3). Near-surface f_O2 recorded by primitive MORB glasses and abyssal peridotites anchor the f_O2 profile of the mantle at depth. If the f_O2-depth relationship of the mantle is known, then the depth of the DCO3 can be predicted. Alternatively, if the DCO3 can be detected geophysically, then its depth can be used to infer physical and chemical characteristics of upwelling mantle.

We present an expanded version of a model of the f_O2-depth profile of adiabatically upwelling mantle first presented by Stagno et al. (2013), kindly provided by D. Frost. The model uses a chemical mass balance and empirical fits to experimental data to calculate compositions and modes of mantle minerals at specified P, T, and bulk Fe³⁺/ƩFe. We added P and T dependences to the partitioning of Al and Ca to better simulate the mineralogical changes in peridotite at depth and included majorite component in garnet to increase the depth range of the model. We calculate f_O2 from the mineral assemblages using the grt-ol-opx oxybarometer (Stagno et al., 2013). The onset of carbonated melting occurs at the intersection of a Fe³⁺/ƩFe isopleth with the DCO3. Upwelling mantle is tied to the DCO3 until all native C is oxidized to form carbonated melts by reduction of Fe³⁺ to Fe²⁺. The depth of intersection of a parcel of mantle with the DCO3 is a function of bulk Fe³⁺/ƩFe, potential temperature, and bulk composition. We predict that fertile mantle (PUM) along a 1400 °C adiabat, with 50 ppm bulk C, and Fe³⁺/ƩFe = 0.05 after C oxidation begins redox melting at a depth of 250 km.

The model contextualizes observations of MORB redox chemistry. Because fertile peridotite is richer in Al₂O₃, the Fe₂O₃-bearing components of garnet are diluted leading to lower f_O2 at a given depth compared to refractory mantle under the same conditions. This may indicate that the negative
correlation observed between enrichment and f_O2 at ridges (Cottrell and Kelley, 2013) is a consequence of the increased fertility of remixing recycled crust into the mantle. Addition of reduced C to the mantle during subduction can also explain this observation. Geophysical detection of the depth of the DCO3 may resolve these hypotheses.