The buffering capacity of lithospheric mantle: implications for diamond formation

Current models for the formation of natural diamond involve either oxidation of a methane-bearing fluid by reaction with oxidized mantle, or reduction of a carbonate-bearing fluid (or melt) by reaction with reduced mantle. Implicit in both models is the ability of the mantle with which the fluid equilibrates to act as an oxidizing or reducing agent, or more simply, to act as a source or sink of O₂. If only redox reactions involving iron are operating, the ability of mantle peridotite to fulfill this role in diamond formation may not be sufficient for either model to be viable. Using the recent experimental recalibration of olivine–orthopyroxene–garnet oxybarometers of Stagno et al. (2013), we re-evaluated the global database of ~200 garnet peridotite samples for which the requisite Fe³⁺/Fe²⁺ data for garnet exist. Relative to the previous calibration of Gudmundsson and Wood (1995), the new calibration yields somewhat more oxidized values of Δlog fO₂ (FMQ), with the divergence increasing from <0.5 units of log fO₂ at ~3 GPa to as much as 1.5 units at 5–6.5 GPa. Globally, there is a range of ~4 log units fO₂ for samples from the diamond stability field at any given pressure. Most samples are sufficiently reduced such that diamond, rather than carbonate, would be stable, and CHO fluids at these conditions would be H₂O-rich (>60 mol%), with CH₄ being the next most abundant species. To ascertain the capacity for mantle peridotite to act as a source or sink of O₂, we developed a new model to calculate the fO₂ for a peridotite at a given P, T, and Fe³⁺/Fe²⁺. The results from this model predict 50 ppm or less O₂ is required to shift a depleted mantle peridotite the observed four log units of fO₂. Coupled with the observed distribution of samples at values of fO₂ intermediate between the most reduced (metal-saturated) and most oxidized (carbonate-saturated) possible values for diamond stability, these results demonstrate that peridotites are very poor sinks or sources of O₂ for possible redox reactions to form diamond. A corollary of the poor redox buffering capacity of cratonic peridotites is that they can be employed as faithful indicators of the redox state of the last metasomatic fluid that passed through them. We propose that diamond formation from CHO fluids is a predictable consequence either of isobaric cooling or of combined cooling and decompression of the fluid as it migrates upward in the lithosphere. This establishes a petrological basis for the observed close connection between subcalcic garnet and diamond: based on high solidus temperatures of harzburgite and dunite effectively precluding dilution of CHO fluids through incipient melts, such highly depleted cratonic peridotites are the preferred locus of diamond formation. Due to a rapid increase in solidus temperature with increasing CH₄ content of the fluid, diamond formation related to reduced CHO fluids may also occur in some cratonic lherzolites.