THE BUFFERING CAPACITY OF CRATONIC MANTLE PERIDOTITE: IMPLICATIONS FOR THE FORMATION OF DIAMOND

Current models for formation of diamond invoke either the oxidation of a methane-bearing fluid by reaction with oxidized mantle, or the 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 interacts to supply or absorb O₂. If only redox reactions involving iron are operating, the ability of mantle peridotite to fulfill this role may not be sufficient for either model to be viable.

Based on the oxybarometer of Stagno et al. (2013 Nature 493:84), there is a range of ~4 log units fO₂ for cratonic garnet peridotites from the diamond stability field. Modeling the relationship between fO₂, P, T, and Fe³⁺/Fe²⁺ for cratonic peridotites, we find that ≤50 ppm O₂ is all that is required to shift a depleted peridotite the observed four log units of fO₂. Such very small amounts of O₂, along 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, imply that these depleted peridotites are very poor sinks or sources of O₂ for redox reactions to form diamond. They are, however, faithful indicators of the redox state of the last metasomatic fluid that passed through them.

We suggest instead that diamond forms from CHO fluids during either isobaric cooling, or combined cooling and decompression as they migrate upward in the lithosphere. Depending on its composition, a CHO fluid could precipitate as much as 45% of its original C during cooling by 100°C to the local geotherm. During ascent of a fluid along the geotherm, the solubility of C in a CHO fluid decreases and C will again be precipitated. This model establishes a petrological basis for the observed close connection between subcalcic garnet and diamond: the high solidus temperatures of harzburgite and dunite preclude dilution of CHO fluids by incipient melting, and thus such highly depleted cratonic peridotites are the preferred locus of diamond formation. Because the solidus temperature increases rapidly with increasing CH₄ content of the fluid, diamond formation related to reduced CHO fluids may also occur in some cratonic lherzolites.