The buffering capacity of lithospheric mantle: implications for diamond formation

• Published in: Contributions to mineralogy and petrology 2014-11, Vol.168 (5), p.1, Article 1083
• Main Authors: Luth, Robert W., Stachel, Thomas
• Format: Article
• Language: English
• Subjects: Calibration; Carbonates; Cooling; Diamond crystals; Diamonds; Earth; Earth and Environmental Science; Earth Sciences; Geology; Lithosphere; Mantle; Methane; Mineral Resources; Mineralogy; Original Paper; Petrology; Redox reactions
• ISSN: 0010-7999; 1432-0967
• DOI: 10.1007/s00410-014-1083-6

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 O 2 . 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 Fe 3+ /Fe 2+ data for garnet exist. Relative to the previous calibration of Gudmundsson and Wood ( 1995 ), the new calibration yields somewhat more oxidized values of Δlog f O 2 (FMQ), with the divergence increasing from 60 mol%), with CH 4 being the next most abundant species. To ascertain the capacity for mantle peridotite to act as a source or sink of O 2 , we developed a new model to calculate the f O 2 for a peridotite at a given P , T , and Fe 3+ /Fe 2+ . The results from this model predict 50 ppm or less O 2 is required to shift a depleted mantle peridotite the observed four log units of f O 2 . Coupled with the observed distribution of samples at values of f O 2 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 O 2 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