The buffering capacity of lithospheric mantle: implications for diamond formation

Abstract

50 ppm or less O 2 is required to shift a depleted mantle peridotite the observed four log units of fO 2 . Coupled with the observed distribution of samples at values of fO 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 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 CH 4 content of the fluid, diamond formation related to reduced CHO fluids may also occur in some cratonic lherzolites.

The oxygen fugacity at which graphite or diamond forms from carbonate-bearing melts in eclogitic rocks

“…As these melts migrate and ultimately cool within eclogitic units, crystallization will concentrate the melt in carbonate components, thus reducing the redox stability field of carbonate melt and encouraging diamond/graphite precipitation. Luth and Stachel (2014) propose a slightly different mechanism for diamond formation which would also only produce diamonds on cooling. The more effective scavenging process may explain why a greater proportion of diamonds are found in eclogitic compared to peridotitic rocks.…”

Ancient mantle metasomatism recorded in subcalcic garnet xenocrysts: Temporal links between mantle metasomatism, diamond growth and crustal tectonomagmatism

“…Richardson et al, 1984;Stachel and Harris, 1997;Luth and Stachel, 2014) and high pressure experiments show that diamond precipitation is especially favorable from (alkali)-carbonate bearing systems (e.g. Litvin et al, 1997;Sokol et al, 2000;Pal'yanov et al, 2002).…”

Stable isotope evidence for crustal recycling as recorded by superdeep diamonds

“…It was suggested by Luth and Stachel (2014) that diamond formation in the lithosphere is unlikely to be controlled by reduction of Fe 3+ to Fe 2+ because there is too little buffering potential in such reactions; diamond precipitation as a result of cooling and depressurising C-H-O fluids was proposed instead.…”

“…The concerns raised by Luth and Stachel (2014) should, however, be taken into consideration when attempting to explain the relationships between δ 13 C and δ…”