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

Abstract

eclogites. The raised prevalence of diamonds in eclogites compared to peridotites may, therefore, reflect more effective scavenging of carbon by melts in these rocks. The ferric iron contents of monomineralic layers of clinopyroxene and garnet contained in the same experiments were also measured using Mössbauer spectroscopy. A preliminary model was derived for determining the fO 2 of eclogitic rocks from the compositions of garnet and clinopyroxene, including the Fe 3+ /ΣFe ratio of garnet, using the equilibrium,The model, which reproduces the independently determined fO 2 of the experimental data to within 0.5 log units, can be used to estimate the fO 2 of ultrahigh-pressure metamorphic eclogites and cratonic eclogitic xenoliths. Although there are very few analyses of garnet Fe 3+ /ΣFe ratios from eclogite samples, the range in fO 2 recorded by available eclogitic xenoliths is similar to that reported for peridotitic xenoliths and generally within the graphite/diamond stability field. Estimates for the average bulk Fe 3+ / ΣFe ratio of modern basaltic oceanic crust, however, are higher than the values for most of these xenoliths, and upon subduction, crustal carbon is likely to remain in the carbonate stability field to depths of at least 250 km. If eclogite xenoliths originated from subducted oceanic crust, then their generally lower fO 2 most likely reflects either lower initial basaltic Fe 3+ /ΣFe ratios, loss of Fe 2 O 3 through partial melting or the initial presence of organic carbon. AbstractThe oxygen fugacity (fO 2 ) at which carbonatebearing melts are reduced to either graphite or diamond in synthetic eclogite compositions has been measured in multi-anvil experiments performed at pressures between 3 and 7 GPa and temperatures between 800 and 1,300 °C using iron-iridium and iron-platinum alloys as sliding redox sensors. The determined oxygen fugacities buffered by the coexistence of elemental carbon and carbonatebearing melt are approximately 1 log unit below thermodynamic calculations for a similar redox buffering equilibrium involving only solid phases. The measured oxygen fugacities normalized to the fayalite-magnetite-quartz oxygen buffer decrease with temperature from ~−0.8 to ~−1.7 log units at 3 GPa, most likely as a result of increasing dilution of the carbonate liquid with silicate. The normalized fO 2 values also decrease with pressure and show a similar decrease with temperature at 6 GPa from ~−1.5 log units at 1,100 °C to ~−2.4 log units at 1,300 °C. In contrast to previous arguments, the stability field of the carbonatebearing melt extends to lower oxygen fugacity in eclogite rocks than in peridotite rocks, which implies a wider range of conditions over which carbon remains mobile in natural Communicated by Chris Ballhaus. Electronic supplementary materialThe online version of this article (

T1 - Immiscible C-H-O fluids formed at subduction zone conditions

“…This is because the experiments here at 1.5 to 2.5 GPa, 600 to 700 °C, and the Co-CoO to the Re-ReO 2 buffer are all saturated with graphite (Table 1), and the bulk composition and species of C-H-O fluids are fixed at graphite saturation at the given P-T-fO 2 conditions (e.g., Zhang and Duan, 2009). In the subducting slab, graphite could be formed by graphitisation of organic matter or by reduction of carbonates (Galvez et al, 2013;Stagno et al, 2015).…”

Quantitative analysis of COH fluids synthesized at HP–HT conditions: an optimized methodology to measure volatiles in experimental capsules

“…; Stagno et al . ). Recently, pioneering more complex models including dissolved species have been developed (Pan et al .…”

“…In most cases, the speciation of COH fluids in experiments has been estimated on the basis of thermodynamic models that rely on equations of state of simple H 2 O-nonpolar gas systems (e.g., equations of state of Cesare 1993 andDuan 2009). These models have been largely employed to constrain the composition of COH fluids in equilibrium with solid phases, and these latter being routinely analyzed by means of electron microprobe (e.g., Poli et al 2009;Malaspina et al 2010;Goncharov et al 2012;Malaspina & Tumiati 2012;Tumiati et al 2013;Stagno et al 2015). Recently, pioneering more complex models including dissolved species have been developed (Pan et al 2013;Facq et al 2014;Sverjensky et al 2014;Galvez et al 2015), to account for the interaction between fluid and coexisting solid phases, but these models still rely on a very limited experimental dataset.…”

Evolution of diamond resorption in a silicic aqueous fluid at 1–3 GPa: Application to kimberlite emplacement and mantle metasomatism

“…Klein-BenDavid et al, 2007a) suggesting a continuous spectrum of diamond-precipitating silicate, aqueous, and carbonatitic fluids and melts (Dasgupta and Hirschmann, 2010). The new estimates of the mantle oxygen fugacity (f O2 ) support aqueous fluid or silicate melt poor in carbonate as the diamond-precipitating agents in peridotite (Stagno and Frost, 2010;Stagno et al, 2013) and CO 2 -fluids or carbonate melts as possible diamond-growth media in eclogite (Stagno et al, 2015). Modeling points out to the possibility of involving organic C-species from subducting plates in diamond crystallization due to pH changes when metasomatizing peridotite (Sverjensky et al, 2014).…”