Example of carbon-iron driven redox reactions in the modern Earth’s

Deep carbon is predominantly stored in accessory phases as a consequence of its low solubility in dominant mantle minerals (e.g., Keppler et al., 2003; Shcheka et al. 2006), where these accessory phases include carbonates, diamonds/graphite, methane and carbides, depending on pressure, temperature, and oxygen fugacity. In highly reducing environments (i.e., low oxygen fugacity), the crystalline form of carbon is graphite or diamond. At more oxidizing conditions, carbonates are favored due to the reaction between elemental carbon and oxygen to form (CO3)^2- groups that bond to other cations such as Ca²⁺, Mg²⁺, Fe²⁺, Ni²⁺ and Na⁺ depending on the composition of the original bulk assemblage. In shallow oceanic mantle, carbonates and carbonatitic melts (carbonatites and/or carbonates silicate melt) could dominate the carbon budget to ~300 km depth (e.g. Dalton and Presnall 1998; Dasgupta and Hirschmann 2006). However, due to redox freezing processes, it is possible that carbonated melts freeze at the expense of diamond/graphite. Starting from a

subducting, locally carbonated, relatively oxidized mafic to ultramafic lithospheres, carbonatite melts will be generated in such lithosphere on thermal relaxation (Rohrbach and Schmidt 2011). This may occur when the lithosphere deflects into the transition zone above the 660-km discontinuity or when stagnating in the lower mantle. On a local scale, oxidized carbonatite melt migrating into the mantle will consume metal (Fe⁰) to first form iron carbide in an intermittent stage, and then further oxidize the Fe and Ni contained in the carbide to leave a mantle domain that contains all iron as Fe²⁺ and Fe³⁺ in silicates (e.g. bridgmanite) and ferropericlase and all carbon as diamond (Rohrbach and Schmidt 2011). Owing to its low viscosity and high wetting properties, any excess carbonatite not consumed by redox reactions would percolate upwards along grain boundaries and exhaust further (Fe,Ni)-metal and carbide until complete redox freezing—that is, immobilization due to reduction of CO2 to C⁰—is achieved. This presumably very efficient process will eventually exhaust all buffering metal and carbide through precipitation of diamond, and result in a metal-free mantle domain where diamonds coexists with Fe³⁺-bearing garnets, perovskite and possibly Fe³⁺-enriched ferropericlase (Fig. 1.3).

Figure 1.3. From Rohrbach and Schmidt (2011). Carbonatitic redox freezing and redox melting caused by redox capacity changes in Earth’s mantle. Main panel, cartoon illustrating a possible sequence of redox freezing and redox melting events driven by oxidation state contrasts between subducted lithosphere and ambient asthenospheric mantle. Right, potential mantle fO2 (red line) and redox buffer capacity (blue line) as function of depth.

On the other hand, redox melting transforms diamond to carbonatite melts, which potentially control the onset of ultra-deep melting.

Figure 1.4. From Stagno et al. (2011). The log fO2 (normalized to IW) buffered by a diamond and magnesite bearing mantle assemblage is shown as a function of pressure for experiments performed between 1500–1700°C (gray diamonds). The data points the higher fO2 region where magnesite is stable from the lower region where diamond forms. Previous measurements of the carbonmagnesite/carbonate melt buffer determined along a mantle adiabat between 3 and 11 GPa (Stagno and Frost, 2010) are also shown (open diamonds). White circles are fO2 measurements from Rohrbach and Schmidt (2011) using IrFe alloy as a redox sensor. The gray shaded regions indicate the fO2 of MORB mantle and the likely fO2 of the transition zone and lower mantle after Frost and McCammon (2008).

However, the depth of reduction is under debate: Stagno and Frost (2010) propose the bottom boundary for redox freezing at 100-150 km, whereas Rohrbach and Schmidt (2011) argued it should be slightly deeper at ~250 km. This difference has surely profound consequences in terms of carbon releasing and outgassing, for instance affecting the composition of the produced melt, which shows kimberlitic affinities

when it forms at ~250 km rather than being a pure carbonatitic melt (Dasgupta et al.

2013a). However, despite the carbonate melt/mineral stability condition due to iron disproportionation-induced redox freezing, local carbonate-rich environments could proceed on their journey through the Earth’ interior entering the Earth’s lower mantle (e.g. Biellmann et al. 1993; Stagno et al. 2011). Interestingly, an experimental determination of the solidus of carbonated peridotite at transition zone and lower mantle depths, shows that following the mantle adiabat for temperature of ~1650 K there could be two crossing of the carbonated peridotite solidus, at ~ 10 GPa and at ~ 30 GPa. This means two things: 1) carbonatite generation happens not only at ~ 300 km depth, but also at around 900 km; 2) the transition zone and the shallow lower mantle may be below the carbonatite solidus, hence they could be considered carbon reservoirs, with carbon stored as iron-magnesite minerals, this proven that the fO2 >

IW+2, Figure 1.4 (Stagno et al. 2011).

Figure 1.5. From Kaminski and Wirth (2011). TEM bright-field image showing a section of the plate-like inclusion in diamond (foil #2053). Note the even interface between iron carbide and diamond, whereas the interface between iron carbide and graphite is invariably irregular. Small fragments of iron carbide of an originally larger grain are present in the graphite matrix.

One should remember however that the diminishing oxygen fugacity with depth (e.g.

Woodland and Koch 2003; McCammon and Kopylova 2004; McCammon 2005;

Stagno and Frost 2010), suggest that diamonds, iron carbides, and more in general iron-nickel carbon-rich alloys may become the stable phase at the expense of carbonates and carbonate melts in the Earth’s lower mantle (e.g. Jacob et al. 2004;

Dasgupta et al. 2009a; Kaminsky and Wirth 2011, Fig. 1.5).