Effect of reducing conditions on melting point

Question

I've been reading a paper^(*) looking at the deep carbon cycle and it mentions a key process that the authors refer to as "redox melting". According to the paper, reducing conditions increase the melting point of the rocks at depth, which affects partial melting. Can anyone explain why the redox state has an impact on the melting point?

^(*) Foley & Fischer (2017) An essential role for continental rifts and lithosphere in the deep carbon cycle, Nature Geoscience 10, 897–902

Answer 1

As you probably know, the melting temperature in the mantle is a function of H₂O contents. The more H₂O you have, the lower the melting point of the rocks (i.e. dry vs wet solidus in the sketch). A less known phenomenon is the decrease of melting point with the addition of CO₂.

The CO₂ melting curve has a weird shape. At low pressures nothing much happens, but there's a certain pressure where all of the sudden CO₂ is soluble in the melt and it greatly depresses the melting point. This kink is known as the "carbonate ledge". The exact depth of the ledge depends on the composition of the rocks and the experimentalist that performed the experiments, but it's somewhere around two to three GPa, corresponding to about 65 to 100 km deep. Low degree melts above the solidus (the yellow area) are proper carbonatites, and higher degree melts are all kinds of carbonated alkaline melts, with more CO₂ dissolved in them at higher pressures. True hybrid carbonate and silicate melts are only possible at pressures of let's say 4-5 GPa and higher. Below that an immiscibiliy gap exists (also dependent on the alkali contents of the melts - more alkalis = more miscibility).

redox melting

Now, for this to happen you obviously need to have CO₂ in the mantle. But CO₂ is the oxidised form of carbon. In general, the deeper you go in the mantle the more reduced the conditions are. The roots of cratonic lithospheric mantle are very reduced and any carbon in there occurs in the form of reduced carbon: diamonds. Diamonds are no good in facilitating carbonated melting of mantle rocks. However, if a process of metasomatism of fluid flow causes oxidation of the diamonds, they transform to CO₂ thus enabling the formation of carbonated mantle melts.