DIAMOND FORMATION DURING PARTIAL MELTING IN THE EARTH’S MANTLE

The C-H-O system is the simplest model for fluids in the Earth’s mantle that can represent both oxidized and reduced fluids coexisting with elemental carbon. At conditions within the diamond stability field, the compositions of fluids coexisting with diamond define the diamond saturation curve in this ternary system. Bulk compositions more C-rich than this curve are two-phase assemblages of diamond + fluid; those less C-rich than the saturation curve are single-phase fluids. The water-maximum fluid at H:O ~ 2 marks the transition between oxidized (dominantly H₂O+CO₂) and reduced (dominantly H₂O+CH₄) fluids.

Removal of H₂O from a C-saturated C-H-O fluid, either by precipitation of hydrous minerals or by dissolution in a melt during partial melting, shifts the residual fluid composition into the two-phase diamond + fluid field. Precipitation of diamond is then necessary – and inevitable – to bring the fluid composition back to the diamond saturation curve.

To focus on the partial melting mechanism, how much diamond forms during such melting will depend on the lithology being melted and the composition of the fluid; a water-maximum fluid will have the greatest effect on lowering the solidus temperature for any lithology. Hydrous melting of harzburgitic mantle will not occur along expected geotherms in the subcontinental lithospheric mantle (Stachel and Luth, Lithos 200-220 2015), so a water-maximum fluid would be stable in harzburgitic mantle. But if that fluid infiltrates either a lherzolitic and eclogitic lithology, it would trigger melting and accompanying formation of diamond in the residue. The amount of diamond formed will depend on the C content of the hydrous fluid, the degree of melting, and the solubility of H₂O in the melt formed. For example, at the hydrous lherzolite solidus at 5 GPa, 20% melting could produce ~640-1200 ppm diamond in the residue for H₂O solubilities of 15-25 wt.%.

Formation of diamond is an inevitable consequence of melting in the diamond stability field in the mantle in the presence of a hydrous, C-bearing fluid. Such a fluid could be derived from dehydration of a subducting lithospheric slab, or be a metasomatic fluid infiltrating the subcratonic lithosphere from greater depths. This mechanism could also play a role in the formation of sublithospheric diamonds.