Upper mantle fluids involved in diamond formation and mantle metasomatism

Abstract:

Diamond formation coupled with metasomatic reactions involving the interaction of fluids with silicate host rocks provides important clues about the deep carbon cycle. However, quantitative modeling of diamond formation with silicate rock metasomatism has not been possible. Here the Deep Water (DEW) model [1] was used to generate equilibrium constants for irreversible chemical mass transfer calculations monitoring evolving fluid chemistry during diamond formation and coupled silicate reactions.

Conceptual models for diamond formation in two environments were constructed for the purpose of illustrating the role of pH in diamond-forming systems. For cratonic diamonds, fluid at 900°C and 5.0 GPa was derived in equilibrium with a carbonated mafic part of a subducting slab consisting of pure diopside, enstatite, pyrope, phlogopite, magnesite, diamond and pyrite. The fluid was assumed to infiltrate and react at constant T and P with a model metasedimentary eclogite (jadeite, pyrope, kyanite and coesite). Abundant diamond was predicted to precipitate as reactant silicate minerals were destroyed and secondary pyrope, jadeite and kyanite were precipitated, which could represent the solid inclusions in natural diamonds. The final fluid chemistry was extremely enriched in Si and depleted in Ca relative to the initial fluid, consistent with the worldwide fluid inclusion trend from carbonatitic fluid to silicic fluid. The logf_O2 changed by only 0.2, whereas pH continuously decreased as reaction with jadeite and kyanite and precipitation of secondary pyrope removed Mg²⁺ and added H⁺ to the fluid. Most of the carbon precipitated as diamond was derived from decreasing concentrations of formate and propionate.

In the UHPM scenario, fluid at 600°C and 5.0 GPa in carbonated peridotite (forsterite, antigorite, clinochlore, magnesite, and pyrrhotite) in a subducting slab was assumed to infiltrate and react at constant T and P with a different model metasedimentary eclogite (jadeite, diopside, pyrope, almandine, coesite, magnesite and aragonite). Two generations of diamond were produced because of the relative rates of change of pH and f_O2. The amount of diamond again correlated closely with the overall decrease in aqueous organic carbon species. [1] Sverjensky, D. A. et al., Geochim. Cosmochim. Acta 129, 125 (2014).