Mixed paragenesis diamond inclusions form during metasomatic fluid evolution

Mineral inclusions in diamond provide the context for diamond formation, yet some diamonds contain disequilibrium ‘mixed paragenesis’ mineral inclusions. Mixed paragenesis diamonds are inferred to somehow record growth of a single diamond in both mafic eclogitic and peridotitic substrates. Here we use thermodynamic modelling with the Extended Deep Earth Water (DEW) model to show that mixed paragenesis mineral inclusions can form during a single episode of diamond growth via fluid-rock metasomatism in the lithospheric mantle. Simulations of fluid infiltration and reactions predict sequential mineral precipitation, with early eclogitic minerals (omphacite and garnet) followed by more peridotitic minerals (e.g., olivine and Mg-rich garnet) during continued diamond growth. Concurrently, modelled fluids evolve from silicic toward carbonatitic compositions and reproduce the compositional range of high density fluids observed in fibrous diamonds. These results demonstrate that mixed paragenesis inclusions and the diversity of diamond-forming fluids can arise from progressive metasomatic reactions during a single diamond-forming event, rather than requiring a diamond to grow first in eclogite and then in peridotite.