Volatile composition during kimberlite emplacement and mantle metasomatism: constrains from diamond resorption morphology

English Abstract:

Substantially complex features on natural diamonds are recognized as oxidation results in CHO fluids or melts and could fingerprint etchant composition during dissolution. Resorption features are categorized as outline of {111} faces, etch pits, striations and hillocks. Diamonds with a combination of some features clustered in some kimberlite pipes or a part of kimberlite provinces. The potential to utilize characteristic combination of resorption features to probe volatile composition motivated us to oxidize natural octahedral diamonds in synthetic systems MgO-SiO₂-H₂O and SiO₂-CO₂±CaO±H₂O at 1.0 - 3.0 GPa and 1150 - 1400 ^oC using a piston-cylinder apparatus in attempt to reproduce and explain the features observed on natural diamonds. Diamonds were examined using an Atomic Force Microscope (AFM). We monitored fluid composition at run conditions by trapping synthetic fluid inclusions in olivine/quartz plates. Diamonds in fluids with H₂O/(CO₂+H₂O)>50 mol.% develop glossy surfaces, ditrigonal {111} faces, sheaf striations, and negative trigons, while circular pits only occur in pure H₂O with low silica content (≤ 4.2 mole/kg) at 1 GPa. Diamonds in fluids with H₂O/(CO₂+H₂O)<50 mol.% showed trigonal {111} face, negative trigons with curved edges, and hexagonal pits. In CO₂-saturated melts, part of resorbed diamonds showed irregular terraces covered with graphite and part showed CO₂-resorption features. Quantitative AFM images of etch pits suggested two stages of their evolution: defect- and condition-oriented, and changes of geometry of etch pits depends on defect sizes and dissolution conditions. Application of our and reported experimental data to natural diamonds suggests that: in kimberlite, circular pits on diamond formed in pure aqueous fluids in the last stages, and irregular terraces in CO₂-statured melts in the middle stages; and, in the mantle metasomatism, presence of hexagonal pits on diamond required CO₂-dominated fluids/carbonatitic melts.