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Fluid inclusions in fibrous and octahedrally-grown diamonds Smith, Evan Mathew


My thesis puts forth new models for diamond formation that explain the difference between octahedral and fibrous diamond growth, as well as the difference between octahedral diamond growth in the lithospheric and the sublithospheric mantle. Diamond growth in the mantle involves reactions between carbon-bearing fluid and the host rocks it infiltrates. This fluid is sometimes included in diamond. Fluids in dendritically-grown, fibrous diamonds from Wawa, Superior craton, were analysed in a novel way, using transmission X-ray diffraction. The technique allows bulk analysis of daughter minerals within fluid inclusions. The mineralogy, major and trace elements, Sr isotopes, volatiles, and nitrogen characteristics of the hydrous saline–high-Mg carbonatitic fluid in these Archean diamonds strongly resemble those of Phanerozoic fibrous diamonds. This implies that some mantle processes, including the formation of fibrous diamonds, can be extended unvaryingly back to 2.7 Ga. Fluid equilibrated with octahedrally-grown diamonds from the Siberian, Kaapvaal, and Congo cratons is trapped in healed fractures in the diamonds. They contain anhydrous CO₂–N₂ fluid inclusions with 40±4 mol% N₂ and inclusions of former silicate melt that had an original N₂ content of ~0.1 wt%, as shown by Raman, electron microprobe, and microthermometry analyses. The liberation of N₂ from the convecting mantle is proposed to be controlled by increasing oxygen fugacity that destabilizes host phases. The observed distinct fluid compositions between hydrous fluids in fibrous and anhydrous fluids in octahedrally-grown diamond entail distinct processes of diamond formation that, ultimately, govern the growth habit. Water may trigger fibrous growth by inhibiting the expansion of {111} layers and lowering the interfacial energy between the diamond and fluid. Certain features in diamond fluids, such as Eu anomalies and potential carbonate–CO₂ isotopic fractionation, show that several mantle processes can produce geochemical signatures that may be mistaken as input from subducted materials. The finding of N₂ in diamond-forming fluids leads to an explanation for the characteristically low N content of sublithospheric diamonds. I propose this compositional trait is due to growth in a metal-saturated environment. Metallic Fe in the mantle below ~250 km should trap N and may be the largest mantle N reservoir.

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