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Metamorphic diamonds hosted by major and accessory phases in ultrahigh-pressure (UHP) metamorphic terranes represent important indicators of deep subduction and exhumation of continental crust at convergent plate boundaries. However, their nucleation and growth mechanisms are not well understood due to their small size and diversity. The Bohemian microdiamond samples represent a unique occurrence of monocrystalline octahedral and polycrystalline cubo-octahedral microdiamonds in two different metasedimentary rock types. By combining new and published data on microdiamonds (morphology, resorption, associated phases, carbon isotope composition) with P–T constraints from their host rocks, we demonstrate that the peak P–T conditions for the diamond-bearing UHP rocks cluster along water activity-related phase transitions that determine the microdiamond features. With increasing temperature, the diamond-forming medium changes from aqueous fluid to hydrous melt, and diamond morphology evolves from cubo-octahedral to octahedral. The latter is restricted to the UHP-UHT rocks exceeding 1100 °C, which is above the incongruent melting of phengite, where microdiamonds nucleate along a prograde P–T path in silicate-carbonate hydrous melt. The observed effect of temperature on diamond morphology supports experimental data on diamond growth and can be used for examining growth conditions of cratonic diamonds from kimberlites, which are dominated by octahedra and their resorbed forms.

The degree and character of diamond dissolution were compared to crystallization temperatures (T) and oxygen fugacities (fO 2) estimated from chromite inclusions in olivine phenocrysts in several kimberlites from Lac de Gras, Northwest Territories, Canada. The T and fO 2 values calculated at an assumed pressure of 1 GPa are in the range of 970-1,140 plus or minus 50 degrees C and 2.8-4.4 log fO 2 units below the nickel-nickel oxide (NNO) buffer. The T and fO 2 vary between kimberlites from northwest and southeastern clusters within 150 degrees C and 1 log unit, respectively. A detailed description of morphological forms and surface dissolution features for diamond parcels from the Panda, Beartooth, Grizzly, Misery and Jay kimberlites (>7000 stones) show that an increase in diamond resorption in the kimberlites corresponds to increase in T. The development of various surface dissolution pits and structures correlates with higher fO 2 of kimberlites and therefore mainly happens in the magma. The two processes of diamond dissolution, volume resorption and surface etching, do not show a strong correlation with each other, since some of the resorption occurs in the mantle. We suggest that the fO 2 of the kimberlite magma plays an important role in both the processes. The proportion of plastically deformed brown diamonds does not correlate with the degree of volume resorption, but does correlate with the development of surface forms. The diamond grade is higher in kimberlites with lower fO 2, confirming that conditions of kimberlite crystallization can have notable effect on diamond dissolution. [PUBLICATION ABSTRACT]