Diamond Dissolution Morphology Yields Clues to Mantle Fluids

Dissolution features commonly observed on natural diamonds hold clues to the conditions the crystals are exposed to while residing in their mantle source and during their ascent to Earth's surface in kimberlite and other diamond-bearing magmas.

Micro-photographs of five diamondsDissolution features commonly observed on natural diamonds hold clues to the conditions the crystals are exposed to while residing in their mantle source and during their ascent to Earth's surface in kimberlite and other diamond-bearing magmas. In these environments the diamonds may undergo partial resorption and re-growth that is recorded in their morphology and zoning patterns. Experimental data demonstrates that the resorption morphology of a diamond greatly depends on the composition of the reacting fluid and thus can be used to constrain the composition of mantle fluids in diamond source areas and magmatic fluid in kimberlite magma.

A recent study [1] used atomic force microscopy (AFM) to quantitatively characterize the crystallographic orientation of micro-faces that form individual etch pits on diamond surfaces produced naturally in a kimberlite magma and experimentally in H2O- and CO2-rich fluids at 1150, 1250 and 1350oC and 1 GPa. The dissolution rates are the same for both fluids but the mechanism of material removal is different. Reaction with H2O-rich fluids develops regular features due to layer-by layer carbon removal. In CO2-rich fluid, deep etch pits with irregular walls form when dissolution is focused around the outcropping dislocations. The size and shape of the etch pits and their association with the outcropping dislocations depend on temperature and could be used to constrain the crystallization conditions of the magma. A diversity of diamond resorption features have been described in other studies but only some of the features were produced in the experiments associated with this study.

Figure:  Micro-photographs under reflected light of the five studied diamonds: (a) H2O-rich fluid at 1150oC, (b) H2O-rich fluid at 1250oC, (c) H2O-rich fluid at 1350oC, (d) CO2-rich fluid at 1250oC, (e) kimberlite-hosted diamond with natural resorption.

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