DIRECTIONAL CHEMICAL VARIATIONS IN DIAMONDS SHOWING OCTAHEDRAL FOLLOWING CUBOID GROWTH

Abstract

A progression from cuboid to octahedral growth has been observed in 16 natural diamonds from Yakutian kimberlites. X-ray and cathodoluminescence topography have revealed that the change in morphology of diamonds with cloudy cuboid cores may occur without mixed-habit growth but via generation of numerous octahedral apices on cuboid surfaces and subsequent gradual transformation into regular octahedral morphology. Nitrogen aggregation in both cuboid and octahedral domains of such diamonds suggests that they have had a long residence time under mantle conditions. Micro-inclusions in the cuboid domains of the diamonds testify to the nucleation and growth of cuboid cores from a hydrous-carbonatitic (oxidized) fluid. The transition from cuboid hummocky growth rich in inclusions to octahedral growth without inclusions may be linked to decreasing upersaturation in the parent fluid. Measurements of δ13C and Nppm by ion microprobe show that the chemical variations observed between inner cuboid domains and outer octahedral zones commonly have a systematic character and as such they are probably not due to purely kinetic effects. The peripheral octahedral zones are always enriched in 13C in comparison with inner cuboid ones, and the total nitrogen content decreases with the change from cuboid to octahedral growth. The octahedral outer zones show a gradual progressive increase in δ13C, with an overall change of up to 5‰ from the cuboid core δ13C usually between -8 and -6‰) to the diamond margin δ13C usually between -4 and -2‰. Decreases in δ13C of this magnitude with a gradual increase in 13C may be attributed to the Rayleigh fractionation operating on a single parent fluid of close to normal mantle δ13C composition with diamond precipitating by the reduction of carbonatitic fluid in a closed system. However, one sample shows a variation of δ13C of approximately -17 to -6‰ and therefore suggests a possible change of fluid source composition from one containing subducted crustal organic carbon to one with common mantle carbon. © Springer-Verlag 2005.