DIAMOND RESORPTION: LINK TO METASOMATIC EVENTS IN THE MANTLE OR RECORD OF MAGMATIC FLUID IN KIMBERLITIC MAGMA?

Abstract

The morphology and surface features on diamond crystals vary tremendously, even within a single body of kimberlite, reflecting a complex history of growth and dissolution. We have developed a methodology to distinguish between diamond-dissolution events happening in the mantle sources of diamond and in the rising kimberlitic magma. We used parcels of microcrystals of diamond (total of 330 stones) from Misery and Grizzly kimberlites, Ekati mine property, Northwest Territories, Canada. The results of morphological studies of resorption features combined with recent experimental data on diamond dissolution allowed us to divide each parcel into several morphological groups: one with kimberlite-induced resorption and several groups with mantle-derived resorption. Kimberlite-induced resorption features are present on ~72–73% of crystals having both octahedral and tetrahexahedral morphology, which have content and aggregation of nitrogen defects very different from those of stones with mantle-derived resorption. The latter make up ~6–13% of the two diamond populations, and their different thermal history in the mantle is supported by the notably lower nitrogen content and higher state of aggregation. Each resorption type represents a single oxidizing event. We define two types of kimberlite-induced resorption: in the presence of an aqueous fluid (Misery parcel) and in fluid-poor kimberlitic magma (Grizzly population). A comparison with the existing experimental data suggests a high H 2 O:CO 2 ratio of the oxidizing fluid, both in the mantle source and in the kimberlitic magma. We propose a new approach for the classification of diamond morphology based on resorption style, with the objective to constrain conditions in diamond-hosting media.