Dissolution features on diamonds from hypabyssal kimberlite facies and the effect of melt composition on diamond resorption

Abstract

Diamonds can preserve features from their time deep in earth’s mantle and their ascent from the mantle to the Earth’s surface in hot kimberlite magma due to their high stability. The record of dissolution textures on diamond surfaces opens a window into the history of the diamonds journey through the kimberlite. Composition of kimberlitic fluid affects dissolution textures on diamonds from volcaniclastic kimberlite facies. The focus of this study is to identify different resorption styles of diamonds from hypabyssal kimberlite facies and use experiments in volatile-undersaturated melt to determine the effect of melt composition on diamond resorption features. The study uses 1300 diamonds from 16 kimberlites from the Ekati Mine, selected hypabyssal dykes, and sills. This data obtained for class 3 kimberlites will be compared to the existing data on diamonds from class 1 kimberlites. SEM, AFM and microscope imaging will be utilized for analysis of surface features. Secondary dissolution corrosion sculpture (CS) features seen on tetrahexahedra (THH) diamond faces will act as a proxy for kimberlite melt composition to allow for the identification of kimberlitic conditions in class 1 and 3 hypabyssal kimberlites. Experiments in a Piston-cylinder apparatus will quantify the effect of melt composition and temperature/pressure variation on the diamond dissolution features. The experiments are conducted between 1000-1200ºC in silicate, carbonate, and silicate-carbonate melts at 1 GPa in “dry” or H2O-undersaturated conditions. Experiments done in exact temperature pressure conditions with either silicate or carbonate melts have shown drastic differences in resorption styles and features. Diamonds in silicate rich melts under 1 GPa of pressure at 1100 ºC demonstrate strong resorption on the {111} and {012} faces whereas the same experiment in a carbonate melt demonstrates strong graphitization and little resorption. The established relationship between dissolution features on diamonds and composition of kimberlitic melt will allow a long-standing question to be addressed, if different kimberlite classes are formed by a uniform kimberlite melt due to difference in the country rock characteristics or due to compositional differences in kimberlite melt. In addition, use of surface features on microdiamonds from hypabyssal kimberlite facies for early identification of kimberlite class will help better planning for drilling programs and diamond grade assessment. This allows for saving on drilling costs and improves the modelling of kimberlite emplacement. KEYWORDS: Kimberlites; Hypabyssal; Diamonds; Dissolution features; Kimberlitic Melt