Geometry of dissolution trigons on diamonds: Implications for the composition of fluid and kimberlite magma emplacement

Abstract

Volatiles, H2O and CO2, play an important role in kimberlite magmatism. They facilitate kimberlite production in the mantle by lowering the solidus, drive the fast ascent of kimberlites, fuel their explosive eruptions, and affect preservation of diamonds in kimberlite magma. However, the volatile contents of kimberlite magmas and variations between different kimberlite localities are poorly constrained. In this study we use a novel way to address this problem by utilizing etch pits on diamonds developed during the interaction with kimberlite magma. Trigonal etch pits (trigons) are the most common dissolution features on diamonds. We use precise measurements of their size and dimensions obtained with an Atomic Force Microscope (AFM) to examine and compare crystallization conditions in seven kimberlites from Northwest Territories, Canada. Parameters of 260 trigons on 39 natural diamonds are compared to the parameters of trigons developed on diamonds in controlled experiments from previous studies to assess XCO2 = [CO2/(CO2 + H2O)], mol % in magmatic fluid, level of volatile saturation in the melt, and crystallization temperature of kimberlites. The solubility model for H2O and CO2 in kimberlite-like melts of Moussallam et al. (2016) is used to discuss the effect of these parameters on kimberlite eruption processes. We found significant variability in the shape and size of the trigons on diamonds from different kimberlite lithologies and locations. Diamonds from kimberlite units comprised of resedimented pyroclastic material show predominantly pointed-bottomed trigons and evolution of small flat-bottomed into larger point-bottomed trigons. These features suggest XCO2 > 0.7 and imply shallow exsolution of volatiles. Diamonds from primary pyroclastic kimberlite units display mostly flat-bottomed trigons and a distinct trend from small point-bottomed to larger flat-bottomed trigons indicating XCO2 < 0.5 and greater depths of volatile exsolution with perhaps partial escape of the fluid. Finally extrusive and hypabyssal kimberlite units host diamonds with very complex and irregular trigons, the flat-bottomed shape of which may also indicate H2O-rich magma and exsolution of the fluid at even greater depths with higher likelihood of fluid loss during the ascent. We use the diameter of the trigons to estimate crystallization temperature of the studied kimberlites. The obtained range of 1150 °C to 1250 °C agrees well with previous estimates based on olivine-spinel geothermometry (TOl-Sp) (Fedortchouk and Canil, 2004). Kimberlites with the highest TOl-Sp host diamonds with larger trigons. Our results confirm the important role of H2O and CO2 content of kimberlite magma in controlling the depth of fluid exsolution, reaching the magma fragmentation threshold, the style of magma eruption, and lithological infill of kimberlite pipes.