Volatiles in kimberlite magma: insights from surface features on diamonds

Séminaire de Yana Fedortchouk, visiting Prof. (Dalhausie University, Halifax, Canada)

Kimberlites are the deepest magmas found on the surface of the Earth, which provide information about the deep mantle region underneath cratonic lithosphere especially presence of volatiles (H2O and CO2). Volatiles are instrumental in generation of kimberlite melts by lowering mantle solidus. Exsolution of fluid drives the exceptionally fast ascent of kimberlite magma through 200 km of lithosphere and its extremely explosive emplacements forming carrot-shaped pipes. However, the content of H2O and CO2 in kimberlite magma are not well constrained due to uncertain composition of kimberlite melt and its crystallization conditions. The depth of fluid exsolution in kimberlite magma is also not well understood. Degassing upon decompression likely occurs at shallow depths. Deep-seated production of fluid can result from CO2 release by reaction of kimberlite magma with mantle silicates (at pressure < 2.5 GPa) and possibly exsolution of aqueous fluid by passing the second critical point (2CP) during magma scent (at unknown depth). Diamonds offer a unique opportunity to examine the fate of volatiles in kimberlite magma by studying the morphology of dissolution features on diamond surface. Kimberlite magmas transport mantle material including diamonds from the base of lithosphere to the surface, and partially dissolve diamonds during the ascent. The geometry of dissolution features on diamonds shows strong dependence of the presence and composition of fluid in kimberlite magma. This talk will show how integration of diamond dissolution experiments with examination of surface features on natural diamonds from various kimberlite lithologies can help estimating the range of CO2 and H2O contents in various kimberlite magmas, examine mechanism(s) of fluid exsolution, and shed more light on the composition of kimberlite melts. This data helps projecting the initial content of H2O and CO2 in primary kimberlite melts at the source and explore the effect of kimberlite composition of diamond weight loss during transportation from the mantle to the surface.