Olivine in kimberlites and related rocks – macrocrysts, dunitic nodules and megacrysts from the same metasomatized source

Abstract

Abstract Kimberlites carry a dense cargo of olivine. Although normally referred to as macrocrysts, much of this olivine is present as multi-grain xenoliths of dunite. Because of their well-rounded outlines, we refer to these objects as ‘nodules’ and apply the term to both multigranular and monogranular varieties. Although compositions within each nodule are very uniform, the entire population displays an enormous variation in Fo content (100*Mg/(Mg + Fe), cation proportions), from close to Fo95 to about Fo82. The change is continuous and there is no indication of separate Fo-rich and Fo-poor populations. Likewise, the sizes, internal textures and habits of Fo-rich and Fo-poor olivines are indistinguishable. Fo-poor nodules have compositions like those of olivines of the megacryst suite and they share similar sizes and internal structures. We propose that the dunite nodules in kimberlites and olivines of the megacryst suite have a common origin. In our interpretation, almost all olivines in kimberlites, except for the marginal rims and rinds, are fragments of dunite that was produced during interaction between mantle peridotite and CO2-rich fluid. Analogues of this process can be found in ophiolites where dunite is the product of reaction between basaltic magma and peridotite. In-situ reactive crystallisation eliminated pyroxene leaving only olivine with Fo contents ranging from higher to lower Fo than in the host peridotite. This interaction occurred near the base of the lithosphere and not during transit of kimberlite towards the surface.