Crystals in Kimberlites: Where are the Phenocrysts?

Kimberlites are among the igneous rocks that we know less about their petrogenesis. This is in part due to their high crystal contents (mainly olivine) and varied provenance of their crystal populations, so that true liquid compositions are difficult to identify. Distinguishing between the different sources of crystals (e.g., phenocrysts vs. xenocrysts) is very important for understanding the formation kimberlites and their associated diamonds. Here we report the results of a petrological and diffusion modeling study of olivine and monticellite from Lac de Gras kimberlites (Canada) which bear implications for the mode and timescales of kimberlite petrogenesis. The rocks contain olivine (Ol) of varied morphology and sizes, from angular through euhedral to rounded, and from about 0.5 cm to 50 μm. In contrast, monticellite (Mo) is euhedral and about 100 μm in diameter. Many large Ol display deformation features such as undulose extinction or kink-bands. Such observations are common to many kimberlites. We distinguish3 types of Ol: (1) unzoned at about Fo93, (2) crystals with cores at Fo86-89 and rims at Fo91, and (3) crystals with cores at ~ Fo93 and rims at Fo91. In addition, any type of crystal may also have a thin (<50 μm) rim of very Mg-rich Ol (Fo98). Despite the different core compositions, all Ol have rims at an intermediate composition (Fo91). This can be the result of mixing between two components and might reflect mantle metasomatic processes related to kimberlite formation. Mo has rather homogenous composition at Fo95-96. Low pressure (< 1 kbar) thermodynamics of Ca-Fe-Mg olivines indicate that this Mo is only in equilibrium with the very Mg-rich Ol (Fo98) found as small rims. Thus, Mo crystallized late, probably when the kimberlite magma was en route to the surface. Diffusion modeling of the Fe-Mg gradients between Ol cores and rims indicates times of a few months, whereas those at the very Fo-rich rims indicates times of a few days. This last estimate overlaps with that of other authors for transfer times of kimberlite magmas from the mantle to the surface. In contrast, our new estimate of several months between magma mixing and eruption have not been yet recognized. Finally, it seems very difficult to attach any genetic information to the word phenocryst in these rocks. The compositional and textural features of the majority of Ol are either the result of open-system processes or occurred very late (Mo and very Fo-rich rims), when the magma was almost at the surface.