Intrusion and Crystallization of a Spinifex-Textured Komatiite Sill in Dundonald Township, Ontario

Although komatiite has been defined as an ultramafic volcanic rock characterized by spinifex texture, there is a growing recognition that similar textures can also form in high-level dykes and sills. Here, we report the results of a petrological and geochemical investigation of a ∼5 m thick komatiite sill in Dundonald Township, Ontario, Canada. This unit forms part of a series of komatiites and komatiitic basalts, some of which clearly intruded unconsolidated sediments. The komatiite sill is differentiated into a spinifex-textured upper part and an olivine cumulate lower part. Features characteristic of the upper sections of lava flows, such as volcanic breccia and a thick glassy chilled margin, are absent and, instead, the upper margin of the sill is marked by a layer of relatively large (1–5 mm) solid, polyhedral olivine grains that grades downwards over a distance of only 2 cm into unusually large, centimetre-sized, skeletal hopper olivine grains. This is underlain by a ∼1 m thick zone of platy spinifex-textured olivine and coarse, complex, dendritic, spinifex-textured olivine. The texture of the olivine cumulate zone in the overlying unit is uniform right down to the contact and a lower chilled margin, present at the base of all lava flows, is absent. The textures in the sill and the overlying unit are interpreted to indicate that the sill intruded the olivine cumulate zone of the overlying unit. Thermal modelling suggests that soon after intrusion, a narrow interval of the overlying cumulate partially melted and that the liquid in the upper part of the sill became undercooled. The range of olivine morphologies in the spinifex-textured part of the sill was controlled by nucleation and crystallization of olivine in these variably undercooled liquids.