Komatiite Lavas and Intrusions

In this Chapter, we consider ultra-low-viscosity lavas. These are represented principally by Archean ultramafic komatiites which form integral components of Archean greenstone belts. Because of their ultramafic, extremely low SiO2 (<45 wt %) and very high MgO contents (>18 wt %), komatiites are renowned for their olivine-rich mineralogy and unique spinifex and cumulate crystalline textures. We briefly review the characteristics of komatiite magmas, their emplacement origins, and their distinctive textures, based on understanding their geology. They were erupted at very high temperatures (up to 1,650 °C) and were also very low-viscosity magmas (~1 Pa s, approximately that of olive oil), which has led previous workers to conclude they had the potential to cause substrate melting and to flow in a turbulent flow state. The komatiite rock suite includes both lavas and subvolcanic intrusions, but differentiating them has sometimes been controversial, and we therefore address the main criteria to distinguish them. Many of the facies characteristics of komatiites are based on the morphology and variations in the morphology of olivine and pyroxene crystals through the profile of komatiite units, and we therefore consider the crystallisation dynamics that produce such variations, which reflect flow and cooling processes in komatiite lavas and intrusions. Both komatiite lavas and intrusions are very important as major sources of nickel sulphide ore deposits. Although they represent a unique silicate magma type, komatiites share some physical properties with other very low-viscosity lavas such as carbonatites and sulphur lavas.