The textures, formation and dynamics of rare high-MgO komatiite pillow lavas

Abstract

Pillow lavas are an abundant morphological lava type both on Earth and on some extraterrestrial bodies. We examine pillows from Kambalda (Western Australia) komatiites, that uniquely preserve pillow necks, inter-pillow cavities, and internal crusts. Our study is the first description of true pillows from an Archaean high-MgO (~31%) komatiite. Their size ranges from 0.5 to 5 cm and most have equal vertical and horizontal axes. Due to very low melt viscosities (~0.01 – 0.1 Pa s), these komatiite pillows are one to two orders of magnitude smaller than modern basaltic counterparts. True high-MgO komatiites, such as those at Kambalda, require a low flow rate, potentially found on distal flow edges. Such low flow rates are in conflict with the high flow velocities generally assumed for komatiites, and hence explains the rarity of komatiite pillow lavas.