KOMATIITES II: EXPERIMENTAL AND THEORETICAL INVESTIGATIONS OF POST-EMPLACEMENT COOLING AND CRYSTALLIZATION

Abstract

When komatiite lavas are emplaced on the sea floor most of the heat transfer occurs through the upper lava-seawater boundary. We have investigated the cooling and crystallization of komatiites using a series of analogue laboratory experiments with aqueous solutions and by theoretical analysis. In komatiites the viscosity is sufficiently low that convection occurs in the interior of the flow and these motions, due both to thermal and compositional variations, have an important influence on the characteristic features of komatiites such as the strong compositional and textural layering. The experiments have been conducted with crystallizing aqueous solutions which display the same overall dynamical processes as solidifying komatiites. The solutions used are simple eutectic systems having the property that crystallization from a solution which is substantially more concentrated than the eutectic composition leaves behind residual fluid which is less dense than the original fluid. This models the decrease in density of komatiite melts on cooling, due to the crystallization of olivine. Such solutions have been cooled strongly through the metal roof of an otherwise insulated container, using a typical fluid depth of 80 mm. Dendritic crystals grew down vertically from the roof and released light fluid, depleted in solute, which rose to form a zone of stagnant fluid at the top of the container, while the tips of the crystals extended just below the bottom of this light layer. A layer of solid eutectic, with a horizontal front, grew more slowly and filled in the space between the vertically oriented crystals. The growth of the crystals and the eutectic layer were monitored visually, and in some experiments the temperatures at the top and in the fluid were recorded, until solidification throughout the layer was complete. The solid block was sampled, and the melted products analysed to give vertical concentration profiles. Both the texture and composition are strongly influenced by the fluid conditions during crystal growth. The top concentration is that of the original solution, rapidly quenched against the roof, and the mean concentration through the region influenced by the stable fluid layer is also close to the original. At the bottom the concentration is high, reflecting the in situ growth of close-packed crystals, and there is a sharp decrease in concentration at an intermediate level, between the upper and lower crystal layers. The experiments and associated theory shed new light on the consolidation of komatiites and the development of their characteristic textures and compositions. Since the lava is convecting within the interior, the early stages of cooling are characterized by a rapid decrease in temperature. Initial cooling rates of 1 to 100 °C h?1 are calculated. At this stage the crust remains thin, but as the spinifex zone develops, convection progressively decreases in vigour and the cooling rate decreases. Spinifex texture is considered to form by constitutional supercooling which is controlled by compositional convection. As the spinifex texture develops, the olivine dendrites form a layer of depleted fluid. The tips of the crystals extend beyond this differentiated layer into a convecting lower region and grow preferentially to produce the characteristic vertically oriented spinifex texture. The composition of spinifex zones is shown to be close, but not identical, to the initial liquid composition. The compositional profiles of the solid products of the experiments are similar to those found in komatiites, with the most evolved rock compositions being found just above the cumulate zone. The experiments also suggest an alternative explanation to crystal settling for the cumulate zone, in which growth of the spinifex zone by compositional convection concentrates crystals suspended within the turbulently convecting lower layer.