DIFFERENTIATION OF KOMATIITE FLOWS

Abstract

Although layered komatiite flows with spinifex-textured upper parts and olivine-enriched lower parts are commonly thought to provide firm evidence of magmatic differentiation by olivine settling, there are reasons to suspect that this may not be true. Komatiite flows almost certainly convect vigorously as they cool, with convective velocities that greatly exceed the settling velocities of olivine grains. Other explanations for the differentiation of komatiite flows into layers with different olivine contents need to be investigated. One such explanation is provided by a detailed study of a komatiite flow from Alexo, Ontario, Canada. This flow is about 16 m thick and has a spinifex-textured upper layer, and a lower layer, called the B-layer, which is composed of closely-packed, equant olivine phenocrysts. The composition of the initial liquid in the flow is given by an upper chill sample, which has 28 per cent MgO. In underlying spinifex lavas, MgO contents range from 20 to 35 per cent MgO, and the B-layer has about 42 per cent MgO. Olivines are most Fo-rich in the chill margin (Fo94.1) and in the B-layer (Fo93.8). In spinifex lavas, olivines are generally less magnesian, ranging from a high of Fo93.8 in unusually MgO-rich chevron spinifex lavas, down to Fo89 in the lower plate spinifex lavas. It is believed that solidification of the upper part of the flow started only after it had become ponded, and that crystallization proceeded with growth of spinifex olivines downwards from the roof of the flow. The manner in which the composition of silicate liquid within the flow changed during growth of the spinifex layer can be calculated using the compositions of olivines in spinifex lavas. These calculations show that all the spinifex lavas are more magnesian than the liquids from which they formed: i.e. they all contain a component of excess olivine. Further calculations indicate, however, that liquid compositions in the lower part of the flow changed more rapidly than can be explained by accumulation of olivine in the spinifex lavas. This additional olivine must have crystallized in the lower part of the flow, and remained there as phenocrysts suspended in the convecting liquid. As the spinifex-textured upper part continued to grow and thicken, olivine continued to crystallize within the flow, and the growing phenocrysts became more and more concentrated in an ever-decreasing volume of liquid. When the concentration exceeded about 50 vol. per cent, the viscosity of the lava became high enough to inhibit convection. The flow then solidified completely, maintaining its olivine distribution with a spinifextextured upper layer and an olivine-enriched lower layer.