KOMATIITES I: ERUPTION AND FLOW

Abstract

Because of their high eruption temperatures and ultrabasic composition, komatiite lavas had low viscosities, which typically ranged from 0-1 to 10 Pa s. A major consequence of this low viscosity is that most lavas erupted as turbulent flows. An analysis of their ascent through the lithosphere suggests ascent velocities in the range of 1 to over 10ms?1 and Reynolds numbers much greater than the critical value of 2000. The lavas would have remained turbulent for most or all of their subsequent flow and emplacement. Typical horizontal flow rates are estimated to range from 0?5 to 100 m2 s?1 per unit width of flow. Such turbulent lava flows would have lost their heat by convection to the surroundings, at rates which are orders of magnitude greater than the rates for laminar flows, which cool by conduction. A quantitative analysis of the cooling of komatiites indicates cooling rates from over 1000 °C hr?1 to a few °C hr?1, while the flows remained turbulent. These rates are in an appropriate range to cause phenomena such as high nucleation rates, strong supersaturation of the lava, delayed nucleation of olivine, and skeletal or dendritic crystal morphologies. Komatiites often flowed over ground composed of rocks with lower melting temperatures. It is proposed that the turbulent lavas melted the ground to form deep thermal erosion channels. Melting rates at the lava source are calculated at several metres per day, and deep troughs with depths of several metres to hundreds of metres and lengths of several kilometres probably formed. Laboratory experiments performed to simulate thermal erosion show qualitative agreement with the theory with channel depth decreasing downstream. The experiments also revealed that the channel margins become undercut during thermal erosion to form overhanging sides of the channel. Some sinuous rilles observed in the mare regions of the Moon are thought to have formed by thermal erosion (Hulme, 1973). They provide analogues of the channels postulated to form in komatiite eruptions, where conditions were in fact more favourable for thermal erosion. An assessment of the role of olivine crystals, precipitated in the cooling, turbulent flows, indicates that they will remain in suspension until the lava has come to rest. Contamination of komatiite lava by underlying rock can be as much as 10 per cent. Some illustrative calculations show how the major element and trace element compositions of residual melts can be significantly modified by combined assimilation and fractional crystallization in a moving flow. Assimilation of tholeiitic basalt into a komatiite can cause incompatible trace element ratios, such as Ti/Zr and Y/Zr, and the rare earth patterns of derivative lavas, to vary substantially. Some of the variations in such geochemical parameters, which are often ascribed to mantle heterogeneity, also could have resulted from assimilation of the ground. Assimilation could have modified the isotope geochemistry of lava suites and led to apparent ages which differ from the true eruption age. The thermal erosion model also provides an explanation of the formation of some nickel sulphide ores found at the bottom of thick komatiite flows. It is proposed that ores can form by assimilation of sulphur-rich sediment, which combines with Ni from the komatiite to form an immiscible liquid.