Komatiites I: Eruption and Flow |
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| Authors: | HUPPERT HERBERT E; SPARKS R STEPHEN J |
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| Institution: | 1Department of Applied Mathematics and Theoretical Physics, University of Cambridge Silver Street, Cambridge CB3 9EW
2Department of Earth Sciences, University of Cambridge Downing Site, Cambridge CB2 3EQ |
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| Abstract: | Because of their high eruption temperatures and ultrabasic composition,komatiite lavas had low viscosities, which typically rangedfrom 0-1 to 10 Pa s. A major consequence of this low viscosityis that most lavas erupted as turbulent flows. An analysis oftheir ascent through the lithosphere suggests ascent velocitiesin the range of 1 to over 10ms–1 and Reynolds numbersmuch greater than the critical value of 2000. The lavas wouldhave remained turbulent for most or all of their subsequentflow and emplacement. Typical horizontal flow rates are estimatedto range from 0?5 to 100 m2 s–1 per unit width of flow.Such turbulent lava flows would have lost their heat by convectionto the surroundings, at rates which are orders of magnitudegreater than the rates for laminar flows, which cool by conduction.A quantitative analysis of the cooling of komatiites indicatescooling rates from over 1000 ?C hr–1 to a few ?C hr–1,while the flows remained turbulent. These rates are in an appropriaterange to cause phenomena such as high nucleation rates, strongsupersaturation of the lava, delayed nucleation of olivine,and skeletal or dendritic crystal morphologies. Komatiites often flowed over ground composed of rocks with lowermelting temperatures. It is proposed that the turbulent lavasmelted the ground to form deep thermal erosion channels. Meltingrates at the lava source are calculated at several metres perday, and deep troughs with depths of several metres to hundredsof metres and lengths of several kilometres probably formed.Laboratory experiments performed to simulate thermal erosionshow qualitative agreement with the theory with channel depthdecreasing downstream. The experiments also revealed that thechannel margins become undercut during thermal erosion to formoverhanging sides of the channel. Some sinuous rilles observedin the mare regions of the Moon are thought to have formed bythermal erosion (Hulme, 1973). They provide analogues of thechannels postulated to form in komatiite eruptions, where conditionswere in fact more favourable for thermal erosion. An assessmentof the role of olivine crystals, precipitated in the cooling,turbulent flows, indicates that they will remain in suspensionuntil the lava has come to rest. Contamination of komatiite lava by underlying rock can be asmuch as 10 per cent. Some illustrative calculations show howthe major element and trace element compositions of residualmelts can be significantly modified by combined assimilationand fractional crystallization in a moving flow. Assimilationof tholeiitic basalt into a komatiite can cause incompatibletrace element ratios, such as Ti/Zr and Y/Zr, and the rare earthpatterns of derivative lavas, to vary substantially. Some ofthe variations in such geochemical parameters, which are oftenascribed to mantle heterogeneity, also could have resulted fromassimilation of the ground. Assimilation could have modifiedthe isotope geochemistry of lava suites and led to apparentages which differ from the true eruption age. The thermal erosionmodel also provides an explanation of the formation of somenickel sulphide ores found at the bottom of thick komatiiteflows. It is proposed that ores can form by assimilation ofsulphur-rich sediment, which combines with Ni from the komatiiteto form an immiscible liquid. |
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