The transport of xenoliths in magmas |
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Authors: | R.S.J. Sparks H. Pinkerton R. Macdonald |
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Affiliation: | Department of Environmental Sciences, Lancaster University, Lancaster LA1 4YQ Great Britain |
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Abstract: | Field measurements of the rheology of Hawaiian and Etnean lavas have shown that, on eruption, they behave as Bingham liquids with yield strengths in the range 70–450 N/m2. Ultramafic nodules entrained in a Bingham liquid cannot settle unless the stress they impose on the liquid exceeds approximately 5–7 times its yield strength. Consequently magmas with yield strengths of 10–1000 N/m2 can transport xenoliths up to 30 cm diameter without settling occurring. The size of nodules commonly observed rarely exceeds 30 cm. A review of experimental data shows that, when conditions are appropriate for settling, the terminal velocities of nodules in magmas are substantially slower if a Bingham rather than Newtonian Model is assumed.The view that large nodules imply fast rates of magma ascent is rejected. A case is presented for slow rates of ascent being more suitable for nodule transport as there is more opportunity for cooling, crystallisation and hence development of a yield strength. The relative abundance of nodules in the alkaline suite may be a consequence of their slow rates of ascent, whereas their absence in tholeiitic melts may be a consequence of rapid ascent rates. This interpretation is compatible with deductions on their relative rates of ascent based on other geological evidence. |
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