The transport of xenoliths in magmas

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.