Crystal population density in not stationary volcanic systems: estimate of olivine growth rate in basalts of Lanzarote (Canary Islands)

We have collected pupulation density data of melanephelinite to olivine tholeiite lavas showing seriated olivine phenocrysts as the first, and often the only, phase occurring on the liquidus. The samples are from the island of Lanzarote, where large volumes of primitive basaltic magmas reach directly the surface from their source regions at a depth between 100 and 50 km. A minimum ascent speed of 1.0 ms-1, corresponding to times of rise of the magmas in the order of 1 day, was estimated from the occurrence of mantle xenoliths. Modal data have been automatically collected from digitalized images of thin sections. The spatial size distribution densities of olivine show an exponential variation for small crystal dimensions and a nearly constant value for large ones. Theoretical analysis shows that this behaviour can be related to variations of nucleation and growth rate with undercooling. Parameters derived for olivine growth rate equation suggest a growth rate of about 3 x 10(-7) cm s-1 for undercooling of about 30-degrees-C.